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Anonymous Inner Class in Java

intermediate· OOP · Nested Classes

Problem

An anonymous inner class is a one-off implementation of an interface or class, written and instantiated in a single expression, with no class name of its own.

Implement a Notifier interface inline, right where a single instance of it is needed, without declaring a separate named class.

Input

notifier.notify("Effective Java")

Output

Notification: "Effective Java" is now available

Java Program

Java

interface Notifier {
    void notify(String bookTitle);
}

public class AnonymousInnerClassDemo {
    public static void main(String[] args) {
        // Implements Notifier inline — no separate named class is declared
        Notifier notifier = new Notifier() {
            @Override
            public void notify(String bookTitle) {
                System.out.println("Notification: \"" + bookTitle + "\" is now available");
            }
        };
        notifier.notify("Effective Java");
    }
}

▶ Run Code

Output

Notification: "Effective Java" is now available

Core Logic

Writing the interface's implementation directly inside the `new Notifier() { ... }` expression skips the step of declaring a separate named class just to create a single instance.

How It Works

1interface Notifier declares one method, notify(String), with no implementation.
2new Notifier() { ... } creates an instance of an unnamed class that implements Notifier right at the point of use.
3The body between the braces supplies notify()'s implementation, the same as a named class would, just without ever being given a name.
4notifier.notify("Effective Java") calls that implementation like any other interface method call.

Calling notifier.notify("Effective Java") runs the body written inline, printing "Notification: \"Effective Java\" is now available".

💡

Key Point: This only makes sense for a single, throwaway instance — needing the same implementation in more than one place is a sign a named class (or a lambda, for a single-method interface) would serve better.

Key Concepts

anonymous classinterface implementationone-off instance

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