Opinionated Final Variables in Java

I use final whenever it is appropriate.

Static final variables has two chances to initialize

1. On declaration
2. In static initializer block

    static final Long MY_ID = 111L;
    static final Long YOUR_ID;
    static {
        YOUR_ID = 222L;
    }
   

Non-static final variables has three chagnes to initialize

1. On declaration
2. In initializer block

3. In the constructor

   Note: any final field must be initialized before the constructor completes.

    final Long MY_ID = 111L;
    final Long YOUR_ID;
    final Long ANOTHER_ID;
       
    {
        // this block has to be upper than constructor
        ANOTHER_ID = 333L; 
    }
       
    MyClass() {
        YOUR_ID = 222L;
    }
   

Final Primitive Type is real final

If you try to reassign a value to an initialized final primitive variable, you will get this:

Error: java: cannot assign a value to final variable i

Final Reference Type has only the reference final

final is only about the reference itself, and not about the contents of the referenced object.

void withCat (final Cat cat) {
    cat.setId(2);
}

public static void main(String[] args) {
    final Cat cat = new Cat();
    cat.setId(1);
}

Question and Answer

1. Improved performance?
   Not always, not really.

2. Why use final?
   Should use final based on clear design and readability.

3. When must use? You have to mark something final so you can access it from within an anonymous inner class.

4. Benefits

   At first, it kind of looks awkward to see a lot of final keywords in your code, but pretty soon you’ll stop noticing the word itself and will simply think, that-thing-will-never-change-from-this-point-on.

References

1. Why would one mark local variables and method parameters as “final” in Java?
2. The “final” Keyword in Java
3. Using the “final” modifier whenever applicable in Java
4. When should one use final for method parameters and local variables