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Java - Anonymous Inner Class

📌 What You Will Learn
  • What an inner class is and why we need it
  • The four types: member, static nested, local, anonymous
  • How to create objects of inner classes
  • Where anonymous inner classes are used in real projects

Anonymous Inner Class is a core concept of the Java language. This lesson explains Anonymous Inner Class with complete, runnable code examples, clear step-by-step explanations, and common mistakes to avoid with exam-style MCQs at the end.

Anonymous Inner Class

An anonymous class has no name. It is created at the same place where it is used, usually to give a quick implementation of an interface or an abstract class. It is very common in GUI event handling and in modern code with functional interfaces.

Trainer's Note: Updated knowledge: In modern Java (Java 8+), the same job is done more simply with a lambda expression: Greeting g = () -> System.out.println("Hello"); We will study lambdas in the Java 8 chapter. But you will still meet anonymous classes in older code and in GUI programs, so understand the idea well.
In simple words: An anonymous inner class is a one-time class with no name. You write the whole implementation right where you need it, usually to override one small behaviour.
Example01
JCode Cell
1interface Greeting {
2 void sayHello();
3}
4 
5class Test {
6 public static void main(String[] args) {
7 // anonymous class - we implement Greeting right here, no named class
8 Greeting g = new Greeting() {
9 public void sayHello() {
10 System.out.println("Hello from anonymous class!");
11 }
12 };
13 g.sayHello();
14 }
15}
Output
Hello from anonymous class!
📝 Key Takeaways
  • Inner class = a class declared inside another class.
  • Member inner class needs an outer object: outer.new Inner().
  • Static nested class is created with Outer.Nested, no outer object needed.
  • Local inner class lives inside a method.
  • Anonymous inner class has no name and is used for one-time implementations.
  • Modern code prefers lambda expressions for one-method interfaces, but anonymous classes still appear in real projects.

🧠 Test Your Knowledge

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