// innerclasses/Callbacks.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Using inner classes for callbacks
// {java innerclasses.Callbacks}
package innerclasses;

interface Incrementable {
 void increment();
}

// Very simple to just implement the interface:
class Callee1 implements Incrementable {
 private int i = 0;
 @Override
 public void increment() {
 i++;
 System.out.println(i);
 }
}

class MyIncrement {
 public void increment() {
 System.out.println("Other operation");
 }
 static void f(MyIncrement mi) { mi.increment(); }
}

// If your class must implement increment() in
// some other way, you must use an inner class:
class Callee2 extends MyIncrement {
 private int i = 0;
 @Override
 public void increment() {
 super.increment();
 i++;
 System.out.println(i);
 }
 private class Closure implements Incrementable {
 @Override
 public void increment() {
 // Specify outer-class method, otherwise
 // you'll get an infinite recursion:
 Callee2.this.increment();
 }
 }
 Incrementable getCallbackReference() {
 return new Closure();
 }
}

class Caller {
 private Incrementable callbackReference;
 Caller(Incrementable cbh) {
 callbackReference = cbh;
 }
 void go() { callbackReference.increment(); }
}

public class Callbacks {
 public static void main(String[] args) {
 Callee1 c1 = new Callee1();
 Callee2 c2 = new Callee2();
 MyIncrement.f(c2);
 Caller caller1 = new Caller(c1);
 Caller caller2 =
 new Caller(c2.getCallbackReference());
 caller1.go();
 caller1.go();
 caller2.go();
 caller2.go();
 }
}
/* Output:
Other operation
1
1
2
Other operation
2
Other operation
3
*/