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					              Chapter 1

Introduction to Computers and Java
   Background information
     » important regardless of programming language

   Introduction to Java
Computer Basics
• Computer system: hardware + software
• Hardware: the physical components
• Software: the instructions that tell the
  hardware what to do
    Common Hardware Components
    Standard Hardware                                           • Processor (CPU)
                                                                    – Central Processing Unit
      Organization                                                  – Interprets and executes the
                                                                • Memory
                                                                    – main & auxiliary
                      Memory                                        – holds data and instructions
                     (main & auxiliary)
                                                                • Input device(s)
                                                                    – mouse, keyboard, etc.
                                           Output               • Output device(s)
  Devices                 (CPU)            Devices                  – video display, printer, etc.
(such as mouse and                          (such as video
    keyboard)                             display or printer)   • CPU and memory are
                                                                  physically housed together
Physical Organization
• Keyboard
• Monitor
• Chassis
   –   CPU
   –   memory
   –   disk drives
   –   I/O connectors
   –   etc.
            Two Kinds of Memory
• Main
   – working area
   – temporarily stores program and data (while program is
• Auxiliary
   – permanent (more or less)
   – saves program and results
   – includes floppy & hard disk drives, CDs, tape, etc.
          Main Memory Organization

• Bit = one binary digit
                                    Address Data Byte
   – Binary digit can have only     3021    1111 0000   Item 1: 2 bytes
      one of two values, 0 or 1                         stored
                                    3022    1100 1100
• Byte = 8 bits
                                    3023    1010 1010   Item 2: 1 byte
• “Byte Addressable”                                    stored
   – Main memory is a list of       3024    1100 1110   Item 3: 3 bytes
      numbered locations that       3025    0011 0001
      contain one byte of data in
                                    3026    1110 0001
      each location
                                    3027    0110 0011   Item 4: 2 bytes
• Number of bytes per data item                         stored
  may vary                          3028    1010 0010

                                    3029    …           Next Item, etc.
Auxiliary Memory Organization
          systems Organization
    [fileAuxiliary Memoryfor users]
            Main (Root) Directory / Folder

        Files       Subdirectory    Subdirectory

    Files       Subdirectory             Subdirectory

        Subdirectory        Files            Files

    Files       Subdirectory

                   Running a Program
  Program—a set of instructions for a computer to follow


                                         Computer          Output
(input for the program)
          Many Types of Programs
• User-created applications
• Existing applications
   –   word-processor/editor
   –   web browser
   –   compiler or assembler
   –   etc.
• Operating System
   – DOS, Microsoft Windows, MacOS, Linux, UNIX, etc.
   Various Types of User Interfaces

• Command-line
   – type in key words and letters
   – DOS and UNIX
• Menu
   – parts of DOS and Windows
• GUI (Graphical User Interface)
   – click on icon
   – also called “event-driven”
   – MacOS, Windows
Programming Language Hierarchy

      High-Level Language (HLL)

         Assembly Lanuage

         Machine Language

                    The highs and lows
               of programming languages ...
High-Level Language (HLL)          Machine Language
   – closest to natural language   (lowest level)
   – words, numbers, and math          – least natural language for
     symbols                             humans, most natural
                                         language for hardware
   – not directly understood by        – just 0s and 1s
                                       – directly understood by
   – “portable” source code              hardware
     (hardware independent)            – not portable (hardware
   – Java, C, C++, COBOL,                dependent)
     FORTRAN, BASIC, Lisp, Ada,
                Assembly Language
                      (middle level)
• a more or less human readable version of machine language
• words, abbreviations, letters and numbers replace 0s and 1s
• easily translated from human readable to machine executable
• like machine code, not portable (hardware dependent)
  Getting from Source to Machine Code
• “Compiling a program”
   translating from a high-level language source code to machine (object, or
      executable) code.
• “Compiler”
   a program that translates HLL source code to machine (object, or
      executable) code.
• “Assembly”
   translating from assemble language source code to machine (object, or
      executable) code.
• “Assembler”
   a program that translates assembly source code to machine (object, or
      executable) code.
• Compilers need to know the specific target hardware
 Compilers vs. Assemblers vs. Interpreters

• Compilers and Assemblers
   – translation is a separate user step
   – translation is “off-line,” i.e. not at run time
• Interpreters - another way to translate source to object code
   – interpretation (from source to object code) is not a separate user step
   – translation is “on-line,” i.e. at run time

             Source                                      Object
              Code              Assembler, or            Code
          Java Program Translation
                              Java Program           Data for Java Program
• Both Compilation and
  Interpretation              Java Compiler         Java
• Intermediate Code:                               Virtual
  “Byte Code”                    Program

   – similar to assembly
                           Byte-Code Interpreter
     code, but hardware
     independent            Machine-Language
• Interpreter translates
  from generic byte code                Computer Execution
  to hardware-specific            of Machine-Language Instructions

  machine code
                                              Output of Java Program
                 Java Byte Code
• generated by Java compiler
   – Instead of generating machine language as most compilers
     do, the Java compiler generates byte code.
• translated to machine language of various kinds of
• executed by Java interpreter
• invisible to programmer
   – You don't have to know anything about how byte code
     works to write a Java program.
          Why Use Byte Code?
• requires both compiler and interpreter
• slower program execution
• portability
   – very important
   – same program can run on computers of different types
     (useful with the Internet)
   – Java compiler for new types of computers can be made
Java Program Translation Including Linker
   Java Program            Previously Compiled Helper Programs       Data for Java Program

   Java Compiler
     Byte-Code                                        Virtual
      Program                                         Machine

Byte-Code Interpreter



                                        Computer Execution
                                  of Machine-Language Instructions

                                     Output of Java Program
   Object-Oriented Programming: OOP

• A design and programming technique
• Some terminology:
   – object - usually a person, place or thing (a noun)
   – method - an action performed by an object (a verb)
   – type or class - a category of similar objects (such as
• Objects have both data and methods
• Objects of the same class have the same data elements and
• Objects send and receive messages to invoke actions
       Example of an Object Class

                 Class: Automobile
Data Items:                  Methods:
   –   manufacturer’s name      – Define data items (specify
   –   model name                 manufacturer’s name,
   –   year made                  model, year, etc.)
   –   color                    – Change a data item (color,
                                  engine, etc.)
   –   number of doors
                                – Display data items
   –   size of engine
                                – Calculate cost
   –   etc.
                                – etc.
                     Why OOP?

• Save development time (and cost) by reusing code
   – once a class is created, it can be used in other

• Easier debugging
   – classes can be tested independently
   – reused objects have already been tested
      Design Principles of OOP

  Three main design principles of Object-Oriented
  Programming (OOP):
• Encapsulation
• Polymorphism
• Inheritance
• Design software
   – can be easily used
   – without knowing the details of how it works.
• Also known as information hiding

An analogy:
• When you drive a car, you don’t have know
   – the details of how many cylinders the engine has or
   – how the gasoline and air are mixed and ignited.
• only have to know how to use the controls.
           Reusable Components
Advantages of using reusable components:
• saves time and money
• components that have been used before
   – often better tested and more reliable than new software

Make your classes reusable:
• encapsulation
• general classes have a better chance of being reused
  than ad hoc classes
• Polymorphism—the same word or phrase can be mean
  different things in different contexts
• Analogy: in English, bank can mean:
   – side of a river or
   – a place to put money
• In Java, two or more classes could each have a method called
• Each output method would do the “right thing” for the class
  that it was in. E.g.
   – display a number (Integer class)
   – display an image (Photo class)
• Inheritance—a way of organizing classes

• Term comes from inheritance of traits like eye color, hair color,
  and so on.

