final Keyword In Java

final, static and others >
Siva Nookala - 20 Feb 2017

The final keyword helps in making a variable unmodifiable after the initialization. If a variable is defined with final keyword, then it can not be modified after the initialization. This kind of check prevents the developers from accidentally modifying a variable, a method or a class.

final keyword can be used on a local variable, member variable, static variable, method or a class. Shown below are the implications of final keyword being applied at various levels.

Local variable: If a variable is declared with the keyword final, then the value can not be changed after initialization. Here the variable pi is declared and initialized to 3.14, if we want to change it 3.45, then it causes a compilation error.

final double pi = 3.14;
pi = 3.45; // Causes a compilation error

Member variable: If a member variable of a class is marked as final, then it has to be initialized while it is declared or initialized in the constructor. Otherwise it causes a compilation error. Here the initializations in class A and B are correct, where as for class C, it is a problem since the variable j is neither initialized when declared nor initialized in the constructor.

class A
{
    final int i = 10;
}

class B
{
    final int k;
    
    B(int k)
    {
        this.k = k;
    }
}

class C
{
    final int j; // Causes a compilation error

    C()
    {
    }
}

Class or static variable: If a static variable of a class is marked as final, then it has to be initialized while it is declared or initialized in the static block. Otherwise it causes a compilation error. Here the initializations in class M and N are correct, where as for class O, it is a problem since the variable j is neither initialized when declared nor initialized in the static block.

class M
{
    static final int i = 10;
}

class N
{
    static final int k;
    
    static
    {
        k = 4 * 20;
    }
}

class O
{
    static final int j; // Causes a compilation error
}

Method: If a method of a class is marked as final, then it can not be overridden in any of its sub-classes. It causes a compilation error, if we try to override a final method.

class X
{
    final void print()
    {
        System.out.println("This method can not be overridden.");
    }
}

class Y extends X
{
    void print() // Causes a compilation error, can not override a final method
    {
    }    
}

Class: If a class itself is marked as final, then it can not be inherited (or extended). It causes a compilation error, if we try to extend a final class. Here class P is marked as final, hence it can not be inherited.

When you use final modifier for class, it means

Class is fully implemented and there should not be any sub-class of this class.

Modifier final is totally opposite of abstract modifier. Hence for a class, final and abstract modifier cannot be used together.

final class P
{
}

class Q extends P
{
    // Causes a compilation error, can not inherit a final class
}

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