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UNIT 3 Java

 Unit-3 Class and objects

3.1 class in Java

A class is a group of objects which have common properties. It is a template or blueprint from which objects are created. It is a logical entity. It can't be physical.

Everything in Java is associated with classes and objects, along with its attributes and methods. A class is container for variable, methods, constructor etc.  For example: in real life, a car is an object. The car has attributes, such as weight and color, and methods, such as drive and brake.

A class in Java can contain:

  • Fields
  • Methods
  • Constructors
  • Blocks
  • Nested class and interface

Syntax to declare a class:

1.      class <class_name>{  

2.          field;  

3.          method;  

4.      }  

3.2 Object in Java

A Java object is a member (also called an instance) of a Java class. Each object has an identity, a behavior and a state. In Java, an object is created from a class.

To create an object of Main, specify the class name, followed by the object name, and use the keyword new:

Example

Create an object called "myObj" and print the value of x:

public class Class_name {

  int x = 5;

  public static void main(String[] args) {

    Class_name myObj = new Class_name();

    System.out.println(myObj.x);

  }

}

We can create multiple objects of one class:

public class Main {

  int x = 5;

  public static void main(String[] args) {

    Main myObj1 = new Main();  // Object 1

    Main myObj2 = new Main();  // Object 2

    System.out.println(myObj1.x);

    System.out.println(myObj2.x);

  }

}

Using Multiple Classes

We can also create an object of a class and access it in another class. This is often used for better organization of classes (one class has all the attributes and methods, while the other class holds the main() method (code to be executed)).

Remember that the name of the java file should match the class name. In this example, we have created two files in the same directory/folder:

●        Main.java

●        Second.java

Main.java

public class Main {

  int x = 5;

}

Second.java

class Second {

  public static void main(String[] args) {

    Main myObj = new Main();

    System.out.println(myObj.x);

  }

}

And the output will be:

5

Modify Attributes

We can also modify attribute values

public class Main {

  int x;

  public static void main(String[] args) {

    Main myObj = new Main();

    myObj.x = 40;

    System.out.println(myObj.x);

  }

}

 

Example

Change the value of x to 25:

public class Main {

  int x = 10;

  public static void main(String[] args) {

    Main myObj = new Main();

    myObj.x = 25; // x is now 25

    System.out.println(myObj.x);

  }

}

If We don't want the ability to override existing values, declare the attribute as final:

Example

public class Main {

  final int x = 10;

  public static void main(String[] args) {

    Main myObj = new Main();

    myObj.x = 25; // will generate an error: cannot assign a value to a final variable

    System.out.println(myObj.x);

  }

}

The final keyword is useful when We want a variable to always store the same value, like PI (3.14159...).

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