在return语句中找不到适合ArrayList <String> .toArray(String [] :: new)的方法

Question

我正在使用Codingbat网站,特别是AP-1中的这种方法

public String[] wordsWithout(String[] words, String target) {
  ArrayList<String> al = new ArrayList<>(Arrays.asList(words));
  al.removeIf(s -> s.equals(target));
  return al.toArray(new String[al.size()]);
}

这个实现工作,以及它当前提交的内容,但是当我将return语句更改为

return al.toArray(String[]::new);

据我所知,这应该工作,给出以下错误:

no suitable method found for toArray((size)->ne[...]size])
method java.util.Collection.<T>toArray(T[]) is not applicable
  (cannot infer type-variable(s) T
    (argument mismatch; Array is not a functional interface))
method java.util.List.<T>toArray(T[]) is not applicable
  (cannot infer type-variable(s) T
    (argument mismatch; Array is not a functional interface))
method java.util.AbstractCollection.<T>toArray(T[]) is not applicable
  (cannot infer type-variable(s) T
    (argument mismatch; Array is not a functional interface))
method java.util.ArrayList.<T>toArray(T[]) is not applicable
  (cannot infer type-variable(s) T
    (argument mismatch; Array is not a functional interface)) line:4

有人会这么好解释为什么这不起作用？

Answer 1

这个:

al.toArray(String[]::new)

仅在java-11之后支持:

default <T> T[] toArray(IntFunction<T[]> generator) {
     return toArray(generator.apply(0));
}

如果你想使用那个方法参考,你必须stream()首先,如:

 al.stream().toArray(String[]::new)