Java Interview Questions (series) - `final` keyword

Beware of the F-word! 😈

Brief

The difference between the three Java F-words is one of the interviewers' favourite these days.

They don't have much in common, practically speaking, but they can be easily mistaken.

Let's take them one by one:

1️⃣  Final - Referring to variables/methods/classes.

2️⃣  Finally - Find it in the context of exception handling.

3️⃣  Finalize - Used to release resources used by objects before they're garbage collected.

❗️  We'll split the three into separate posts so you're able to separate them better.

Implementation

1️⃣   Final

Final variable

If you don't want a variable to change its value over the lifecycle of your application, you should add final in front of it. ⚓️

Example:

final int x = 1;  // variable initialization         
...         
x = 2;            // changing its value will result in a compile-time error

❗️ Keep in mind: If a final variable references an object, it won't keep the object's internals from changing!

final List<Integer> list1 = new ArrayList<>();
list1.add(2);               // will work

final List<Integer> list2 = new ArrayList<>();
...
list2 = new ArrayList();   // will not work

Final method

Add the final keyword in front of a method that you don't want to be overridden by extending classes. 🔒

Example:

public class Animal {
    public final int getAge() {
        ...
    }
    public void move(){
        ...
    }
}

public class Dog extends Animal {

    @Override
    public int getAge() {      // Won't work! Will result in a compile-time error.
        ...
    }
    
    @Override
    public void move() {       // Will work!
        ...
    }
}

Final class

Mark a class to be final when you don't want it to be extended. 🔒

Moreover, it's the first step in creating an immutable class.

public final class Animal {
    public final int getAge() {
        ...
    }
    public void move(){
        ...
    }
}

public class Dog extends Animal { // Won't work! Will result in a compile-time error.
   ...
}

Check out the next post where we'll go in-depth about the finally keyword! 🚀