Object oriented programming

Be sure to read pages 82 -100

Summary of Fridays info

• We created a class Dog with:

– Two instance variables

– Four methods (two accessors and two mutators)

– This class was put into a file called Dog.java

• Then we created a class with a main( ) method so we could instantiate objects of type Dog

Looking at the Java syntax

• public class Dog
{ // instance variables
private int age; // the age of the dog
private String color; // the color of the dog

// mutators change the value of the instance variable
public void setColor(String col)
{ color = col; }

•
public void setAge(int dogAge)
{ age = dogAge; }
//accessors return the value of an instance variable
public String getColor()
{ return color; }

•
public int getAge()
{ return age; }
}

• public class TestDogClass
{
public static void main(String args[ ])
{ Dog zelda, yourDog, strayDog;
zelda = new Dog();
zelda.setAge(10);
zelda.setColor("brown");
yourDog = new Dog();
yourDog.setAge(32);
yourDog.setColor("red");
System.out.print("Zelda is " + zelda.getColor())
System.out.println(" and " + zelda.getAge ( ) + “ years old. “

• } // end main
} // end class

Using arguments to pass information

• The mutator method call in main has information about the dog’s color

• This information needs to be saved in the class object

• This is done using an argument in the mutator method

• The method call from main( )
zelda.setColor("brown");

• The method itself from the Dog class that saves the information
public void setColor(String col)
{ color = col; }

Adding to our Dog class

• Add an instance variable for the dog’s name

• This means adding a new accessor and mutator

• Allow user input to get the data

• Use a constructor to initialize the objects

Interactive input

• Import the Scanner class before the class declaration in the file using input

• This would be the file with the main( ) method

import java.util.Scanner;

• Declare an object of type Scanner

Scanner in = new Scanner(System.in);

• Use that object to call nextInt( ) or next( )

int myNum= in.nextInt( );

String myStr = in.next( );

Constructors: used to initialize objects

• Research has found that a very common error in programs is for variables or objects to have unknown values

• Constructors give objects initial values, which then the user can change if they wish

• Constructors always have the same name as the class

• They never have a return type

• The constructor is called when the object is created, initializing it.

• There can be more than one constructor for a class

• The difference between them is the number of arguments they have.

Examples of the interactive I/O and a constructors will be found

On the files that are linked to the lecture page.