'final' keyword

A 'final' keyword can be applied on a variable
Maybe a class variable or an instance variable. It means that once the variable is initialized, its value cannot be modified, or changed. Meaning that its value is considered as a constant. Therefore, a final variable must be initialized, as it is declared,  Otherwise, compiler will pop up an error. It won't be automatically initialized as a null or a zero by Java.

final class variable should be initialized in a static block; final instance variable can be initialized in a constructor.

A 'final' keyword can be applied on a method 
Maybe a class method or an instance method. Final methods can be inherited by sub-classes. However, for a class method it means that the method cannot be hidden; for an instance method it means that the method cannot be overridden.

A 'final' keyword applied on a class 
It means that the class cannot be extended; there is no kids allowed.

/**
 *
 * @author YNZ
 */
public final class KeyFinal {

    public static final int A = 10;
    public static final int B;
    public final int x;

    static {
        B = 30;
    }

    public KeyFinal() {
        this.x = 20;
    }

    public final void doSome() {
        System.out.println("" + this.x);
    }

    public static void main(String[] args) {
        KeyFinal kf = new KeyFinal();
        System.out.println("" + A);
        kf.doSome();

    }

}