What is an anonymous inner class?
Jul 07, 2025 am 02:18 AM
Anonymous internal classes are used in Java to create subclasses or implement interfaces on the fly, and are often used to override methods to achieve specific purposes, such as event handling in GUI applications. Its syntax form is a new interface or class that directly defines the class body, and requires that the accessed local variables must be final or equivalent immutable. Although they are convenient, they should not be overused. Especially when the logic is complicated, they can be replaced by Java 8's Lambda expressions.
An anonymous inner class in Java is essentially a way to create a subclass or implement an interface on the fly, without explicitly giving it a name. It's useful when you need a quick, one-off implementation that won't be reused elsewhere.
When Are They Used?
Anonymous inner classes are typically used when you want to override methods of a class or interface for a specific purpose, especially when creating listeners in GUI applications (like event handlers). For example:
- Implementing a
Runnableinline for threading - Creating custom behavior for buttons in Swing or Android UIs
They allow you to keep your logic close to where it's being used, which can make code more readable and concise.
What Does the Syntax Look Like?
The syntax might look a bit strange at first, but it follows a pattern. Here's a simple example:
new InterfaceOrClass() {
// Override methods here
}; So if you're implementing an interface like Runnable , it would look like this:
new Runnable() {
public void run() {
System.out.println("Running...");
}
};You can also pass arguments to constructors if you're extending a class:
new MyClass(10) {
// Additional overrides or initializations
};It looks like an object instantiation, but you're actually defining a new class inline.
How Do They Capture Variables?
One important thing to note is that anonymous inner classes can access variables from their enclosing scope — but only if those variables are effectively final. This means:
- Local variables must be final or not modified after assignment
- Instance variables from the outer class are accessible directly
For example:
int value = 42;
new MyTask() {
void doSomething() {
System.out.println(value); // Allowed because value is effectively final
}
}; If you try to modify value later, the compiler will throw an error. This restriction helps avoid concurrency issues and keeps things predictable.
This feature is handy but shouldn't be overused — especially when the logic gets complex. In modern Java (8 ), lambda expressions often offer a cleaner alternative for simple functional interfaces.
Basically that's it.
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