【拾遗】Java中的隐式参数

今天翻《Java 8 编程参考官方教程》的时候看到一段代码：

public class InstanceMethWithObjectRefDemo {
    private static <T> int counter(T[] vals, MyFunc<T> f, T v2) {
        int count = 0;
        
        for (T v1 : vals) {
            if (f.func(v1, v2)) count++;
        }
        
        return count;
    }
    
    
    public static void main(String[] args) {
        int count;
        
        HighTemp[] weekDayHighs = {
                                    new HighTemp(89), new HighTemp(82),new HighTemp(90), new HighTemp(89),  
                                    new HighTemp(89), new HighTemp(91),new HighTemp(84), new HighTemp(83) 
                                    };
        
        count = counter(weekDayHighs, HighTemp::sameTemp, new HighTemp(89));
        
        System.out.println(count + " days had a high of 89");
        
        count = counter(weekDayHighs, HighTemp::lessThanTemp, new HighTemp(89));

        System.out.println(count + " days had a high less than of 89");
    }
}

@FunctionalInterface
interface MyFunc<T> {
    boolean func(T v1, T v2);
}

class HighTemp {
    private int hTemp;

    public HighTemp(int hTemp) {
        this.hTemp = hTemp;
    }

    boolean sameTemp(HighTemp ht2) {
        return this.hTemp == ht2.hTemp;
    }

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

这里有一段：

count = counter(weekDayHighs, HighTemp::sameTemp, new HighTemp(89));

看到的时候有点疑问，HighTemp#sameTemp怎么就和MyFunc函数式接口兼容了呢？MyFunc需要两个T参数和一个boolean返回值。而HighTemp#sameTemp是一个参数一个返回值。愣了一会儿，想起隐式参数这回事，恍然大悟。
一般情况下方法都会有super和this，所以相当于sameTemp方法的声明是：

boolean sameTemp(HighTemp this, HighTemp ht2);

这样就和MyFunc兼容了。

接口中唯一抽象方法的命名并不重要，因为函数式接口就是对某一行为进行抽象，主要目的就是支持Lambda表达式。

总结

有些知识点看的时候理解，但不到实际应用，还真容易忘，但是看过的东西总比没有学习过要好，因为在你需要它的时候，你能想起来，帮助你解决问题。