In Java, an anonymous class is a class that is declared and instantiated at the same time, without a specific class name. Anonymous classes are often used to create objects that implement an interface or extend a class without defining a new class. They are a convenient way to create one-off objects with custom behavior.

Here is an example of an anonymous class that implements the Runnable interface:

Thread t = new Thread(new Runnable() {
    public void run() {
        // code here
    }
});

In this example, an anonymous class is created and passed as an argument to the Thread constructor. The anonymous class implements the Runnable interface and defines the behavior of the thread's run() method.

Anonymous classes can also be used to extend a class:

SomeClass obj = new SomeClass() {
    public void someMethod() {
        // code here
    }
};

In this example, an anonymous class is created and assigned to a variable of type SomeClass. The anonymous class extends SomeClass and provides its own implementation of the someMethod() method.

Anonymous classes are often used in event handling and user interface programming, where a one-off object with custom behavior is required. They can help to reduce the amount of boilerplate code needed to create custom objects, and they can improve the readability of the code by keeping related code together. However, they should be used judiciously, as they can make code more difficult to read and debug.