在Java 8中使用函数类型congruent lambda表达式 [英] Usage of function type congruent lambda expressions in Java 8

问题描述

我很难定义和使用

Stream.collect(Supplier<R> supplier, BiConsumer<R,? super T> accumulator, BiConsumer<R,R> combiner)

方法。

方法签名包括BiConsumer 键入的参数。 BiConsumer FunctionalInterface定义了一种功能方法 accept（Object，对象）。据我所知，我现在可以使用任何全等到这个功能界面。

The method signature includes BiConsumer typed parameters. The BiConsumer FunctionalInterface defines one functional method accept(Object, Object). As far as I understand I can now use any lambda expression that is congruent to this functional interface.

但是Stream.collect中提到的例子 JavaDoc的是例如

But the example mentioned in the Stream.collect JavaDoc is e.g.

 List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);

我不明白为什么ArrayList.add(E e) （单个参数）与 BiConsumer.accept（T t，U u）方法（两个参数）并且可以在收集方法中用作累加器函数。

I do not understand why ArrayList.add(E e) (single parameter) is congruent with the BiConsumer.accept(T t, U u) method (two parameters) and can be used as the accumulator function in the collect method.

如你所见，我显然缺乏理解并理解任何解释。

As you see I obviously have a lack of understanding and appreciate any explanation.

推荐答案

累加器BiConsumer的两个参数是（1）列表本身和（2）要添加的项目。这：

The accumulator BiConsumer's two parameters are (1) the list itself and (2) the item to add to it. This:

List<String> asList = stringStream.collect(
    ArrayList::new,
    ArrayList::add,
    ArrayList::addAll
);

相当于：

List<String> asList = stringStream.collect(
    () -> new ArrayList<>(),
    (list, item) -> list.add(item),
    (list1, list2) -> list1.addAll(list2)
);

会得到与此相同的结果：

which will give the same result as this:

List<String> asList = stringStream.collect(
    new Supplier<ArrayList<String>>() {
        @Override
        public ArrayList<String> get() {
            return new ArrayList<>();
        }
    },

    new BiConsumer<ArrayList<String>,String>() {
        @Override
        public void accept(ArrayList<String> list, String item) {
            list.add(item);
        }
    },

    new BiConsumer<ArrayList<String>,ArrayList<String>>() {
        @Override
        public void accept(ArrayList<String> list1, ArrayList<String> list2) {
            list1.addAll(list2);
        }
    }
);

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