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Introduction to Java Classes

This tutorial will take you step by step in uderstanding the basics of a class , how to define it and how to use it. The best way to learn is to compile and run these programs yourself (copy, paste, compile and run !). Comments such as /* this is a comment */ or // this is another comment are inserted to explain what does the line of code do. The programs are kept simple for the purpose of concentrating on the main idea in question.

Example 1: simple class

 

// this class is called Room1
class Room1{ 

// it defines 3 instance variables: length, width and height 
   double length;
   double width;
   double height;

   public static void main(String t[]){
   }
}
      

The above program defines a template for the class Room1, no object of type Room1 is created yet.

Example 2: how to create an object of type Room1 ?

  
class Room1{ 

   double length;
   double width;
   double height;

   public static void main(String t[]){

// this statement creates an object of type Room1 and myroom1 is
// an instance of Room1.
   Room1 myroom1 = new Room1();

// this assigns values to the instance variables length, 
// width and height of the object referenced by myroom1.
   myroom1.length= 10.0; // Note the use of the dot(.) operator.
   myroom1.width = 8.0;
   myroom1.height = 5.5;

// this creates another object of type Room1.
   Room1 myroom2 = new Room1();

// this assigns values to the instance variables length, 
// width and height of the object referenced by myroom2.
   myroom2.length= 9.0;
   myroom2.width = 7.0;
   myroom2.height = 4.5;
   }
}   

You can create as many objects as you wish and assigns to each one of them different values of the instance variables as we have seen above. Note that each object has its own copies of the instances variables assigned.

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