Constructors

							Programming 2

  Constructor
                Instantiation
• When talking about variables, we use two
  key words, declare and initialize
  – Declaring a variable is to create the variable
    somewhere in the computers memory
  – Initializing the variable means to give it a value
• When we create an object of a class, we do
  not called it declaration anymore, we call it
  instantiating the class object. We will see
  this once we have a class to use for an
  example.
                          Class Definition
   • In the last presentation, we saw our
     first class definition. Here is another:
public class Car                               •Notice that there are three class variables
{                                              speed, mpg and fuel that are private. Most of
             private int speed;                the time, we create class data members to be
             private int mpg;                  private.
             private int fuel;                 •Remember, that means that only objects of
             public void setCar( )             that class can use the variables. Outside of
             {                                 the class, you cannot access them.
                DEFINED SET OF INSTRUCTIONS    •Notice that there are two methods
             }                                 associated with the class Car. Both setCar();
             public void printCar();           and printCar(); are void functions which
             {                                 means they will pass data to one another or
                 DEFINED SET OF INSTRUCTIONS   other classes.
             }
}
• We have a class definition for car, we then create the method definitions.
• They will look like this:
public void setCar()
{
   String s = JOptionPane.showInputDialog( “How fast can your car go?”
                                         + ”\nEnter the speed: ”);
   String m = JOptionPane.showInputDialog( “How many miles per gallon can
                          your car go?\nEnter the miles per gallon: ”);
   String f = JOptionPane.showInputDialog( “How much fuel does your car
                          have?\nEnter the number of gallons: ”);

    speed = Integer.parseInt(s);   //sets the local variables that are entered
    mpg = Integer.parseInt(m);     //equal to the private data members of
    fuel = Integer.parseInt(f);    //the class Car. This way the entire class
}                                  //has access to the data
public void printCar()
{
   String output = “\nThe speed of your car is “ + speed + “.”
         + “\nThe miles per gallon that your car gets is “ + mpg + “.”
         + “\nThe amount of fuel in your tank is “ + fuel +“ gallons.”
         + “\nYou should be able to drive “ + fuel*mpg + “ miles!”;
   JOptionPane.showMessageDialog( null, output, “Print the Car”,
                          JOptionPane.INFORMATION_MESSAGE );
}


• A thing to remember:
    – Since speed, mpg and fuel are private members of the
      class, only the methods of the class can access them.
              Constructors
• Of special note is the fact that the data
  members of Car (speed, fuel and mpg) are
  not initialized. In fact, you should not
  initialize them in the class definition.
• A constructor is a special method of a class.
  The in the constructor is where any
  initialization takes place.
• A constructor MUST have the same name
  as the class. Therefore, our constructor must
  be named Car().
• If you go back to the class definition, we
  did not create a constructor.
• If you do not create a constructor of your
  own, Java creates one for you. We call this
  the default constructor.
• We used the default constructor in the last
  presentation, so this time lets create our
  own.
 • We first go back to the class definition and
   modify it so that it reads one of these ways:
public class Car                             public class Car
{                                            {
             private int speed;                           private int speed;
             private int mpg;                             private int mpg;
             private int fuel;                            private int fuel;
             public Car()                                 public Car()
             {                                            {
                           speed = 0;                         speed = mpg = fuel = 0;
                           mpg = 0;                       }
                           fuel = 0;                      public void setCar( )
             }                                            {
             public void setCar( )                          DEFINED SET OF INSTRUCTIONS
             {                                            }
               DEFINED SET OF INSTRUCTIONS                public void printCar();
             }                                            {
             public void printCar();                        DEFINED SET OF INSTRUCTIONS
             {                                            }
               DEFINED SET OF INSTRUCTIONS   }
             }
}
• Notice that we did not specify the
  constructors return type. In fact a
  constructor cannot return a value.
• You can send data to a constructor, but we
  will see that later.
• We also have to define the constructor’s
  implementation ( that is what we call the
  method definition)

public Car( )
{
    speed = 0;
    mpg = 0;
    fuel = 0;
}
• When a class object is instantiated (created), the
  constructor is automatically called with the line
      Car newcar = new Car( );
• In this case, our constructor initializes all class
  data members to zero.
• The constructor can be overloaded, which means
  there can be multiple constructors that are sent
  different things.
• Arguments can be passed to the constructor

• Lets take a look at main
public static void main(String args[]) {
     Car newCar = new Car( );

     newCar.setCar();
     newCar.printCar();
     System.exit(0);
}
• We can put this all together to see the
  program.
  – See Source Code
  – See Application
                     Assignment
• In main try a line like:     Car newcar = new Car();
                               newcar.fuel = 2;
  See what the computer says.
• Create a program the will use a class called Human.
  Objects of class Human should have data fields to store
  the height, weight and name of the person. There should
  be a constructor that initializes the data members to be
  zero. There are to be four methods, one that asks for
  each piece of data and one that prints it out.