Passing a Method to Another Class to be Called By That Class

Question:
Is it possible to pass a method to another class,so that method can be called by that class at a later date?For example, in C you can pass an event handler to afunction, and that handler can be later called by that function if aspecific event occurs.

Answer:
This issue comes up frequently with object-oriented languagesand fortunately Java provides a very clean solution for accomplishingthis: Interfaces.

An interface is basically a collection of method definitions withoutthe implementations. A class is said to implement an interface whenit provides implementations for the methods listed in that interface.

For example, the Runnable interface in Java looks like this:

public interface java.lang.Runnable       {               public abstract void run();       }

It specifies a single method called ‘run’ that doesn’t take anyarguments and doesn’t return anything. It’s just a specificationat this point — no body has been defined for the method.

When a class implements the Runnable interface, it has to providethe implementation for the run() method as it sees fit. Forexample:

       public class Foo implements Runnable {               public void run() {                       // Foo’s implementation of run() goes here               }       }

It just so happens that the Thread class can be initializedusing an object that implements the Runnable interface. Becauseclass Foo implements the Runnable interface, you cancreate a Thread in this manner:

Thread t = new Thread(newFoo()); 

Now the Thread class is guaranteed to be able to call therun() method of Foo.

Let’s see how this mechanism can solve your problem. Let’s say youhave a class Chef and a class Food. You wantclass Chef to be able to call the cook() methodin class Food, and you somehow want to ‘pass’ thecook() method in Food to the Chef. Here’s how toaccomplish this:

       // define an interface called cookable that       // specifies the cook() method       public interface Cookable {               public abstract void cook();       }       // Let class Food implement that interface       // by providing the body for the cook() method       public class Food implements Cookable {               public void cook() {                       // get cooked               }       }       // Let the Chef call cook() on the Cookable object it gets       // All the Chef knows is that the object is Cookable.       public class Chef {               public makeDish(Cookable dish) {                       dish.cook();               }       }

Now, if you had a Kitchen class, you could easily have theChef cook some food:

       Chef chefWang = new Chef();       Food cashewChicken = new Food();       chefWang.makeDish(cashewChicken);       Food noodles = new Food();       chefWang.makeDish(noodles);

Since noodles and cashewChicken both implementthe Cookable interface, chefWang can just call theircook() methods and the dishes will be ready in no time.