• Classes with attributes in common can be grouped so that
  their common attributes are only defined once.
           An Inheritance Hierarchy

       Automobile                  Motorcycle                       Bus

 Sedan              Sports Car                  School Bus                Luxury Bus

What properties does each vehicle inherit from the types of vehicles above it in the
• Algorithm - a set of instructions (steps) for solving a
   – must be precise
   – must be complete

• May be in a number of different formats
  – natural language (such as English)
  – a specific programming language
  – a diagram, such as a flow chart
  – pseudocode - a mix of natural and programming
          Example of an Algorithm
Algorithm that determines the total cost of a list of items:

1. Write the number 0 on the blackboard.

2. Do the following for each item on the list:
--Add the cost of the item to the number on the blackboard.
--Replace the old number on the board by this sum.

3. Announce that the answer is the number written on the board
         Program Design Process
•    Design, then code (not code, then design)
•    Design process
    1.   define the problem clearly
    2.   design objects your program needs
    3.   develop algorithms for the methods of objects
    4.   describe the algorithms, usually in pseudocode

•    Writing/Coding
    1. write the code
    2. test the code
    3. fix any errors and retest
       Testing and Debugging

• Even with careful programming, your code
  could still contain errors and must be
  thoroughly tested.

• Bug—a mistake in a program

• Debugging—fixing mistakes in a program
           Types of Errors
• Syntax

              • Run-Time

                             • Logic
• Syntax: the set of grammar rules for a
  programming language.

• The compiler checks your program to make
  sure it follows the grammar/syntax

• Violating the syntax => error
                 Syntax Errors
• caught by compiler (“compiler-time error”)
• automatically found, usually the easiest to fix
• cannot run program until all syntax errors are
• error message may be misleading

  Misspelling a command, for example “rtrn”
  instead of “return”
                   Run-Time Errors
•   An execution error (during run-time)
•   The program cannot continue to run
•   Not always so easy to fix
•   Error message may or may not be helpful
•   Not detected by the compiler.

    Division by zero - if your program attempts to divide by zero it
    automatically terminates and prints an error message.
                       Logic Errors

Just because it compiles and runs without getting an error
   message does not mean the program is correct!

• An error in the design (the algorithm) or its implementation
   – Program compiles without errors
   – no run-time error messages
   – but incorrect action or data occurs during execution
• Generally the most difficult to find and fix
• Need to be alert and test thoroughly
   – think about test cases and predict results before executing the
            Logic Error Examples
• Algorithm Error:
   – circleArea = radius * radius;
     (pi * radius * radius)

• Implementation Error:
   – typed in wrong symbol in source code -
     sum = a - b;
     (should be sum = a + b;)
       Finally! Now, a taste of Java!
• 1991 - James Gosling, Sun Microsystems, Inc.
• originally a language for programming home appliances
• later (1994) used for World Wide Web applications
   – byte code can be downloaded and run without compiling it
• eventually used as a general-purpose programming
  language (it is object-oriented)
• Why the name “Java”? Not sure - it may just be a name
  that came during a coffee break and it had not been
  copyrighted, yet.
     Applets vs. Java Applications
• Applets
   – Java programs intended to be downloaded via the WWW
     and run immediately
   – “little applications”
   – run in a web browser
• Applications
   – Java programs intended to be installed then run
   – often larger applications
• Slightly different programming for each
import java.util.*;
public class FirstProgram
                                      A Sample Java Program
    public static void main(String[] args)
        System.out.println("Hello out there.");
        System.out.println(“I will add two numbers for you");
        System.out.println(“Enter two whole numbers on a line:");

        int n1, n2;

        Scanner keyboard = new Scanner(;
        n1 = keyboard.nextInt();
        n2 = keyboard.nextInt();

        System.out.println(“The sum of those two numbers is:”);
        System.out.println(n1+ n2);
           Explanation of Code ...
• Code to begin the program (to be explained later):
   public class FirstProgram
       public static void main(String[ ] args)

• Java applications all have similar code at the beginning
   – The name of the class differs from one program to another.
                Explanation of Code ...
• display text strings to the screen:
System.out.println("Hello out there.");
System.out.println(“I will add two numbers for you.");
System.out.println(“Enter two whole numbers on a line.");

    –   Note the “dot” operator
    –   System.out is an object
    –   println is a method that it carries out
    –   double-quoted text inside the parentheses is an argument to the method
    –   general syntax: Object_Name.Method_Name(Arguments)
       … Explanation of Code ...

• Code to create two variables named n1, n2 to contain
  two whole numbers (integer):
   int n1, n2;

• They store the user’s response.
        … Explanation of Code ...
• Creating an object called keyboard of the Scanner

  Scanner keyboard = new Scanner(;

• is the keyboard, but the Scanner
  class has easier methods to use.
         … Explanation of Code ...
• Read two integers typed in from the keyboard and store
  them in the variables n1 and n2:

   n1 = keyboard.nextInt();
   n2 = keyboard.nextInt();
         … Explanation of Code
• Printing the sum to the console:

System.out.println(“The sum of those two numbers is:");
System.out.println(n1 + n2);
            Compiling and Running
               a Java Program
• Compile
  – javac <file>.java
• Run (and link)
  – java <file>
  – <file> must have a main method
• BlueJ has two similar steps by mouse clicking
  (discussed in the labs).
                           Part 1

• A computer’s main memory holds both the program that
  is currently running and its data.
• Main memory is a series of numbered locations, each one
  containing a single byte.
• Auxiliary memory is for more or less permanent storage.
• A compiler is a program that translates a high-level
  language, like java, into a lower level format (“byte-code”
  for java).
• Actual translation of Java byte-code to the hardware’s
  specific machine code occurs at run time (it is
                             Part 2

• An algorithm is a set of instructions for solving a problem (it
  must be complete and precise).
• An object is something that has both data and actions
  (methods) associated with it.
• A class defines a type of object; all objects of the same class
  have the same methods.
• Three OOP design principles are encapsulation,
  polymorphism, and inheritance.
• In a java program, a method invocation has the general form
Primitive Types, Strings, and
         Console I/O
            Chapter 2
• become familiar with Java primitive types
  (numbers, characters, etc.)
• learn about assignment statements and
• learn about strings
• become familiar with classes, methods, and
• learn about simple keyboard input and screen
•   Primitive Types and Expressions
•   The Class String
•   Keyboard and Screen I/O
•   Documentation and Style
          Variables and Values
• Variables store data such as numbers and
   – Think of them as places to store data.
   – They are implemented as memory locations.
• The data stored by a variable is called its value.
   – The value is stored in the memory location.
• Its value can be changed.
        Variables and Values
• variables

• assigning values
  eggsPerBasket = 6;
  eggsPerBasket = eggsPerBasket - 2;
 Naming and Declaring Variables

• Choose names that are helpful such as count
  or speed, but not c or s.
• When you declare a variable, you provide its
  name and type.
  int numberOfBaskets, eggsPerBasket;
• A variable‘s type determines what kinds of
  values it can hold (int, double, char, etc.).
• A variable must be declared before it is used.
       Syntax and Examples
• syntax
  type variable_1, variable_2, …;

