Classes and Objects

Classes in Java

A class is a blueprint from which individual objects are created.

Example

public class Dog {

   String breed;

   int age;

   String color;

   void barking() {

   }

   void hungry() {

   }

   void sleeping() {

   }

}

A class can contain any of the following variable types.

  • Local variables: Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.

  • Instance variables: Instance variables are variables within a class but outside any method. These variables are initialized when the class is instantiated. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.

  • Class variables: Class variables are variables declared within a class, outside any method, with the static keyword.

A class can have any number of methods to access the value of various kinds of methods. In the above example, barking(), hungry() and sleeping() are methods.

 

 Constructors

When discussing about classes, one of the most important topic is constructors. Every class has a constructor. If we do not explicitly write a constructor for a class, the Java compiler builds a default constructor for that class.

Each time a new object is created, at least one constructor will be invoked. The main rule of constructors is that they should have the same name as the class. A class can have more than one constructor.

Example

public class Cat{

   public Cat() {

   }

   public Cat(String name) {

      // This constructor has one parameter, name.

   }

}

 

 Creating an Object

An object is created from a class. In Java, the new keyword is used to create new objects.

Example

public class Cat{

   public Cat(String name) {

      // This constructor has one parameter, name.

      System.out.println("Passed Name is :" + name );

   }

   public static void main(String []args) {

      // Following statement would create an object myCat

      Cat myCat = new Cat( "White Cat" );

   }

}


Output:

Passed Name is : White Cat