Anonymous inner class using an interface in Java

Question

So, when looking into lambda expressions and using them to substitute anonymous inner classes for EventHandlers in Java, I came across a few anonymous inner classes that mad

Answer 1

1) Why are we able to create an anonymous inner class directly from an interface rather than having to create one through a class that implements the interface?

2) Why am I unable to create an anonymous inner class that implements ActionListener instead of directly from it as I show in my second code snippet?

When you create a class using implements XXXX, you are defining a class(inner or non-inner), and you will have to give it a name, sure we can do that and this is what we often do . While anonymous inner class dose not have a name, and it is more like an expression.

---

I copy this from http://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html

And I think this will help you to understand what anonymous class is.

An anonymous class is an expression. They are like local classes except that they do not have a name

. The syntax of an anonymous class expression is like the invocation of a constructor, except that there is a class definition contained in a block of code.

Consider the instantiation of the frenchGreeting object:

    HelloWorld frenchGreeting = new HelloWorld() {
        String name = "tout le monde";
        public void greet() {
            greetSomeone("tout le monde");
        }
        public void greetSomeone(String someone) {
            name = someone;
            System.out.println("Salut " + name);
        }
    };

The anonymous class expression consists of the following:

• The new operator

• The name of an interface to implement or a class to extend. In this example, the anonymous class is implementing the interface HelloWorld.

• Parentheses that contain the arguments to a constructor, just like a normal class instance creation expression. Note: When you implement an interface, there is no constructor, so you use an empty pair of parentheses, as in this example.

• A body, which is a class declaration body. More specifically, in the body, method declarations are allowed but statements are not.

Because an anonymous class definition is an expression, it must be part of a statement. In this example, the anonymous class expression is part of the statement that instantiates the frenchGreeting object. (This explains why there is a semicolon after the closing brace.)