• examples
  int styleChoice, numberOfChecks;
  double balance, interestRate;
  char jointOrIndividual;
                 Types in Java
• A class type
  – a class of objects and has both data and methods.
  – “Think Whirled Peas” is a value of class type String
• A primitive type
  – simple, nondecomposable values such as an
    individual number or individual character.
  – int, double, and char are primitive types.
         Naming Conventions
• Class types
  – begin with an uppercase letter (e.g. String).
• Primitive types
  – begin with a lowercase letter (e.g. int).
• Variables of both class and primitive types
  – begin with a lowercase letters (e.g. myName,
  – Multiword names are ―punctuated‖ using uppercase
   Where to Declare Variables
• Declare a variable
  – just before it is used or
  – at the beginning of the section of your program that is
    enclosed in {}.
    public static void main(String[] args)
    { /* declare variables here */


               Java Identifiers
• An identifier
   – a name, such as the name of a variable.
• Identifiers may contain only
   – letters
   – digits (0 through 9)
   – the underscore character (_)
   – and the dollar sign symbol ($) which has a special
   – but the first character cannot be a digit.
        Java Identifiers, cont.
• identifiers may not contain any spaces, dots (.),
  asterisks (*), or other characters:
        7-11   util.* (not allowed)
• Identifiers can be arbitrarily long.
• Since Java is case sensitive, stuff,    Stuff, and
  STUFF are different identifiers.
 Keywords or Reserved Words
• Words such as if are called keywords or
  reserved words and have special, predefined
• Keywords cannot be used as identifiers.
• See Appendix 1 for a complete list of Java
• other keywords: int, public, class
• Appendix 1
                 Primitive Types
• four integer types (byte,    short, int,   and long)
   – int is most common

• two floating-point types (float and double)
   – double   is more common
• one character type (char)
• one boolean type (boolean)
Primitive Types, cont.
  Examples of Primitive Values
• integer types
      0   -1   365     12000

• floating-point types
      0.99     -22.8   3.14159 5.0

• character type
      `a`    `A`   `#`   ` `
• boolean type
      true     false
      Assignment Statements
• An assignment statement is used to assign a
  value to a variable.
  answer = 42;

• The ―equal sign‖ is called the assignment
• We say, ―The variable named answer is assigned
  a value of 42,‖ or more simply, ―answer is
  assigned 42.‖
 Assignment Statements, cont.
• Syntax
 variable = expression ;
 where expression can be
  – another variable,
  – a literal or constant (such as a number),
  – or something more complicated which combines
    variables and literals using operators (such as + and -)
        Assignment Examples
amount = 3.99;
firstInitial = „W‟;
score = numberOfCards + handicap;
eggsPerBasket = eggsPerBasket - 2;

(last line looks weird in mathematics, why?)
       Assignment Evaluation
• The expression on the right-hand side of the
  assignment operator (=) is evaluated first.
• The result is used to set the value of the variable
  on the left-hand side of the assignment operator.
  score = numberOfCards + handicap;   eggsPerBasket =
  eggsPerBasket - 2;
Specialized Assignment Operators
• Assignment operators can be combined with
  arithmetic operators (including -, *, /, and %,
  discussed later).
  amount = amount + 5;

  can be written as
  amount += 5;
  yielding the same results.
        Simple Screen Output
  System.out.println(“The count is “ + count);

• outputs the Sting literal ―The count is ― followed by
  the current value of the variable count.
• + means concatenation if one argument is a

 (an example of which of the three properties of
  OO languages?)
                Simple Input
• Sometimes the data needed for a computation
  are obtained from the user at run time.
• Keyboard input requires
  import java.util.*

  at the beginning of the file.
          Simple Input, cont.
• Data can be entered from the keyboard using
     Scanner keyboard =
           new Scanner(;

  followed, for example, by
  eggsPerBasket = keyboard.nextInt();
  which reads one int value from the keyboard
  and assigns it to eggsPerBasket.
            Simple Input, cont.
• class EggBasket2
           Number Constants
• Literal expressions such as 2, 3.7, or ‟y‟ are
  called constants.
• Integer constants can be preceded by a + or -
  sign, but cannot contain commas.
• Floating-point constants can be written
  – with digits after a decimal point or
  – using e notation.
                   e Notation
• e notation is also called scientific notation or
  floating-point notation.
• examples
   – 865000000.0 can be written as 8.65e8
   – 0.000483 can be written as 4.83e-4

• The number in front of the e does not need to
  contain a decimal point, eg. 4e-4
    Assignment Compatibilities
• Java is said to be strongly typed.
  – You can‘t, for example, assign a floating point value to
    a variable declared to store an integer.
• Sometimes conversions between numbers are
  doubleVariable = 7;
  doubleVariable = intVariable ;

  is possible even if doubleVariable is of type double,
  for example.
  Assignment Compatibilities, cont.
• A value of one type can be assigned to a
  variable of any type further to the right
  byte --> short --> int --> long
  --> float --> double

  but not to a variable of any type further to the
• You can assign a value of type char to a variable
  of type int.
                Type Casting
• A type cast creates a value in a new type from
  the original type.
• For example,
  double distance;
  distance = 9.0;
  int points;
  points = (int)distance;

  (illegal without (int))
          Type Casting, cont.
• The value of (int)distance is 9, but the value of
  distance, both before and after the cast, is 9.0.
• The type of distance does NOT change and
  remains float.
• Any nonzero value to the right of the decimal
  point is truncated, rather than rounded.
       Characters as Integers
• Characters are actually stored as integers
  according to a special code
  – each printable character (letter, number, punctuation
    mark, space, and tab) is assigned a different integer
  – the codes are different for upper and lower case
  – for example 97 may be the integer value for ‗a‘ and 65
    for ‗A‘
• ASCII and Unicode are common character
       Unicode Character Set
• Most programming languages use the ASCII
  character set.
• Java uses the Unicode character set which
  includes the ASCII character set (Appendix 3)
• The Unicode character set includes characters
  from many different alphabets other than English
  (but you probably won‘t use them).
 32   33          34    35         36   37           38       39        40   41
          !        “       #        $       %        &        ‘         (     )

48    …       57       …       65       …       90        …        97       … 122
0             9                A                Z                  a              z
     Casting a char to an int
• Casting a char value to int produces the
  ASCII/Unicode value
• For example, what would the following display?
   char answer = ’y’;
• >y
          Initializing Variables
• A variable that has been declared, but no yet
  given a value is said to be uninitialized.
• Uninitialized class variables have the value null.
• Uninitialized primitive variables may have a
  default value.
• It‘s good practice not to rely on a default value,
  which could be arbitrary.
     Initializing Variables, cont.
• To protect against an uninitialized variable (and
  to keep the compiler happy), assign a value at
  the time the variable is declared.
• Examples:
  int count = 0;
  char grade = ‟A‟;   // default is an A
    Initializing Variables, cont.
• syntax
 type variable_1 = expression_1, variable_2 =
 expression_2, …;
        Binary Representation
• Assume an 8-bit type
• 5 as an integer
   – 00000101
• ‗5‘ as a character
   – 00110101 (53 decimal, ASCII)
• 5.0 as a floating point number
   – How?
   – What about 5.5?
         Binary Real Numbers
     …         23   22   21   20       2-1   …
• 5.5
   – 101.1
• 5.25
   – 101.01
• 5.125
   – 101.001
• 5.75
   – 101.11
                        8 bits only
       25      24       23      22      21      20      2-1   2-2

• 5.5
   – 101.1 -> 000101 10
• 5.25
   – 101.01 -> 000101 01
• 5.125
   – 101.001 -> ??
• With only 2 places after the point, the precision is .25
• What if the point is allowed to move around?
     Floating-point Numbers
• Decimal
   – 54.3
   – 5.43 x 101     [scientific notation]

• Binary
   – 101.001
   – 10.1001 x 21    [more correctly: 10.1001 x 101]
   – 1.01001 x 22    [more correctly: 1.01001 x 1010]

