Function Pointers in Java

Question

The Java idiom for function-pointer-like functionality is an an anonymous class implementing an interface, e.g.

Collections.sort(list, new Comparator<MyClass>(){
    public int compare(MyClass a, MyClass b)
    {
        // compare objects
    }
});

Update: the above is necessary in Java versions prior to Java 8. Now we have much nicer alternatives, namely lambdas:

list.sort((a, b) -> a.isGreaterThan(b));

and method references:

list.sort(MyClass::isGreaterThan);

You can substitue a function pointer with an interface. Lets say you want to run through a collection and do something with each element.

public interface IFunction {
  public void execute(Object o);
}

This is the interface we could pass to some say CollectionUtils2.doFunc(Collection c, IFunction f).

public static void doFunc(Collection c, IFunction f) {
   for (Object o : c) {
      f.execute(o);
   }
}

As an example say we have a collection of numbers and you would like to add 1 to every element.

CollectionUtils2.doFunc(List numbers, new IFunction() {
    public void execute(Object o) {
       Integer anInt = (Integer) o;
       anInt++;
    }
});

You can use reflection to do it.

Pass as parameter the object and the method name (as a string) and then invoke the method. For example:

Object methodCaller(Object theObject, String methodName) {
   return theObject.getClass().getMethod(methodName).invoke(theObject);
   // Catch the exceptions
}

And then use it as in:

String theDescription = methodCaller(object1, "toString");
Class theClass = methodCaller(object2, "getClass");

Of course, check all exceptions and add the needed casts.

No, functions are not first class objects in java. You can do the same thing by implementing a handler class - this is how callbacks are implemented in the Swing etc.

There are however proposals for closures (the official name for what you're talking about) in future versions of java - Javaworld has an interesting article.

This brings to mind Steve Yegge's Execution in the Kingdom of Nouns. It basically states that Java needs an object for every action, and therefore does not have "verb-only" entities like function pointers.