Java equivalent of C# Delegates (queues methods of various classes to be executed)

Question

TLDR:

Is there a Java equivalent of C#'s delegates that will allow me to queue up methods of various classes and add them to the queue dynamically? Language constructs instead of the code for it.

Context:

I have used Unity 3D before and I love things like the Update method of scripts. Just declaring the method adds it to the list of methods executed each frame. I want to create something like that in my LWJGL game. For this, I would want to use delegates (or something equivalent to it). Is there any Java language construct that would allow me to do this? I would prefer answers that include two or more (so that I can pick and choose which will be the most optimal for my code) constructs and a way of using them. I don't want the code, I just want to know where to start. The most fun part of programming is working the problem out and I don't want to be deprived of that. Also, I don't want to be told EXACTLY how to do it. I want to be guided in the right direction instead of being thrown in that direction onto that destination. How would I learn? :-)

Answer 1

Actually there is no exact counterpart for delegates in Java. But there are constructs that mimic their behavior.

Java 8

Functional interfaces

The concept that comes closes to delegates in Java 8 is that of functional interfaces.

For example, if you have a C# delegate:

delegate void Runnable();

in Java, you would create a functional interface like:

@FunctionalInterface
public interface Runnable {
    void run();
}

The nice thing about functional interfaces is they can be used easily in lambda expressions.

Example

So, let's suppose you have the following class:

public class SomeClass {
    public static void someStaticMethod() {
    }

    public void someMethod() {
    }
}

Lambda expressions and method references

With Java 8, you get lambda expressions.

List<Runnable> queue = new ArrayList<>();
queue.add(() -> someMethod());
queue.add(() -> someStaticMethod());

There is a short-hand named method reference for this, if you actually simply call a method:

List<Runnable> queue = new ArrayList<>();
queue.add(this::someMethod);
queue.add(SomeClass::someStaticMethod);

Java 7

With Java 7, the only thing you can use is anonymous classes:

List<Runnable> queue = new ArrayList<>();
queue.add(new Runnable() {
    public void run() {
        someMethod();
    }
});
queue.add(new Runnable() {
    public void run() {
        someStaticMethod();
    }
});

I hope this was not too much information, so you can still learn. ;-) However, I like my answer to be useful also for other people looking up this question.