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Final Variable

Final Instance Variable

  • If the value of a variable changes from object to object, such a variable is called an instance variable.
  • For every object, a separate copy of the instance variable will be created.
  • Instance variables do not require explicit initialization; JVM always provides default values.
Example: 
class Test {
    int x;        // instance variable

    public static void main(String... args) {
        Test t = new Test();
        System.out.println(t.x);   // output: 0
    }
}
  • If the instance variable is declared as final, then explicit initialization is mandatory. JVM will NOT provide default values.
Example: 
class Test {
    final int x;
}

Compile-time Error: variable x might not have been initialized.

Rule: A final instance variable must be initialized before constructor completion.

Possible places for initialization:

  • 1. At the time of declaration
class Test {
    final int x = 10;
}
  • 2. Inside an instance block
class Test {
    final int x;

    {
        x = 10;
    }
}
  • 3. Inside the constructor
class Test {
    final int x;

    Test() {
        x = 10;
    }
}

These are the only valid places for initialization. Assigning anywhere else results in an error.

Invalid Example: 
class Test {
    final int x;

    public static void m() {
        x = 10;   // Invalid
    }
}

Compile-time Error: cannot assign a value to final variable x.


Final Static Variables

If a variable's value does not vary from object to object, it should be declared at the class level using the static modifier.

Static variables have a single copy shared among all objects and are automatically given default values by the JVM.

Example: 
class Test {
    static int x;

    public static void main(String... args) {
        System.out.println(x);   // output: 0
    }
}

If a static variable is declared as final, then explicit initialization is mandatory.

Example: 
class Test {
    final static int x;
}

Compile-time Error: variable x might not have been initialized.

Rule: A final static variable must be initialized before class loading completes.

Possible places for initialization:

  • 1. At the time of declaration
class Test {
    final static int x = 10;
}
  • 2. Inside a static block
class Test {
    final static int x;

    static {
        x = 10;
    }
}

These are the only valid places. Assigning anywhere else causes an error.

Invalid Example: 
class Test {
    final static int x;

    public void m() {
        x = 10;   // Invalid
    }
}

Compile-time Error: cannot assign a value to final variable x.


Final Local Variables

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