Why Use the Final Keyword with Anonymous Inner Classes in Java?

What is the purpose of the final keyword in relation to anonymous inner classes in Java?

class OuterClass {
    void createAnonymousClass() {
        final String message = "Hello from anonymous class!";
        Runnable runnable = new Runnable() {
            public void run() {
                System.out.println(message);
            }
        };
        new Thread(runnable).start();
    }
}

In Java, anonymous inner classes allow you to instantiate a class without explicitly naming it. However, when accessing variables from the enclosing scope, certain restrictions apply. The final keyword plays a crucial role in these scenarios. It ensures that the variables used within these anonymous inner classes cannot be modified, thereby maintaining consistency and preventing errors during execution.

public class Main {
    public static void main(String[] args) {
        final int number = 42; // final variable
        Runnable myRunnable = new Runnable() {
            public void run() {
                System.out.println("Number is: " + number);
            }
        };
        new Thread(myRunnable).start();
    }
}

Causes

• Anonymous inner classes need to access local variables of the enclosing method.
• Java allows access to local variables only if they are declared as final (or effectively final) to ensure stability.

Solutions

• Declare local variables accessed in anonymous inner classes as final to make them accessible.
• Java 8 introduced effectively final, allowing variables without explicit final declaration as long as they are not modified.

Mistake: Declaring variables as non-final when accessing them in anonymous inner classes.

Solution: Declare such variables as final or ensure they are effectively final.

Mistake: Attempting to modify a final variable inside an anonymous inner class.

Solution: Use effectively final variables, or avoid modifying them within the anonymous class.