   – What can we say about the most significant bit?
      Floating-point Numbers
• General form: sign 1.mantissa x 2exponent
  – the most significant digit is right before the dot
     • Always 1 [no need to represent it]
  – (more details are not discussed here: mantissa has
    no sign, but sign is embedded in exponent…)
• 1.01001 x 22
  – Sign: positive (0)
  – Mantissa: 01001
  – Exponent: 10 (decimal 2)
     • [IEEE standard: ―biased exponent‖
         – exponent + 2numBits-1 - 1
         – Example: 2 + 22-1 – 1 = 3 => 11 in binary]
  Java Floating-point Numbers

sign     exponent            mantissa

• Sign:
   – 1 bit [0 is positive]
• Mantissa:
   – 23 bits in float
   – 52 bits in double
• Exponent:
   – 8 bits in float
   – 11 bits in double
   Imprecision in Floating-Point
• Floating-point numbers often are only
  approximations since they are stored
  with a finite number of bits.
• Hence 1.0/3.0 is slightly less than 1/3.
• 1.0/3.0 + 1.0/3.0 + 1.0/3.0 could be
  less than 1.
         Arithmetic Operations
• Arithmetic expressions can be formed using the
  +, -, *, and / operators
  – together with variables or numbers referred to as
  – When both operands are of the same type
     • the result is of that type.
  – When one of the operands is a floating-point type and
    the other is an integer
     • the result is a floating point type.
   Arithmetic Operations, cont.
• Example
  If hoursWorked is an int to which the value 40 has
  been assigned, and payRate is a double to which
  8.25 has been assigned

  hoursWorked * payRate

  is a double with a value of 500.0.
   Arithmetic Operations, cont.
• Expressions with two or more operators can be
  viewed as a series of steps, each involving only
  two operands.
  – The result of one step produces one of the operands
    to be used in the next step.
• example
    balance + (balance * rate)
   Operators with integer and floating
            point numbers
• if at least one of the operands is a floating-point
  type and the rest are integers
   – the result will be a floating point type.
• The result is the rightmost type from the
  following list that occurs in the expression.
  byte --> short --> int --> long
  --> float --> double
          The Division Operator
• The division operator (/) behaves as expected
  – if one of the operands is a floating-point type.
• When both operands are integer types
  – the result is truncated, not rounded.
  – Hence, 99/100 has a value of 0.
  – called integer division or integer divide
            The mod Operator
• The mod (%) operator is used with operators of
  integer type to obtain
  – the remainder after integer division.
• 14 divided by 4 is 3 with a remainder of 2.
  – Hence, 14 % 4 is equal to 2.
• The mod operator has many uses, including
  – determining if an integer is odd or even
  – determining if one integer is evenly divisible by
    another integer.
  Case Study:Vending Machine
• requirements
  – The user enters an amount between 1 cent and 99
  – The program determines a combination of coins
    equal to that amount.
  – For example, 55 cents can be two quarters and
    one nickel.
             Case Study, cont.
• sample dialog
  Enter a whole number from 1 to 99.
  The machine will determine a combination of coins.
  87 cents in coins:
    3 quarters
    1 dime
    0 nickels
    2 pennies
           Case Study, cont.
• variables needed
  int amount, quarters, dimes, nickels, pennies;
                Case Study, cont.
•    algorithm - first version
    1.   Read the amount.
    2.   Find the maximum number of quarters in the amount.
    3.   Subtract the value of the quarters from the amount.
    4.   Repeat the last two steps for dimes, nickels, and pennies.
    5.   Print the original amount and the quantities of each coin.
             Case Study,cont.
• The algorithm doesn‘t work properly, because
  the original amount is changed by the
  intermediate steps.
  – The original value of amount is lost.
• Change the list of variables
  int amount, originalAmount, quarters, dimes,
  nickles, pennies;

• and update the algorithm.
          Case Study, cont.
1. Read the amount.
2. Make a copy of the amount.
3. Find the maximum number of quarters in the
4. Subtract the value of the quarters from the amount.
5. Repeat the last two steps for dimes, nickels, and
6. Print the original amount and the quantities of each
           Case Study, cont.
• Write Java code that implements the algorithm
  written in pseudocode.
            Case Study, cont.
• How do we determine the number of quarters (or
  dimes, nickels, or pennies) in an amount?
  – There are 2 quarters in 55 cents, but there are also 2
    quarters in 65 cents.
  – That‘s because
     55 / 2 = 2 and 65 / 25 = 2.
            Case Study, cont.
• How do we determine the remaining amount?
  – using the mod operator
     55 % 25 = 5 and 65 % 25 = 15
  – similarly for dimes and nickels.
  – Pennies are simply amount % 5.
          Case Study, cont.
• class ChangeMaker
          Case Study—testing the
• The program should be tested with several
  different amounts.
• Test with values that give zero values for each
  possible coin denomination.
• Test with amounts close to
  – extreme values such as 0, 1, 98 and 99
  – coin denominations such as 24, 25, and 26
     • Boundary values.
  Increment (and Decrement)
• used to increase (or decrease) the value of a
  variable by 1
• easy to use, important to recognize
• the increment operator
   count++ or ++count

• the decrement operator
   count-- or --count
  Increment (and Decrement)
• ―mostly‖ equivalent operations
  count = count + 1;

  count = count - 1;
     Increment (and Decrement)
      Operators in Expressions
• after executing
   int m = 4;
   int result = 3 * (++m)

   result hasa value of 15 and m has a value of 5
• after executing
   int m = 4;
   int result = 3 * (m++)

   result has   a value of 12 and m has a value of 5
      Increment and Decrement Operator
common code
int n = 3;
int m = 4;
int result;
What will be the value of m and result after each of these
(a) result = n * ++m; //preincrement m
(b) result = n * m++; //postincrement m
(c) result = n * --m; //predecrement m
(d) result = n * m--; //postdecrement m
    Answers to Increment/Decrement
         Operator Questions
(a) 1) m = m + 1;        //m = 4 + 1 = 5
    2) result = n * m;   //result = 3 * 5 = 15

(b) 1) result = n * m;   //result = 3 * 4 = 12
    2) m = m + 1;        //m = 4 + 1 = 5

(c) 1) m = m - 1;        //m = 4 - 1 = 3
    2) result = n * m;   //result = 3 * 3 = 9

(b) 1) result = n * m;   //result = 3 * 4 = 12
    2) m = m - 1;        //m = 4 - 1 = 3
      Summary of Operators
• +, -, *, /
• %
• ++, --
  Parentheses and Precedence
• Parentheses can communicate the order in
  which arithmetic operations are performed
• examples:
  (cost + tax) * discount
  cost + (tax * discount)

• Without parentheses, an expression is evaluated
  according to the rules of precedence.
Precedence Rules
 Precedence Rules—Binary Operators

• The binary arithmetic operators *, /, and %
  – have lower precedence than the unary operators +, -,
    ++, --, and !
  – but have higher precedence than the binary arithmetic
    operators + and -. (Appendix 2)
• When binary operators have equal precedence
  – the operator on the left has higher precedence than
    the operator(s) on the right.
 Precedence Rules—Unary Operators

• When unary operators have equal precedence
  – the operator on the right has higher precedence than
    the operation(s) on the left
      • opposite order to binary operators
  – if x is 10
      • -++x is -11 and x is 11 afterwards
      • same as –(++x)
  – if x is 10
      • -x++ is -10 and x is 11 afterwards
      • same as –(x++)
           Use Parentheses
• Even when parentheses are not needed,
  they can be used to make the code
  balance + (interestRate * balance)
• [Spaces also make code clearer
  balance + interestRate*balance
  but spaces do not dictate precedence.]
Sample Expressions
          The Class String
• We‘ve used constants of type String already.
  “Enter a whole number from 1 to 99.”

