Is there a method reference for a no-op (NOP) that can be used for anything lambda?

Question

This may sounds like a strange question, but is there a way to refer to a standard no-op (aka null operation, null-pattern method, no-operation, do-nothing method) method for a Lambda in Java 8.

Currently, I have a method that takes a, say, void foo(Consumer<Object>), and I want to give it a no-op, I have to declare:

foo(new Consumer<Object>() { 
  public void accept(Object o) { 
    // do nothing 
  }
}

where I would like to be able to do something like:

foo(Object::null)

instead. Does something like exist?

Not sure how that would work with multi-parameter methods -- perhaps this is a deficiency in the lambdas in Java.

Solution 1

This is no deficiency.

Lambdas in Java are instances of functional interfaces; which, in turn, are abstracted to instances of Java constructs which can be simplified to one single abstract method, or SAM.

But this SAM still needs to have a valid prototype. In your case, you want to have a no-op Consumer<T> which does nothing whatever the T.

It still needs to be a Consumer<T> however; which means the minimal declaration you can come up with is:

private static final Consumer<Object> NOOP = whatever -> {};

and use NOOP where you need to.

Solution 2

In your particular case you could simply do:

foo(i -> {});

This means that the lambda expression receives a parameter but has no return value.

The equivalent code for a BiConsumer<T, U> would be:

void bifoo(BiConsumer<Object, Object> consumer) { ... }

bifoo((a, b) -> {});

Solution 3

Could Function.identity() fit your needs?

Returns a function that always returns its input argument.

Solution 4

If you want a method reference for a method that does nothing, the easiest way is to write a method that does nothing. Notice that in this example I have used Main::doNothing when a Consumer<String> is required.

class Main {

    static void doNothing(Object o) { }

    static void foo(Consumer<String> c) { }

    public static void main(String[] args) {
        foo(Main::doNothing);
    }
}

You could also overload doNothing by providing a version using varargs.

static void doNothing(Object... o) { }

This signature will accept literally any sequence of parameters (even primitives, as these will get autoboxed). That way you could pass Main::doNothing whenever the functional interface's method has void return type. For example you could pass Main::doNothing when an ObjLongConsumer<Integer> is needed.

Solution 5

You can have your own NOOP implementation, similar to Function.Identity.

static <T> Consumer<T> NOOP() {
    return t -> {};
}

View more solutions

Comments

[Stuart Marks]

Related: stackoverflow.com/a/26553481/1441122

[paul_h]

"Null Object" pattern is the name of this (per Martin Fowler's Refactoring book - 1997 and updated in 2018). And yes, it'd be great if most interfaces in the JDK shipped with a public static inner class NULL or NullObject.

[paul_h]

I mean .. Most interfaces in the JDK should ship with a inner class that is a Null Object implementation, and a public static var NULL or NullObject which is an instance of that for all to use, like so: foo = Consumer.NULL;

[Davids037]

Can be extends for any number of arguments too: foo((a,b) -> {}); etc

[Holger]

Well, you can say Consumer<ArbitraryType> c=Function.identity()::apply; but the question is whether this is really better than c=ignore->{};…

[Radiodef]

Can be generic for even more applicability. static <T, U> U noop(T... a) {return null;}

[Paul Boddington]

@Radiodef I thought about mentioning that in my answer but decided I didn't like it because you can pass noop even in the case where the functional interface's method returns a primitive type, which is asking for a null pointer exception.

[Paul Boddington]

@Radiodef The other unfortunate thing is that the signatures of the versions returning U and void have the same erasure, so either the methods would have to have different names, or they'd have to be in different classes. Personally I think this is a deficiency, and something like noop or doNothing would fit natually in the Objects class.

[Radiodef]

All true. I didn't think of the unboxing problem.

[jk.]

actually i think i like Function.identity()::apply as it doen't intriduce an unused parameter

[xorcus]

Would be really nice to have this in the Consumer interface itself.

[Zoltán Umlauf]

might not have been what op was looking for, but this is what i was looking for, thanks!

[Antoniossss]

I know this is stupid, but I just used Function.identity()::apply; over (v)->{} as formatter leaves it in the same line instead of hardbreaking { } XD