关于双参数方法参考的困惑

Question

我对这个方法参考语法有点困惑。

counter() 期望 BiFunction 然而 HighTemp::lessThanTemp 是一个有效的参数，尽管 HighTemp.lessThanTemp() 只接受一个参数。

行中到底发生了什么：if (f.func(vals[i], v))?

MCVE:

import java.util.function.BiFunction;

class Demo {
  static class HighTemp {
    private int hTemp;

    HighTemp(int ht) { hTemp = ht; }

    boolean lessThanTemp(HighTemp ht2) {
      return hTemp < ht2.hTemp;
    }
  }

  static <T> int counter(T[] vals, BiFunction<T,T,Boolean> f, T v) {
    int count = 0;

    for (int i=0; i < vals.length; i++) {
      if (f.apply(vals[i], v)) { // THIS LINE
        count++;
      }
    }

    return count;
  }

  public static void main(String args[]) {    
    HighTemp[] weekDayHighs2 = { new HighTemp(32), new HighTemp(12),
                                 new HighTemp(24), new HighTemp(19),
                                 new HighTemp(18), new HighTemp(12),
                                 new HighTemp(-1), new HighTemp(13) };

    int count = counter(weekDayHighs2, HighTemp::lessThanTemp, new HighTemp(19));
    System.out.println(count + " days had a high of less than 19");
  }
}

Answer 1

看看 relevant documentation，其中注释：

The equivalent lambda expression for the method reference String::compareToIgnoreCase would have the formal parameter list (String a, String b), where a and b are arbitrary names used to better describe this example. The method reference would invoke the method a.compareToIgnoreCase(b).

换句话说，HighTemp::lessThanTemp等价于lambda表达式：

(a, b) -> a.lessThanTemp(b)

关于双参数方法参考的困惑

Confusion about two-argument method reference

java

lambda

java-8

method-reference