问题
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.
回答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.
回答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.
回答3:
Could Function.identity() fit your needs?
Returns a function that always returns its input argument.
回答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.
回答5:
You can have your own NOOP implementation, similar to Function.Identity.
static <T> Consumer<T> NOOP() {
return t -> {};
}
来源：https://stackoverflow.com/questions/29851525/is-there-a-method-reference-for-a-no-op-nop-that-can-be-used-for-anything-lamb