Closure and Callbacks

Question

Is there any other way in java to implement call backs apart from inner classes? What is the difference between callbacks and closures?

Answer 1

Closure is how you build it, callback is how you use it.

A callback can be implemented as a closure (in languages that have them) or an implementation of an interface (in Java, as an anonymous inner class or a regular class).

Callback means that you pass a piece of code to a function, so that the function can call that piece of code later. It is a special kind of parameter.

The piece of code can be a function pointer or a closure or an object with well-known methods, depending on what the language offers.

Answer 2

Both closures and anonymous inner classes (and others) can be used as callbacks. A callback is just some code which is passed as an argument to other code.

A big difference of closures, compared to Java's anonymous inner classes, is that (in imperative languages) a closure can modify the variables of the surrounding scope. Wikipedia gives the following example:

var f, g;
function foo() {
  var x = 0;
  f = function() { return ++x; };
  g = function() { return --x; };
  x = 1;
  alert('inside foo, call to f(): ' + f()); // "2"
}
foo();
alert('call to g(): ' + g()); // "1"
alert('call to f(): ' + f()); // "2"