Final is keyword that can be used before a class or method or a variable.

Final Class

They can not be subclassed and the method in it are by default final.

public final class MyFinalClass {...}

public class ThisIsWrong extends MyFinalClass {...}

Final Method

They can be override.This is done because to prevent unexpected behaviour from a subclass that is altering a method.

public class Base
{
    public       void m1() {...}
    public final void m2() {...}

    public static       void m3() {...}
    public static final void m4() {...}
}

public class Derived extends Base
{
    public void m1() {...}  // Ok, overriding Base#m1()
    public void m2() {...}  // forbidden

    public static void m3() {...}  // OK, hiding Base#m3()
    public static void m4() {...}  // forbidden
}

Final Variable

A final variable can be only initialized once.There is no need to again initialize at the time of declaration.

The value of a final variable may not be known at the compile time.

It is usually defined by a capital letter. public class Sphere {

    // pi is a universal constant, about as constant as anything can be.
    public static final double PI = 3.141592653589793;

    public final double radius;
    public final double xPos;
    public final double yPos;
    public final double zPos;

    Sphere(double x, double y, double z, double r) {
         radius = r;
         xPos = x;
         yPos = y;
         zPos = z;
    }

    [...]
}