final variables

final is used to create two types of constants

    1. Creating final constants

    2. Creating final reference variables

1. Creating final constants.

Below post discussed about the final constants

http://selflearningjava.blogspot.in/2014/02/keyword-final.html

2. Creating final reference variables

When you assign an object to final reference variable, then the reference variable points to the same object, the object can change its state, I.e, its values. Assigning some other object or reference to the final reference variable is not allowed.

Example

class FinalReferenceEx{
  int width, height;

  public static void main(String args[]){
    final FinalReferenceEx ref1 = new FinalReferenceEx();

    ref1.width = 100;
    ref1.height = 200;
    System.out.println("Width is " + ref1.width + " height is " + ref1.height);

    ref1.width = 200;
    ref1.height = 300;
    System.out.println("Width is " + ref1.width + " height is " + ref1.height);
  }
}

   

Output

Width is 100 height is 200

Width is 200 height is 300

As you observe the output, the final reference variable is allowed to change the properties of the object that it is pointing. But it is not allowed to refer other object.

Example

class FinalReferenceEx{
  int width, height;

  public static void main(String args[]){
    final FinalReferenceEx ref1 = new FinalReferenceEx();

    ref1.width = 100;
    ref1.height = 200;
    System.out.println("Width is " + ref1.width + " height is " + ref1.height);

    ref1.width = 200;
    ref1.height = 300;
    System.out.println("Width is " + ref1.width + " height is " + ref1.height);

    ref1 = new FinalReferenceEx();
  }
}

I am trying to assign a new object to the final reference variable ref1, which is not acceptible, so compiler throws below error.

FinalReferenceEx.java:16: error: cannot assign a value to final variable ref1

ref1 = new FinalReferenceEx();

^

1 error

The same applicable for Arrays also, since Arrays also reference types.

Some Points to Remember

1. If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object.

2. What is blank final variable in Java ?

Blank final variable in Java is a final variable which is not initialized while declaration, instead they are initialized on constructor or initializer blocks or static initialization blocks.

Related Links

http://selflearningjava.blogspot.in/2014/02/keyword-final.html

https://self-learning-java-tutorial.blogspot.com/2014/08/blank-final-variable.html

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