01 Note - Java Language Basics

							Java Language Basics
Expectations:

The student will:

   1. be able to define the term: variable
   2. explain the primitive variable types
   3. declare a primitive variable
   4. assign a value to a primitive variable
   5. know the various comparison operators
   6. know the various types of math operations
   7. know the shortcut assignment operators
   8. know how to define constants
   9. explain casting
   10. be familiar with methods of the Math class
   11. know the general pattern for coding a class



A variable is a named storage location in RAM

JAVA has the following primate variable types:

Numbers without Decimal Portion:

Java Variable Type Name Size
byte                        1 byte
short                       2 bytes
int                         4 bytes
long                        8 bytes

Numbers with Decimal Portion:

Java Variable Type Name Size
float                   4 bytes
double                  8 bytes

One Alphanume ric Character

Java Variable Type Name Size
char                    2 bytes (2 bytes allow for international symbol set)
True or false flag

Java Variable Type Name Values
boolean                 true and false only

Declaring Variables

      creates storage place in RAM
      must be done before a variable can be used
      can be done just before variable is used (no need to put all declares at top of
       methods)
      Java is case sensitive, types must be all in lowercase

General Format:

Java-Variable-Type-Name Variable-Name Semi Colon

Variable Naming Convention

      primitive variable names start with a lower case letter
      name may contain letters, numbers and underscore
      all letters are lower case, except where a name is composed of multiple words,
       then the subsequent words have their first letter capitalized

Examples :

boolean happy;
char gender;
double robsPay;

Assignment Statement

      Assigns a value to a variable.
      Java uses =

Examples:

int age;
age = 16;

long age;
age = 197L; (L indicates a long value)

double robsPay;
robsPay = 13.50;
float robsPay;
robsPay = 13.50f; (f indicates a float value)

char gender;
gender = ‘m’; (Note: single quotes around the character value)

boolean happy;
happy = false;

Primitive Variable Type Comparative Ope rators

Ope rator             Symbol Example
equals                ==       if (x = = y)
not equals            !=       if (x != y)
less than             <        if (x < y)
greater than          >        if (x > y)
less than or equal    <=       if (x <= y)
greater than or equal >=       if (x >= y)

Math Operations
Java follows BEDMAS and left-to-right evaluation when two or more operations of same
precedent occur without brackets

Ope ration             Symbol Example
addition               +      a = b + 2;
subtraction            -        a = b - 2;
multiplication         *        a = b * 2;
                                a = b / 2;
division               /        Note: if b is integer, then the answer has all decimals
                                truncated, even if a is a float or double
integer division
                       %        a = b % 2;
remainder (mod)
                                a++;
                                adds 1 to a, after operations involving a are completed eg.
post increment         ++
                                if (a++ < 2) -> a is incremented after the comparison is
                                performed
                                ++a;
                                adds 1 to a, before operations involving a are completed
pre increment          ++
                                eg. if (++a < 2) -> a is incremented before the comparison
                                is performed
post decrement         --       a--;
                                 same comment as a++
                                 --a;
pre decrement          --
                                 same comment as ++a

Note: Java does division according to the operand types:

double answer;
answer = 7/2;

answer is 3 not 3.5 !!
because 7 and 2 are integers

answer = 7.0 / 2;

answer is 3.5
because 7.0 is a double

ShortCut Assignment Ope rators

Operator              Symbol Example
                                 a += 2;
assign addition       +=
                                 means: a = a + 2;
                                 a -= 2;
assign subtraction    -=
                                 means: a = a - 2;
                                 a *= 2;
assign multiplication *=
                                 means: a = a * 2;
                                 a /= 2;
assign division       /=
                                 means: a = a / 2;
                                 a %= 2;
assign mod            %=
                                 means: a = a % 2;

Other math functions are built into the Math Class

See the Math Class documentation for : cos, sin, tan, absolute value, PI, max, min,
random number, square root

To raise x to the power of 5 :

double x = 3;
double answer;
answer = Math.pow(x,5);
Constants in Java

        A constant is similar to a variable, except its value may not be changed.
        The Java keyword final is used to create a constant.
        They are used to make your code easier to understand.
        They are used to make your code easier to change (it is easier to change the value
         for GST in one place than many)
        No numbers of any kind are to appear in code - always use constants

Example

private final double GST = 0.07;

private final double MINIMUM_WAGE=6.75;

Coding Standards - Constant names have all letters capitalized



Conve rting Between Variable Types:

Safe

It is always safe to convert from a variable type that can hold less information to a type
that can hold more information, so the following conversions are safe to perform:

byte -> short -> int -> long -> float -> double

Example:

int age;
long bigAge;
age = 17;
bigAge = age;

Unsafe

When converting from a variable type that can hold more information to a variable type
that can hold less information, you may lose information (for example, decimals may be
truncated), so this is unsafe conversion and you must explicitly tell Java you desire to
perform the conversion.
Casting is the technique used when performing an unsafe conversion.

Casting syntax is: (Convert-to-Variable-Type)

Example:
double age = 17;
int newAge;
newAge = age; WRONG (type cast error)

newAge = (int)age; CORRECT Casting



Java Class Pattern

public class MyClass
{

    public int getMyVariable1()***
    {
       return myVariable1;
    }

    public void setMyVariable1(int newMyVariable1)**
    {
       myVariable1 = newMyVariable1;
    }


    private int myVariable1; *
    private int myVariable2; *
}

Notes:

* 2 variables declared

        the attributes of the class (often called instance variables)
        can be used in any of the class's methods (essentially global variables inside the
         class)
        they are private so only methods inside the class can see and change them, they
         are invisible to any code outside this class (such as another class )

** mutator method

        used to change the value of an instance variable of the class
        name of method starts with ‘set’ is public so other classes can run this method
        void means nothing is returned from this type of method
        (int newMyVariable1) is the parameter type and local name of the parameter
         which is sent in from the calling class

*** accessor method

        used by other classes to get the value of an instance variable
      name of method starts with ‘get’ is public so other classes can run this method
      int is type of variable returned by this method
      () no parameters sent in

Public / Private General Rules

All attributes (instance variables) are private

All methods are public

      Unless it is a ‘helper’ methods that other public methods will use, but never need
       to be used by outside classes, in which case it is private
      Example : A simple calculation used by a number of other methods

Orde r of Class Contents

   1. all public methods, in alpha order by method name
   2. all private methods, in alpha order by method name
   3. all instance variables