方法引用 - 无效方法引用 - 不能从静态上下文引用

Question

我有以下一段代码

StringJoiner joiner = new StringJoiner(", ");
joiner.add("Something");
Function<StringJoiner,Integer> lengthFunc =  StringJoiner::length;
Function<CharSequence,StringJoiner> addFunc = StringJoiner::add;

最后一行导致错误

Error:(54, 53) java: invalid method reference
  non-static method add(java.lang.CharSequence) cannot be referenced from a static context

我知道这个方法不能以静态方式使用,我应该有类似的东西:

Function<CharSequence,StringJoiner> addFunc = joiner::add;

代替.但是我无法理解为什么第三行,StringJoiner::length;用于java编译完全正确.someboedy可以解释一下为什么会这样吗？

Answer 1

因为StringJoiner.length接受零参数,所以方法引用整体取一个StringJoiner(任意实例)并返回一个Integer.换句话说,第一个指定的方法ref相当于:

Function<StringJoiner, Integer> lengthFunc = new Function<StringJoiner, Integer>() {

       @Override
        public Integer apply(StringJoiner stringJoiner) {
            return stringJoiner.length;
        }
}

你会这样称呼Function如下:

StringJoiner sj1 = ...  // an arbitrary StringJoiner
int sjLength1 = lengthFunc.apply(sj1);

在契约中,StringJoiner.add(CharSequence)采用一个论证,所以总体上Function必须采取(1)arbirary实例StringJoiner,(2)CharSequence并返回a StringJoiner.

您可以改为将引用分配给BiFunction:

BiFunction<StringJoiner, CharSequence, StringJoiner> addFunc = StringJoiner::add;

这相当于:

BiFunction<StringJoiner, CharSequence, StringJoiner> addFunc = new BiFunction<StringJoiner, CharSequence, StringJoiner>() {

       @Override
        public StringJoiner apply(StringJoiner stringJoiner, CharSequence charSequence) {
            return stringJoiner.add(charSequence);
        }
}

并将使用如下:

StringJoiner sj1 = ...  // an arbitrary StringJoiner
sj1 = addFunc.apply(sj1, "a"); // no need to re-assign, but just to show the return type

Answer 2

Function<StringJoiner,Integer> lengthFunc =  StringJoiner::length;

lengthFunc是一个函数,它接受StringJoiner并返回一个Integer.因此,任何实例方法StringJoiner都不需要,并返回Integer匹配此接口.调用该方法的实例将是StringJoiner所需的实例Function<StringJoiner,Integer>.

另一方面

Function<CharSequence,StringJoiner> addFunc = StringJoiner::add

addFunc是一个Function需要CharSequence和回报StringJoiner.没有StringJoiner匹配此接口的实例方法,因为该函数没有StringJoiner要应用该add方法的输入实例.

你需要一个BiFunction匹配签名StringJoiner::add:

BiFunction<StringJoiner,CharSequence,StringJoiner> addFunc = StringJoiner::add;