Equivalent to C# delegate in Java

Java
1

Hello there.

As a trained C# dev I learnt about delegates and the predefined ones such as Func<>. I was wondering if there is a way to reproduce such behaviour with Java.

I did write a markdown file with further details on GitLab.

It is likely what I try to achieve does not exist in Java and may not be replicable for now. So it is fine if no one actually answers? But I’m just dropping a message in a bottle someone could maybe answer.

Kind Regards.

2

Sounds like what you are looking for is mostly called lambdas, method references and functional interfaces in Java. Most of that stuff has been available since Java 8 and has gotten a little nicer with time. I’ll try to find you a good reference but if I cannot find one I can certainly give you a demonstration.

1 Like
3

Using the code in your GitLab, it would look like this:

public static void main(String[] args) {
  for (ThreePeople myFunc : 
       List.of(
         FuncEquivalentDemo::sumAges,
         FuncEquivalentDemo::averageAges)) {
    System.out.println(myFunc.doWithAges(person1, person2, person3));
  }
}

You do have to call the doWithAges method on the object, but that is how it works. You can call a function like askFunc with either a method reference or a lambda:

askFunc(FuncEquivalentDemo::sumAges);
askFunc(FuncEquivalentDemo::averageAges);
askFunc((p1, p2, p3) -> -1);

Since everything in Java is an object, there is no way to get this exact syntax though:

var myFunc = ...;
myFunc(person1, person2, person3);

You have to call the method for the interface in order to run the code.

1 Like
4

This is about the best description I can find describing functional interfaces:

https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html#package.description

1 Like
5

Hi.

Thank you very much for the quick answer.
I learnt about references and I tried a thing or two but I assumed that because I was in the type where the sumAges() and averageAges() were, I did not need to prefix with type name.

C# was inspired by Java so everything in C# is Object too.
I knew I could not get a perfectly identical solution but that’s close enough to me. Simply in C# you can prepare a delegate and call it when needed while the askFunc(FuncEquivalentDemo::sumAges); statement call it right on. But that’s not a big deal. Your solution works like a charm.
I will update my notes and give you credit (Just 1st letter on name unless you’re OK I put it fully) for the help. :slight_smile:

Like we say in France “Rendre à César ce qui appartient à César” (i.e. Give back to Caesar what belong to Caesar).

Best Regards.

EDIT:

Got through the ref you provided.
Tried something inspired from their Assignation context example and it is fair enough:

public static void furtherDemo() {
//	    Predicate<String> isEmpty = String::isEmpty;
//	    System.out.println(isEmpty.test("Hello"));
//	    System.out.println(isEmpty.negate().test("World"));

	ThreePeople tp = FuncEquivalentDemo::sumAges;

	System.out.println(tp
		.doWithAges(new Person("Jake", 57), 
			new Person("Bill", 25), 
			new Person("Laura", 18)));
}

Output

100

Thanks again.