• A value of type String is a sequence of
  characters treated as a single item.
  Declaring and Printing Strings
• declaring
  String greeting;
   greeting = “Hello!”;
   String greeting = “Hello!”;
   String greeting = new String(“Hello!”);

• printing
     Concatenation of Strings
• Two strings are concatenated using the +
  String greeting = “Hello”;
  String sentence;
  sentence = greeting + “ officer”;

• Any number of strings can be concatenated
  using the + operator.
 Concatenating Strings and
String solution;

solution = “The temperature is “ + 72;
System.out.println (solution);

The temperature is 72
• A class is a type used to produce objects.
• An object is an entity that stores data and can
  take actions defined by methods.
• An object of the String class stores data
  consisting of a sequence of characters.
• The length() method returns the number of
  characters in a particular String object.
  int howMany = solution.length()
   Objects, Methods, and Data
• Objects within a class
  – have the same methods
  – have the same kind(s) of data but the data can have
    different values.
• Primitive types have values, but no methods.
String Methods
      The Method length()
• The method length() returns an int.
• You can use a call to method length() anywhere
  an int can be used.
  int count = solution.length();
  spaces = solution.length() + 3;
         Positions in a String
• positions start with 0, not 1.
   – The „J‟ in “Java is fun.” is in position 0
 Positions in a String, cont.
• A position is referred to an an index.
   – The „f‟ in “Java is fun.” is at index 9.
Indexing Characters within a String
• charAt(position)method
   – returns the char at the specified position
• substring(start, end) method
   – returns the string from start upto excluding end
• For example:
   String greeting = "Hi, there!";
   greeting.charAt(0)      returns H
   greeting.charAt(2)      returns ,
   greeting.substring(4,7) returns the

  H      i     ,            t     h     e         r   e   !
  0      1     2     3      4     5     6         7   8   9
    Using the String Class
• class StringDemo
          Escape Characters
• How would you print
  “Java” refers to a language.?

• The compiler needs to be told that the quotation
  marks (“) do not signal the start or end of a
  string, but instead are to be printed.
  “\”Java\” refers to a language.”);
        Escape Characters

• Each escape sequence is a single character
  even though it is written with two symbols.
char singleQuote = „\‟‟;
   The Unicode Character Set
• Most programming languages use the ASCII
  character set.
• Java uses the Unicode character set which
  includes the ASCII character set
  – Backward compatible to ASCII
• The Unicode character set includes characters
  from many different alphabets (but you probably
  won‘t use them).
      Keyboard and Screen
          I/O: Outline
• Screen Output
• Keyboard Input
              Screen Output
• We‘ve seen several examples of screen output
• System.out is an object that is part of Java.

• println() is one of the methods available to the
  System.out object.
         Screen Output, cont.
• The concatenation operator (+) is useful when
  everything does not fit on one line.
  System.out.println(“When everything “ +
    “does not fit on one line, use the” +
    “ concatenation operator (\‟+\‟)”);
  – Do not break the line except immediately before or
    after the concatenation operator (+).
         Screen Output, cont.
• Alternatively, use print()
   System.out.print(“When everything “);
   System.out.print(“does not fit on “);
   System.out.print(“one line, use the “);
   System.out.print(“\”print\” ”);

   ending with a println().
           Screen Output, cont.
• syntax
  System.out.println(output_1 + output_2 + ...+ output_n);
• example
  System.out.println (1967 + “ “ + “Oldsmobile” + “
    “ + 442);
  1967 Oldsmobile 442
printf (or format) Method for Output
• Heavily influenced by C
• outputStream.printf(formatString, args…)
   – System.out.printf(…)
   – smileyOutStream.printf(…)
• formatString specifies how to format args
• System.out.printf(“%s %d %f%n”, name, id, gpa);
   – System.out.println(name + “ “ + id + “ “ +
• Useful for “right justified” numbers
• Numbers in println and print are “left justified”
            Formatting String
• % width conversion
• width specifies how many slots are available for output
• If width > number of characters, spaces are printed first
  before the characters—“right justified”
• printf(“%5d”, count)
   – Count
       • 32901: 3 2 9 0 1
       • 2004:          2 0 0 4

       • 22:                2 2

       • 6747280: all digits are printed, width is ignored
             Conversion Characters
Conversion   Argument                     Description
d            integer                      Decimal integer
f            floating point               Decimal float
s            general (String, Boolean, …) String
n                                         New line
c            character                    Character (unicode)
e            floating point               Decimal scientific notation
o            integer                      Octal integer
x            integer                      Hexadecimal integer
%                                         %    (%% to output %)
           Floating-point Precision
• width.precision conversion
• printf(“%5.2f”, PI)

                      3        .       1       4
• printf(“%7.4f”, PI)

                  3       .        1       4       1   5
                  Left Justified
• Spaces are added (padded) on the right
• Minus (-) sign before the width
• …printf(“%-7s %-4d”, name, age)

          J o h n                     2 0

• Why are there 4 spaces after ―John‖ instead of 3?
               Keyboard Input
• Starting from Java 5.0
  – Java has reasonable facilities for handling keyboard
• Scanner   class in the java.util package
  – A package is a library of classes.
      Using the Scanner Class
• Near the beginning of your program, insert
  import java.util.*

• Create an object of the Scanner class
  Scanner keyboard =
     new Scanner(

• Read data (an int or a double, for example)
  int n1 = keyboard.nextInt();
  double d1 = keyboard.nextDouble();
         Some Scanner Class
•   syntax    Methods
    Int_Variable =    Object_Name.nextInt();
    Double_Variable = Object_Name.nextDouble();
    Float_Variable = Object_Name.nextFloat();

    String_Variable =;
    String_Variable = Object_Name.nextLine();

    Boolean_Variable = Object_Name.nextBoolean();

    nextByte(), nextShort(), nextLong()
 Some Scanner Class Methods,
• examples
  int count = keyboard.nextInt();
  double distance = keyboard.nextDouble();
  String word =;
  String wholeLine = keyboard.nextLine();

• Remember to prompt the user for input, e.g.
  System.out.print(“Enter an integer: “);
  Keyboard Input Demonstration
• class ScannerDemo
  nextLine()Method Caution
• The nextLine() method reads the remainder of
  the current line, even if it is empty.
nextLine()Method Caution, cont.
• example
  int n;
  String s1, s2;
  n = keyboard.nextInt();
  s1 = keyboard.nextLine();
  s2 = keyboard.nextLine();
  or bust

  n isset to 5440
  but s1 is set to the empty string.
              The Empty String
• String with zero characters

         String s3 = “”;

• Good for String initialization
        Other Input Delimiters
• Characters for separating ―words‖
  – Default is ―whitespace‖: space, tab, newline
• Change the delimiter to “##”


