Java 8 Lambdas与泛型的使用

Question

    //in client 
    MathUtility.sum(listOfInts, (Integer i)->{return (i<3);});

   //in utility
    class MathUtility<T extends Number> {
        public static <T extends Number> T sumWithCondition(List<T> numbers, Predicate<T> condition) {
            return numbers.parallelStream()
                    .filter(condition)
                    .map(i -> i)
                    .reduce(0, T::sum); //compile time error
        }
        public static <T extends Number> T avgWithCondition(List<T> numbers, Predicate<T> condition) {
            //another function
        }
        //lot many functions go here
    }

Answer 1

有没有一种方法可以做到这一点，而不用为我所期望的每一种可能的t类型编写求和函数？

正如Aaron Davis在上面的评论中所述，您可以将缩减参数传递给方法本身。

public static <T> T sumWithCondition(List<T> numbers, Predicate<T> condition, T identity, BinaryOperator<T> accumulator) {
    return numbers.parallelStream().filter(condition).reduce(identity, accumulator);
}

一个例子是：

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

System.out.println(sumWithCondition(list, i -> i > 1, 0, (a, b) -> a + b));

>> 14

List<BigInteger> list2 = Arrays.asList(BigInteger.ONE, BigInteger.ONE);

System.out.println(sumWithCondition(list2, i -> true, BigInteger.ZERO, (a, b) -> a.add(b)));

>> 2