What's the nearest substitute for a function pointer in Java?

Anonymous inner class

Say you want to have a function passed in with a String param that returns an int.
First you have to define an interface with the function as its only member, if you can't reuse an existing one.

interface StringFunction {
    int func(String param);
}

A method that takes the pointer would just accept StringFunction instance like so:

public void takingMethod(StringFunction sf) {
   int i = sf.func("my string");
   // do whatever ...
}

And would be called like so:

ref.takingMethod(new StringFunction() {
    public int func(String param) {
        // body
    }
});

EDIT: In Java 8, you could call it with a lambda expression:

ref.takingMethod(param -> bodyExpression);

For each "function pointer", I'd create a small functor class that implements your calculation. Define an interface that all the classes will implement, and pass instances of those objects into your larger function. This is a combination of the "command pattern", and "strategy pattern".

@sblundy's example is good.

When there is a predefined number of different calculations you can do in that one line, using an enum is a quick, yet clear way to implement a strategy pattern.

public enum Operation {
    PLUS {
        public double calc(double a, double b) {
            return a + b;
        }
    },
    TIMES {
        public double calc(double a, double b) {
            return a * b;
        }
    }
     ...

     public abstract double calc(double a, double b);
}

Obviously, the strategy method declaration, as well as exactly one instance of each implementation are all defined in a single class/file.

You need to create an interface that provides the function(s) that you want to pass around. eg:

/**
 * A simple interface to wrap up a function of one argument.
 * 
 * @author rcreswick
 *
 */
public interface Function1<S, T> {

   /**
    * Evaluates this function on it's arguments.
    * 
    * @param a The first argument.
    * @return The result.
    */
   public S eval(T a);

}

Then, when you need to pass a function, you can implement that interface:

List<Integer> result = CollectionUtilities.map(list,
        new Function1<Integer, Integer>() {
           @Override
           public Integer eval(Integer a) {
              return a * a;
           }
        });

Finally, the map function uses the passed in Function1 as follows:

   public static <K,R,S,T> Map<K, R> zipWith(Function2<R,S,T> fn, 
         Map<K, S> m1, Map<K, T> m2, Map<K, R> results){
      Set<K> keySet = new HashSet<K>();
      keySet.addAll(m1.keySet());
      keySet.addAll(m2.keySet());

      results.clear();

      for (K key : keySet) {
         results.put(key, fn.eval(m1.get(key), m2.get(key)));
      }
      return results;
   }

You can often use Runnable instead of your own interface if you don't need to pass in parameters, or you can use various other techniques to make the param count less "fixed" but it's usually a trade-off with type safety. (Or you can override the constructor for your function object to pass in the params that way.. there are lots of approaches, and some work better in certain circumstances.)

You can also do this (which in some RARE occasions makes sense). The issue (and it is a big issue) is that you lose all the typesafety of using a class/interface and you have to deal with the case where the method does not exist.

It does have the "benefit" that you can ignore access restrictions and call private methods (not shown in the example, but you can call methods that the compiler would normally not let you call).

Again, it is a rare case that this makes sense, but on those occasions it is a nice tool to have.

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

class Main
{
    public static void main(final String[] argv)
        throws NoSuchMethodException,
               IllegalAccessException,
               IllegalArgumentException,
               InvocationTargetException
    {
        final String methodName;
        final Method method;
        final Main   main;

        main = new Main();

        if(argv.length == 0)
        {
            methodName = "foo";
        }
        else
        {
            methodName = "bar";
        }

        method = Main.class.getDeclaredMethod(methodName, int.class);

        main.car(method, 42);
    }

    private void foo(final int x)
    {
        System.out.println("foo: " + x);
    }

    private void bar(final int x)
    {
        System.out.println("bar: " + x);
    }

    private void car(final Method method,
                     final int    val)
        throws IllegalAccessException,
               IllegalArgumentException,
               InvocationTargetException
    {
        method.invoke(this, val);
    }
}

If you have just one line which is different you could add a parameter such as a flag and a if(flag) statement which calls one line or the other.

You may also be interested to hear about work going on for Java 7 involving closures:

What’s the current state of closures in Java?

http://gafter.blogspot.com/2006/08/closures-for-java.html
http://tech.puredanger.com/java7/#closures

The Google Guava libraries, which are becoming very popular, have a generic Function and Predicate object that they have worked into many parts of their API.

One of the things I really miss when programming in Java is function callbacks. One situation where the need for these kept presenting itself was in recursively processing hierarchies where you want to perform some specific action for each item. Like walking a directory tree, or processing a data structure. The minimalist inside me hates having to define an interface and then an implementation for each specific case.

One day I found myself wondering why not? We have method pointers - the Method object. With optimizing JIT compilers, reflective invocation really doesn't carry a huge performance penalty anymore. And besides next to, say, copying a file from one location to another, the cost of the reflected method invocation pales into insignificance.

As I thought more about it, I realized that a callback in the OOP paradigm requires binding an object and a method together - enter the Callback object.

Check out my reflection based solution for Callbacks in Java. Free for any use.

Sounds like a strategy pattern to me. Check out fluffycat.com Java patterns.

None of the Java 8 answers have given a full, cohesive example, so here it comes.

Declare the method that accepts the "function pointer" as follows:

void doCalculation(Function<Integer, String> calculation, int parameter) {
    final String result = calculation.apply(parameter);
}

Call it by providing the function with a lambda expression:

doCalculation((i) -> i.toString(), 2);

The open source safety-mirror project generalizes some of the above mentioned solutions into a library that adds functions, delegates and events to Java.

See the README, or this stackoverflow answer, for a cheat sheet of features.

As for functions, the library introduces a Fun interface, and some sub-interfaces that (together with generics) make up a fluent API for using methods as types.

Fun.With0Params<String> myFunctionField = "   hello world   "::trim;`  
Fun.With2Params<Boolean, Object, Object> equals = Objects::equals;`  
    
public void foo(Fun.With1ParamAndVoid<String> printer) throws Exception {
    printer.invoke("hello world);
}  

public void test(){
    foo(System.out::println);
}  

Notice:

1. that you must choose the sub-interface that matches the number of parameters in the signature you are targeting. Fx, if it has one parameter, choose Fun.With1Param.
2. that Generics are used to define A) the return type and B) the parameters of the signature.

Also, notice that the signature of the Method Reference passed to the call to the foo() method must match the the Fun defined by method Foo. If it do not, the compiler will emit an error.