  – whitespace will no longer be a delimiter for keyboard2
      Other Input Delimiters, cont.
• class DelimitersDemo
    Documentation and Style: Outline
•   Meaningful Names
•   Self-Documentation and Comments
•   Indentation
•   Named Constants (ALL CAPITAL LETTERS)
     Documentation and Style
• Most programs are modified over time to
  respond to new requirements.
• Programs which are easy to read and
  understand are easy to modify.
• Even if it will be used only once, you have to
  read it in order to debug it .
Meaningful Names for Variables
• A variable‘s name should suggest its use.
• Observe conventions in choosing names for
  – Use only letters and digits.
  – Use more than one character.
  – ―Punctuate‖ using uppercase letters at word
    boundaries (e.g. taxRate).
  – Start variables with lowercase letters.
  – Start class names with uppercase letters.
 Documentation and Comments
• The best programs are self-documenting.
  – clean style
  – well-chosen names
• Comments are written into a program as needed
  explain the program.
  – They are useful to the programmer, but they are
    ignored by the compiler.
       When to Use Comments
• Begin each program file with an explanatory
  –   what the program does
  –   the name of the author
  –   contact information for the author
  –   date of the last modification.
• Provide only those comments which the
  expected reader of the program file will need in
  order to understand it.
         Comments Example
• class CircleCalculation
• A program can usually be broken into
  segments/blocks based on the algorithm, e.g. in
   –   Prompt the user for input
   –   Input from the keyboard
   –   Calculation
   –   Output to the screen
• Blank line between two segments
• A description (comment) before each segment
         Pseudocode and Comments
•   Solving a problem
    1.   Devise an algorithm (steps to solve the problem)
    2.   Write the algorithm in pseudocode (semi English, semi Java)
    3.   English part of pseudocode becomes comments in your program
•   Tip:
    1.   type the English part of pesudocode as comments into your program
    2.   write the detailed Java instructions to satisfy/implement the
•   Advantages:
    1.   Each line of pseudocode helps you focus on a small task
    2.   Pseudocode tells you what steps you want to achieve
    3.   No need to add comments later on
• A comment can begin with //.
  – Everything after these symbols and to the end of the
    line is treated as a comment and is ignored by the
    double radius; //in centimeters
            Comments, cont.
• A comment can begin with /* and end with */
  – Everything between these symbols is treated as a
    comment and is ignored by the compiler.
  /* the simplex method is used to
    calculate the answer*/
            Comments, cont.
• A javadoc comment, begins with /** and ends
  with */.
  – It can be extracted automatically from Java software.
  /** method change requires the number of coins to
    be nonnegative */
• Indentation should communicate nesting clearly.
• A good choice is four spaces for each level of
• Indentation should be consistent.
• Indentation should be used for second and
  subsequent lines of statements which do not fit
  on a single line.
           Indentation, cont.
• Indentation does not change the behavior of the
• Improper indentation can miscommunicate the
  behavior of the program.
            Named Constants
• To avoid confusion, always name constants (and
  circumference = PI * radius;
  is clearer than
  circumference = 3.14159 * 6.023;

• Place constants near the beginning of the
        Named Constants, cont.
• The value of a constant cannot be changed
  once it is initialized

   public static final double INTEREST_RATE = 6.65;

• Consider the interest rate is used many times in the
   – What if you type 6.65 in some places, but 6.56 in others?
   – What if the interested rate has changed to 7.3?
   – Is balance * INTEREST_RATE easier to read than
        balance * 6.65 ?
           Declaring Constants
• syntax
  public static final Type Name = Constant;

• examples
  public static final double PI = 3.14159;
  public static final String MOTTO = “The customer
    is always right.”;
  – By convention, uppercase letters are used for
           Named Constants
• class CircleCalculation2
• You have become familiar with Java primitive
  types (numbers, characters, etc.).
• You have learned about assignment statements
  and expressions.
• You have learned about stings.
• You have become familiar with the basics of
  classes, methods, and objects.
• You have learned about simple keyboard input
  and screen output.
Flow of Control
     Chapter 3
• learn about Java branching statements
• learn about loops
• learn about the type boolean
           Flow of Control
• Flow of control is the order in which a
  program performs actions.
   – Up to this point, the order has been
• A branching statement chooses between two
  or more possible actions.
• A loop statement repeats an action until a
  stopping condition occurs.
Branching Statements: Outline
• The if-else Statement
• Introduction to Boolean Expressions
• Nested Statements and Compound
• Multibranch if-else Statements
• The switch Statament
• (optional) The Conditional Operator
    The if-else Statement
• A branching statement that chooses between
  two possible actions.
• syntax
  if (Boolean_Expression)
   The if-else Statement,
• example
  if (count < 3)
      total = 0;
      total = total + count;
The if-else Statement, cont.
• class BankBalance
     Compound Statements
• To include multiple statements in a branch,
  enclose the statements in braces.
  if (count < 3)
      total = 0;
      count = 0;
      Omitting the else Part
• If the else part is omitted and the expression
  after the if is false, no action occurs.
• syntax
   if (Boolean_Expression)

• example
   if (weight > ideal)
       caloriesPerDay -= 500;
     Introduction to Boolean
• The value of a boolean expression is either
  true or false.

• examples
  time < limit
  balance <= 0
Java Comparison Operators
        Compound Boolean
• Boolean expressions can be combined using
  the ―and‖ (&&) operator.
• example
  if ((score > 0) && (score <= 100))

• not allowed
  if (0 < score <= 100)
        Compound Boolean
        Expressions, cont.
• syntax
  (Sub_Expression_1) && (Sub_Expression_2)
• Parentheses often are used to enhance
• The larger expression is true only when both
  of the smaller expressions are true.
       Compound Boolean
       Expressions, cont.
• Boolean expressions can be combined using
  the ―or‖ (||) operator.
• example
  if ((quantity > 5) || (cost < 10))

• syntax
  (Sub_Expression_1) || (Sub_Expression_2)
        Compound Boolean
        Expressions, cont.
• The larger expression is true
   – when either of the smaller expressions is
   – when both of the smaller expressions are
• ―or‖ in Java is inclusive or
  – either or both to be true.
• exclusive or
  – one or the other, but not both to be true.
        Negating a Boolean
• Boolean negation
  – ―not‖ (!) operator.
• syntax
• Example:
  Boolean walk = false;
Truth Tables
        Primary Logical Operators
• Primary logical operators: and, or, not
• Any logical expression can be composed
• Example: exclusive or
   (a || b) && !(a && b)
• Either work or play:
   (work || play) && !(work && play)
• ^ is exclusive-or in Java
   – work ^ play
   – not a logical operator in most languages
                    Using ==
• ==   is appropriate for determining if two
    integers or characters have the same value.
    if (a == 3)
     where a is an integer type
•   == is not appropriate for determining if two
    floating point values are equal.
    – Use < and some appropriate tolerance instead.
       if (Math.abs(b - c) < epsilon)
    – b, c, and epsilon are of floating point type
             Using ==, cont.
• ==is not appropriate for determining if two
  objects have the same value.
  – if (s1 == s2)
       • determines only if s1 and s2 are at the same
         memory location.
  – If s1 and s2 refer to strings with identical
    sequences of characters, but stored in
    different memory locations
       • (s1 == s2)   is false.
              Using ==, cont.
• To test the equality of objects of class String,
  use method equals.
• To test for equality ignoring case, use method
         equals and
• syntax
  Testing Strings for Equality
• class StringEqualityDemo
        Lexicographic Order
• Lexicographic order is similar to alphabetical
  order, but is it based on the order of the
  characters in the ASCII (and Unicode)
  character set.
   – All the digits come before all the letters.
   – All the uppercase letters come before all
     the lower case letters.
   Lexicographic Order, cont.
• Strings consisting of alphabetical characters
  can be compared using method compareTo and
  method toUpperCase or method toLowerCase.
  String s1 = “Hello”;
  String lowerS1 = s1.toLowerCase();
  String s2 = “hello”;
  if (lowerS1.compareTo(s2) == 0)

