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javalambdafunctional-programmingjava-stream

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

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

Solution

  • 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().