Why can I pass a getter reference to stream().mapToInt(...)?

Question

In the stream, I use mapToInt, in which I must pass a reference to a method with type ToIntFunction

the signature is simple

@FunctionalInterface
public interface ToIntFunction<T> {
    int applyAsInt(T value);
}

This means that the method to which I can pass a reference to mapToInt must contain an input parameter. However, I manage to pass a getter that takes no incoming parameters and returns an int

myList.stream().mapToInt(Transaction::getValue).average().getAsDouble();

Here is the getter

public int getValue() {
return value;
}

Why is it possible to pass a reference to a getter to where the ToIntFunction type is expected where the applyAsInt method has an input parameter

@AlexanderIvanchenko Thank you very much, it really looks like it. Very useful — Thomazzz, Feb 17 '22 at 06:18

score 2 · Accepted Answer · answered Feb 17 '22 at 05:41

2

In a method reference to an instance method, the receiver is treated as the first parameter.

So Transaction::getValue is converted to the equivalent of (Transaction t) -> t.getValue().

answered Feb 17 '22 at 05:41

Louis Wasserman

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Correctly I understand that a reference to a static method is a real reference to a method. And a reference to a class instance method is syntactic sugar? – Thomazzz Feb 17 '22 at 05:53
1

No. It's still a real reference to a method. – Louis Wasserman Feb 17 '22 at 16:17

Why can I pass a getter reference to stream().mapToInt(...)?

1 Answers1