  //or use s1.compareToIgnoreCase(s2)
       Method compareTo
• syntax
• Method compareTo returns
  – a negative number if String_1 precedes
  – zero if the two strings are equal
  – a positive number of String_2 precedes
  – Tip: Think of compareTo is subtraction
        Comparing Numbers vs.
          Comparing Strings

Integer and floating-      String objects
    point values
         ==                  equals( )
         !=             equalsIgnoreCase( )

          >                compareTo( )
          <              [lexicographical
        Nested Statements
• An if-else statement can contain any sort of
  statement within it.
• In particular, it can contain another if-else
   – An if-else may be nested within the ―if‖
   – An if-else may be nested within the ―else‖
   – An if-else may be nested within both parts.
   Nested Statements, cont.
• syntax
  if (Boolean_Expression_1)
    if (Boolean_Expression_2)
    if (Boolean_Expression_3)
           Nested if Example
if (temperature > 90) // int temperature
  if (sunny)            // boolean sunny
  if (sunny)
   Nested Statements, cont.
• Each else is paired with the nearest
  unmatched if.
• Indentation can communicate which if goes
  with which else.
• Braces are used to group statements.
    Nested Statements, cont.
• Different indentation
  first form            second form
  if (a > b)          if (a >   b)
    if (c > d)          if (c   > d)
      e = f;              e =   f;
  else                  else
    g = h;                g =   h;

  Same to the compiler!
   Nested Statements, cont.
• Are these different?
  first form           second form
  if (a >   b)        if (a > b)
  {                     if (c > d)
    if (c   > d)          e = f;
      e =   f;        else
  }                     g =h;
      g =   h;
    Nested Statements, cont.
• Proper indentation and nested if-else
  ―else‖ with outer ―if‖ ―else‖ with inner ―if‖
   if (a >   b)          if (a > b)
   {                       if (c > d)
     if (c   > d)            e = f;
       e =   f;            else
   }                         g =h;
       g =   h;
      Compound Statements
• When a list of statements is enclosed in
  braces ({}), they form a single compound
• syntax
 Compound Statements, cont.
• A compound statement can be used
  wherever a statement can be used.
• example
  if (total > 10)
     sum = sum + total;
     total = 0;
      Multibranch if-else
• syntax
  if (Boolean_Expression_1)
  else if (Boolean_Expression_2)
  else if (Boolean_Expression_3)
  else if …
Multibranch if-else Statements, cont.
• class Grader
        Multibranch if-else
         Statements, cont.
• equivalent logically
    if (score >= 90)
      grade = ‘A’;
    if ((score >= 80) && (score < 90))
      grade = ‘B’;
    if ((score >= 70) && (score < 80))
      grade = ‘C’;
    if ((score >= 60) && (score < 70))
      grade = ‘D’;
    if (score < 60)
      grade = ‘F’;
        switch Statement
• The switch statement is a multiway branch
  that makes a decision based on an integral
  (integer or character) expression.
• The switch statement begins with the keyword
  switch followed by an integral expression in
  parentheses and called the controlling
    switch Statement, cont.
• A list of cases follows, enclosed in braces.
• Each case consists of the keyword case
  followed by
   – a constant called the case label
   – a colon
   – a list of statements.
• The list is searched for a case label matching
  the controlling expression.
   switch Statement, cont.
• The action associated with a matching
  case label is executed.
• If no match is found, the case labeled
  default is executed.
  – The default case is optional, but
    recommended, even if it simply prints a
• Repeated case labels are not allowed.
    switch Statement, cont.
• class MultipleBirths
    switch Statement, cont.
• The action for each case typically ends with
  the word break.
• The optional break statement prevents the
  consideration of other cases.
• The controlling expression can be anything
  that evaluates to an integral type (integer or
The switch Statement, cont.
• syntax
  switch (Controlling_Expression)
     case Case_Label:
     case Case_Label:
Switch with char Type
char grade = 'A';
  case 'A':
  case 'B':
  case 'C':
  case 'D':
  case 'W':
  case 'I':
       Conditional Operator
  if (n1 > n2)
    max = n1;
    max = n2;
   can be written as
   max = (n1 > n2) ? n1 : n2;
• The ? and : together is called the conditional
  operator (a ternary operator).
• Note (n1 > n2) ? n1 : n2 is an expression that
  has a value unlike the ―normal‖ if statement
  Conditional Operator, cont.
• The conditional operator can be useful with
  print statements.
  System.out.print(“You worked “ + hours + “ “ +
     ((hours > 1) ? “hours” : “hour”));
       Summary of branching
• if statement (1 or 2 branches)
• Multi-branch if-else-if statement (3 or
  more branches)
• Multi-branch switch statement
• Conditional operator ? :
          Loop Statements
• A portion of a program that repeats a
  statement or a group of statements is called a
• The statement or group of statements to be
  repeated is called the body of the loop.
• A loop could be used to compute grades for
  each student in a class.
• There must be a means of exiting the loop.
            Loop Structure
1. Control of loop: ICU
   1. Initialization
   2. Condition for termination (continuing)
   3. Updating the condition
2. Body of loop
          Loop Statements
• the while Statement
• the do-while Statement
• the for Statement
          while Statement
• also called a while loop
• a controlling boolean expression
  – True -> repeats the statements in the loop body
  – False -> stops the loop
  – Initially false (the very first time)
     • loop body will not even execute once
    while Statement, cont.
• syntax
  while (Boolean_Expression)
  while (Boolean_Expression)
while Statement, cont.
     while Statement, cont.
• class WhileDemo
        do-while Statement
• also called a do-while loop (repeat-until loop)
• similar to a while statement
   – except that the loop body is executed at least
• syntax
   while (Boolean_Expression);

   – don’t forget the semicolon at the end!
   do-while Statement, cont.
• First, the loop body is executed.
• Then the boolean expression is checked.
   – As long as it is true, the loop is executed
   – If it is false, the loop exits.
• equivalent while statement
   while (Boolean_Condition)
do-while Statement, cont.
   do-while Statement, cont.
• class DoWhileDemo
     Programming Example:
         Bug Infestation
• given
   – volume of a roach: 0.0002 cubic feet
   – starting roach population
   – rate of increase: 95%/week
   – volume of a house
• find
   – number of weeks to exceed the capacity of
     the house
   – number and volume of roaches
Programming Example: Bug Infestation, cont.
• class BugTrouble
             Infinite Loops
• A loop which repeats without ever ending
• the controlling boolean expression (condition
  to continue)
  – never becomes false
• A negative growth rate in the preceding
  problem causes totalBugVolume always to be
  less than houseVolume
  – the loop never ends.
            for Statement
• A for statement executes the body of a loop a
  fixed number of times.
• example
  for (count = 1; count < 3; count++)
       for Statement, cont.
• syntax
  for (Initialization; Condition; Update)
  – Body_Statement
     • a simple statement or
     • a compound statement in {}.
• corresponding while statement
  while (Condition)
for Statement, cont.
      for Statement, cont.
• class ForDemo
    Multiple Initialization, etc.
• example
  for (n = 1, p = 1; n < 10; n++)
      p = p * n

• Only one boolean expression is allowed, but
  it can consist of &&s, ||s, and !s.
• Multiple update actions are allowed, too.
  for (n = 1, p = 100; n < p; n++, p -= n)

