java inner class

1. Inner class are helper classes that can access member variables of  something outer (class, function, blocks and expression ) and are for local use only. They are no need to be aware of by outside world. 

2. Three types of inner classes:
inner class: inside a class
local class: inside any block ( expression, statement or block )
anonymous class: are expressions

Not matter how you categorize them, basically they are local to something outer.

I want to talk a little more about anonymous class. Basically it's 2-in-1: implementation + initialization

    interface HelloWorld {
        public void greet();
        public void greetSomeone(String someone);
    }

    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);
        }
    };

It's the same as 
 class FrenchGreeting implements HelloWorld { same blah above...}

 FrenchGreeting frenchGreeting = new FrenchGreeting();

However, by doing 2-in-1, you not only get rid of creating 

new class name FrenchGreeting, which is why it's called anonymous 

class,  but also initialize an object of it. So it's much simpler.