JS当中的一段实现“加法器”的代码。

function makeAdder(a) {
  return function(b) {
    return a + b;
  };
}
var fiveAdder = makeAdder(5);
var tenAdder = makeAdder(10);

fiveAdder(6); // ?
tenAdder(7); // ?

显然，fiveAdder是用来每次加5的加法器，而tenAdder是用来每次加10的加法器。

一个很直接的问题便是，fiveAdder的5和tenAdder的10被存在哪里？

Inner class之于class，就好比是first class function（一等函数）之于函数。只不过这里是first class class（一等类）。你可以直接把一个类的实现返回出来，而不再是返回一个简单的数值。

来自Thinking in Java的一个例子：

interface Incrementable {
    void increment();
}

class Callee1 implements Incrementable {
    private int i = 0;

    public void increment() {
        i++;
        System.out.println(i);
    }
    
}

class MyIncremental {
    public void increment() {
        System.out.println("other operations");
    }
    
    static void f(MyIncremental mi) {
        mi.increment();
    }
}

class Callee2 extends MyIncremental implements Incrementable {
    private int i = 0;
    
    @Override
    public void increment() {
        super.increment();
        i++;
        System.out.println(i);      
    }
    
    // The non-trivial part!
    private class Closure implements Incrementable {

        public void increment() {
            Callee2.this.increment();           
        }       
    }
    
    Incrementable getCallbackRef() {
        return new Closure();
    }
}


// The initiator of calling.
class Caller {
    private Incrementable callbackRef;
    
    Caller(Incrementable cbh) {
        callbackRef = cbh;
    }
    
    void go() {
        // ...... do lots of other things.
        
        // at the end:
        callbackRef.increment();
    }
}

public class Callbacks {
    
    public static void main(String... args) {

        Callee1 c1 = new Callee1();
        Callee2 c2 = new Callee2();
        
        MyIncremental.f(c2);
        
        Caller caller1 = new Caller(c1);
        caller1.go();
        caller1.go();
        Caller caller2 = new Caller(c2);
        caller2.go();
        caller2.go();
    }
}