• rarely used
  Choosing a Loop Statement
• If you know how many times the loop will be
  iterated, use a for loop.
• If you don‘t know how many times the loop
  will be iterated, but
   – it could be zero, use a while loop
   – it will be at least once, use a do-while loop.
• Generally, a while loop is a safe choice.
  Summary of loop statements
• while loop
• do-while loop
• for loop
  break Statement in Loops:
     NOT recommended
• A break statement can be used to end a loop
• The break statement ends only the innermost loop
  that contains the break statement.
• break statements make loops more difficult to
   – Loop could end at different places (multiple
     possible exit points), harder to know where.
• Always try to end a loop at only one place--makes
  debugging easier (only one possible exit point)
 Misuse of break Statements in loops (p.
• “Because of the complications they introduce,
  break statements in loops should be avoided.
• Some authorities contend that a break statement
  should never be used to end a loop,
• but virtually all programming authorities agree that
  they should be used at most sparingly.”
            exit Method
• Sometimes a situation arises that makes
  continuing the program pointless.
• A program can be terminated normally by

• example
  if (numberOfWinners == 0)
     System.out.println(“/ by 0”);
     Programming with Loops:
•   The Loop Body
•   Initializing Statements
•   Ending a Loop
•   Loop Bugs
•   Tracing Variables
               Loop Body
• To design the loop body, write out the actions
  the code must accomplish.
• Then look for a repeated pattern.
   – The pattern need not start with the first
   – The repeated pattern will form the body of
     the loop.
   – Some actions may need to be done after
     the pattern stops repeating.
       Initializing Statements
• Some variables need to have a value before
  the loop begins.
   – Sometimes this is determined by what is
      supposed to happen after one loop
   – Often variables have an initial value of zero
      or one, but not always.
• Other variables get values only while the loop
  is iterating.
            Ending a Loop
• If the number of iterations is known before the
  loop starts, the loop is called a count-
  controlled loop.
   – use a for loop.
• Asking the user before each iteration if it is
  time to end the loop is called the ask-before-
  iterating technique.
   – appropriate for a small number of iterations
   – Use a while loop or a do-while loop.
         Ending a Loop, cont.
• For large input lists, a sentinel value can be
  used to signal the end of the list.
   – The sentinel value must be different from
     all the other possible inputs.
   – A negative number following a long list of
     nonnegative exam scores could be
       Ending a Loop, cont.
• example - reading a list of scores followed by
  a sentinel value
  int next = keyboard.nextInt();
  while (next >= 0)
     next = keyboard.nextInt();
        Ending a Loop, cont.
• class ExamAverager
             Nested Loops
• The body of a loop can contain any kind of
  statements, including another loop.
• In the previous example
   – the average score was computed using a
     while loop.

   – This while loop was placed inside a do-while
     loop so the process could be repeated for
     other sets of exam scores.
                           Nested Loops
• The body of a loop can have any kind of statements,
  including another loop.
 for (line = 0; line < 4; line++)
                                                              body of
                                                             outer loop
       for (star = 0; star < 5; star++)
             System.out.print('*');                          body of
                                                            inner loop

• Each time the outer loop body is executed, the inner loop
  body will execute 5 times.
• 20 times total                   *****  Output:
  Declaring Variables Outside
         Loop Bodies
• Declaration of variables   while (…)
  inside a loop body is      {
  repeated with each           int x;
  execution of the loop
  body--can be inefficient
• Declaration of variables
  can generally be moved
  outside the loop body.
                Loop Bugs
• common loop bugs
   – unintended infinite loops
   – off-by-one errors
   – testing equality of floating-point numbers
• subtle infinite loops
   – The loop may terminate for some input
     values, but not for others.
   – For example, you can‘t get out of debt
     when the monthly penalty exceeds the
     monthly payment.
          Off-by-One Errors
• The loop body is repeated one too many
  times or one too few times.
• examples
   – < is used when <= should be used or <= is
     used when < should be used
   – using the index of the last character of a
     string instead of the length of the string (or
     vice versa)
• easy to overlook
                Off by One

int i = 0;
while (i <= 10)
      Empty for Statement
• What is printed by
  int product = 1, number;
  for (number = 1; number <= 10; number++);
      product = product * number;

• The last semicolon in
  for (number = 1; number <= 10; number++);

  produces an empty for statement.
    Empty while Statement
  int product = 1, number = 1;
  while (number <= 10);
      product = product * number;
• The last semicolon in
  while (number <= 10);
 produces an empty while loop body.
  Testing Equality of Floating-
        point Numbers
• == works satisfactorily for integers and
• == is not reliable for floating-point numbers
  (which are approximate quantities).
   – Can cause infinite loops
   – Use <= or >= rather than == or !=.
          Tracing Variables
• Tracing variables means watching the
  variables change while the program is
   – Simply insert temporary output statements
     in your program to print of the values of
     variables of interest
   – or, learn to use the debugging facility that
     may be provided by your system.
          Tracing Variables, cont.
float creditCardBalance = 9000.0;
while (creditCardBalance > 0)
  … // input payment
  creditCardBalance -= payment;

    … // calculate penalty
    creditCardBalance += penalty;

           Type boolean
• Boolean Expressions and Variables
• Truth Tables and Precedence Rules
• Input and Output of Boolean Values
       Type boolean, cont.
• The type boolean is a primitive type with only
  two values: true and false.
• Boolean variables can make programs more
  if (systemsAreOK)
  instead of
  if((temperature <= 100) && (thrust >= 12000)
    && (cabinPressure > 30) && …)
   Boolean Expressions and
• Variables, constants, and expressions of type
  boolean all evaluate to either true or false.

• A boolean variable can be given the value of
  a boolean expression by using an assignment
  boolean isPositive = (number > 0);
  if (isPositive) ...
   Naming Boolean Variables
• Choose names such as isPositive or
• Avoid names such as numberSign or
         Precedence Rules
• Parentheses should be used to indicate the
  order of operations.
• When parentheses are omitted, the order of
  operation is determined by precedence rules.
    Precedence Rules, cont.
• Operations with higher precedence are
  performed before operations with lower
• Operations with equal precedence are done
  left-to-right (except for unary operations
  which are done right-to-left).
Precedence Rules, cont.

              Comparison operators:
              <, >, <=, >=
              ==, !=

              Logical operators:
    Precedence Rules, cont.
• In what order are the operations

score < min/2 - 10 || score > 90
score < (min/2) - 10 || score > 90
score < ((min/2) - 10) || score > 90
(score < ((min/2) - 10)) || score > 90
(score < ((min/2) - 10)) || (score > 90)
      Short-circuit Evaluation
• Sometimes only part of a boolean expression
  needs to be evaluated to determine the value
  of the entire expression.
   – If the first operand of || is true
      • entire expression is true
   – If the first operand of && is false
      • entire expression is false
• This is called short-circuit or lazy evaluation.
 Short-circuit Evaluation, cont.
• Short-circuit evaluation is not only efficient,
  sometimes it is essential!
• A run-time error can result, for example, from
  an attempt to divide by zero.
   if ((number != 0) && (sum/number > 5))

• Complete evaluation can be achieved by
  substituting & for && or | for ||.
        Short-circuit Evaluation

int count = 1;
if ( … && (++count < 10) )
 Input and Output of Boolean
• example
  boolean boo = false;
  System.out.print(“Enter a boolean value: “);
  Scanner keyboard = new Scanner (;
  boo = keyboard.nextBoolean();
 Input and Output of Boolean
         Values, cont.
• dialog
  Enter a boolean value: true
 Using a Boolean Variable to
         End a Loop
• example
  boolean numbersLeftToRead = true;
  while (numbersLeftToRead)
     next = keyboard.nextInt();
     if (next < 0)
           numbersLeftToRead = false;
Using a Boolean Variable to End a Loop, cont
• class BooleanDemo
• You have learned about Java branching
• You have learned about loops.
• You have learned about the type boolean.

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