Final

A value cannot be changed. For example 10 is 10. We cannot change 10 to 20. With a variable, we can assign new values—but a value is final.

With the final keyword, we indicate a field or local is constant (a value). A final thing is in its terminal state. It cannot ever become something new.

First example

Here we introduce a class with 2 final ints on it. The Box class has width and height final values on it. These can be accessed like fields.

class Box {
    // This class has final ints on it.
    final int width = 10;
    final int height = width * 2;

    public int area() {
        return width * height;
    }
}

public class Program {

    public static void main(String[] args) {
        // Create and use class with final ints.
        Box box = new Box();
        System.out.println(box.area());
    }
}
200

Final static

In Java the static keyword means a field that is based on the type, not the instance of the type. It only exists once at a time in memory.

public class Program {

    // This String is both final and static.
    final static String description = "Hello world program";

    public static void main(String[] args) {
        // Print string.
        System.out.println(description);
    }
}
Hello world program

Final String

Here we use a final String. We then create a string with the same value (Hyperion) and compare the two. We must use equals() to compare two strings based on data.

public class Program {
    public static void main(String[] args) {

        // This is a final String.
        final String name = "Hyperion";

        // Construct another String.
        String test = "Hyper";
        String test2 = test + "ion";

        System.out.println(name);

        // Equals test the two strings.
        System.out.println(name.equals(test2));

        // The string references are different.
        System.out.println(name == test2);
    }
}
Hyperion
true
false

Final error

We cannot assign to a final value. We receive a "cannot be assigned" error in this case. The program will not compile.

public class Program {
    public static void main(String[] args) {

        final int size = 10;
        // This fails with a compilation error.
        size = 200;
    }
}
The final local variable size cannot be assigned.
It must be blank and not using a compound assignment

The final keyword is not related to the finally keyword. Final indicates constancy. Meanwhile finally is part of an exception handling construct.