CODEWITHSUNDEEP
javalambda
WORMSS
WORMSS

Reputation: 1664

Lambda Function get turned into BiConsumer

I am trying to work out how this works.
I am going to list what I believe is happening and hopefully someone can tell me I am wrong in places. Or tell me how/why this works.

public BiConsumer<List<T>, T> accumulator() {
    return List::add;
}
  1. A method called accumulator that accepts nothing and returns a BiConsumer method.
  2. accumulator returns a reference to the add method on the context of a List as the BiConsumer
  3. The actual add method on a List is actually a Function<T, Boolean>, takes in T and returns a boolean

This is where I get into the guessing realm.

  1. It is calling the add method, on the instance of the List that is the first argument of the BiConsumer.

I know internally it must be doing.

public BiConsumer<List<T>, T> accumulator() {
    return (list, item) -> list.add(i);
}

I understand that it can ignore the return type of the add method, so it can pretend Function<A, B> is Consumer<A>

But I just don't understand the black magic that turns a BiConsumer runs a Consumer or Function on the instance of the first argument of the BiConsumer.

Upvotes: 3

Views: 749

Answers (1)

Sweeper
Sweeper

Reputation: 271685

It's not black magic. It is well specified here.

The following is an example of a reference to an instance method of an arbitrary object of a particular type:

String[] stringArray = { "Barbara", "James", "Mary", "John",
    "Patricia", "Robert", "Michael", "Linda" };
Arrays.sort(stringArray, String::compareToIgnoreCase);

The equivalent lambda expression for the method reference String::compareToIgnoreCase would have the formal parameter list (String a, String b), where a and b are arbitrary names used to better describe this example. The method reference would invoke the method a.compareToIgnoreCase(b).

When you have something like SomeType::instanceMethod, it can be converted to a function accepting two arguments, the first being the instance to call the method on. It just can, because the language spec says so.

Upvotes: 2

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