Java final Keyword

final is a Keyword in Java, and it can be used to decorate,

• class
• variable
• method

final on Class

final can be used to decorate a class, and we called a class with final defind as final class.

public final class Test {
   ...
}

If a class is defined as final, then, this class can't be inherited by any sub class.

Usually, an immutable class always declared as final, such as String class.

final on Variable

final also can be used to decorate a variable.

public final class Test {
    private final String name = "aa";

    private final int age;

    public Test(int age) {
        this.age = age;
    }

    public static void main(String[] args) {
        final var app = new Test(12);
        ...
    }
}

• final can be used to define class variable, once that final variable is assigned, it's value can't be changed.

  The final variable can be assigned in its declaration, or in a later static code block, constructor or any method, but it only can be assigned once.

• final also can be used to define a local variable, once that final variable is assigned, it's value can't be changed.

For example,

public final class Test {
    private final String name = "aa";

    private final int age;

    public Test(int age) {
        this.age = age;
        this.name = "new name"; // compile error
    }

    public static void main(String[] args) {
        final var app = new Test(12);
        app = new Test(13); // compile error
    }
}

final on Method

final also can be used on a method. When a method is defined as final, that method can't be overridden in sub class.

For example,

class Parent {
    public final void hello() {
        System.out.println("hello world");
    }
}

public class Child extends Parent {

    @Override
    public void hello() { // compile error

    }

}