java - 执行 map-reduce 操作的通用方法。 (Java-8)

Question

如何在 Java 8 中使用泛型参数重载函数？

public class Test<T> {

    List<T> list = new ArrayList<>();

    public int sum(Function<T, Integer> function) {
        return list.stream().map(function).reduce(Integer::sum).get();
    }


    public double sum(Function<T, Double> function) {
        return list.stream().map(function).reduce(Double::sum).get();
    }
}

Error: java: name clash: sum(java.util.function.Function<T,java.lang.Double>) and sum(java.util.function.Function<T,java.lang.Integer>) have the same erasure

Answer 1

Benji Weber once wrote of a way to circumvent this.您需要做的是定义扩展参数类型的自定义功能接口(interface):

public class Test<T> {

    List<T> list = new ArrayList<>();

    @FunctionalInterface
    public interface ToIntFunction extends Function<T, Integer>{}
    public int sum(ToIntegerFunction function) {
        return list.stream().map(function).reduce(Integer::sum).get();
    }


    @FunctionalInterface
    public interface ToDoubleFunction extends Function<T, Double>{}
    public double sum(ToDoubleFunction function) {
        return list.stream().map(function).reduce(Double::sum).get();
    }
}

另一种方法是改用 java.util.function.ToIntFunction 和 java.util.function.ToDoubleFunction:

public class Test<T> {

    List<T> list = new ArrayList<>();

    @FunctionalInterface
    public int sum(ToIntFunction function) {
        return list.stream().mapToInt(function).sum();
    }

    public double sum(ToDoubleFunction function) {
        return list.stream().mapToDouble(function).sum();
    }
}