Left and Right identity of a BiFunction?-CodePudding

When I create a Map from stream, I use:

Map<Bar, Foo> fooBar = stream
    .collect(Collectors.toMap(
        Foo::getBar,
        Function.identity(),
        (x, y) -> x
    ));

The merge function I need is just the first. We have Function.identity() that we can pass as a lambda. Do we have something similar for BiFunction, such as BiFunction.left()? External library can work as well.

CodePudding user response：

No. It's simplest, and most efficient, to do exactly what you have done. (Honestly, x -> x is often simpler and clearer than Function.identity() as well.)

CodePudding user response：

If you have a look at the documentation of BinaryOperator you'll find out that the only methods it defines are maxBy() and minBy(). And it also inherits apply() and andThen() from BiFunction.

Nothing similar to what you're looking for.

If you feel like you really need such method left() you can extend BinaryOperator and define one:

public interface MyBinaryOperator<T> extends BinaryOperator<T> {
    static <T> BinaryOperator<T> left() {
        return (left, right) -> left;
    }
}

Applying to your example:

foos.stream()
    .collect(Collectors.toMap(
        Foo::getBar,
        Function.identity(),
        MyBinaryOperator.left()
    ));