说明

这里只探究稍微特殊点的两个函数式接口：UnaryOperator 和 BinaryOperator

UnaryOperator<T>

对类型为 T 参数应用操作,进行一元运算并返回 T 类型的结果

@FunctionalInterface
public interface UnaryOperator<T> extends Function<T, T> {

    /** a function that always returns its input argument */
    static <T> UnaryOperator<T> identity() {
        return t -> t;
    }
}

样例

探究 identity 方法

代码

public class TestDemo {
    /** 测试 */
    public static void main(String[] args) {
        LocalDateTime now = LocalDateTime.now();
        Stream<LocalDateTime> stream = Stream.of(now, now.plusDays(1L), now.plusDays(2L));
        List<LocalDateTime> collect = stream.map(UnaryOperator.identity()).collect(Collectors.toList());
        collect.forEach(System.out::println);
    }
}

输出

2021-12-30T17:12:34.273
2021-12-31T17:12:34.273
2022-01-01T17:12:34.273

BinaryOperator<T>

对类型为 T 参数应用操作,进行二元运算并返回 T 类型的结果

@FunctionalInterface
public interface BinaryOperator<T> extends BiFunction<T,T,T> {

    public static <T> BinaryOperator<T> minBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) <= 0 ? a : b;
    }

    public static <T> BinaryOperator<T> maxBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) >= 0 ? a : b;
    }
}

样例

探究 maxBy 和 minBy 方法

代码

public class TestDemo {
    /** 测试 */
    public static void main(String[] args) {
        int[] arr1 = {9, 1, 7, 3, 5};
        Integer maxResult = arr1[0];
        Integer minResult = arr1[0];
        for(int i = 1; i < arr1.length; i ++){
            maxResult = getMaxInt(maxResult, arr1[i], Integer::compareTo);
            minResult = getMinInt(minResult, arr1[i], Integer::compareTo);
        }
        System.out.println("最大值: " + maxResult);
        System.out.println("最小值: " + minResult);
    }
    /** 定义操作方法：获取最大值 */
    public static int getMaxInt(int a, int b, Comparator<Integer> comparator){
        return BinaryOperator.maxBy(comparator).apply(a, b);
    }
    /** 定义操作方法：获取最小值 */
    public static int getMinInt(int a, int b, Comparator<Integer> comparator){
        return BinaryOperator.minBy(comparator).apply(a, b);
    }
}

输出

最大值: 9
最小值: 1

上一节：Java 8 函数式接口(三) 四大内置的核心函数式接口的样例