带有泛型的Java 8 Lambdas的使用 [英] Use of Java 8 Lambdas with Generics

问题描述

我有一个客户端类，它利用MathUtility类提供的函数。无论Mathmatical操作如何，它只应在MathUtility类中发生。

  //在客户端
中MathUtility.sum（listOfInts，（Integer i） - > {return（i< 3）;}）; 
 
 //在实用程序中
 class MathUtility< T extends Number> {
 public static< T extends Number> T sumWithCondition（List  numbers，Predicate< T> condition）{
 return numbers.parallelStream（）
 .filter（condition）
 .map（i  - > i）
 .reduce（0，T :: sum）; //编译时错误
} 
 public static< T extends Number> T avgWithCondition（List< T>数字，Predicate< T>条件）{
 //另一个函数
} 
 //很多函数在这里
} 
  

现在失败，出现此错误 - 方法reduce（T，BinaryOperator< T>）in类型Stream< T>不适用于参数（int，T :: sum）

注意：我不想为不同的数字类型编写求和函数

解决方案

有没有办法在不写每个可能类型的<






$ b $正如Aaron Davis在上述评论中指出的那样，您可以将缩减参数传递给方法本身。

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

一个例子是：

 列表<整数> list = Arrays.asList（1，2，3，4，5）; 
 
 System.out.println（sumWithCondition（list，i  - > i> 1,0，（a，b） - > a + b））; 
 
>> 14 
 
列表< BigInteger> list2 = Arrays.asList（BigInteger.ONE，BigInteger.ONE）; 
 
 System.out.println（sumWithCondition（list2，i  - > true，BigInteger.ZERO，（a，b） - > a.add（b）））; 
 
>> 2 
  

Is it possible to do this using Predicate interface.

I have a client class that utilizes functions provided by a MathUtility class. Whatever the Mathmatical operation it should happen only within the MathUtility class.

    //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
    }

Right now it fails with this error - The method reduce(T, BinaryOperator<T>) in the type Stream<T> is not applicable for the arguments (int, T::sum)

Note: I do not want to write sum functions for different Number types

解决方案

Is there a way to do it without writing a sum function for every possible type of T that i'm expecting?

As Aaron Davis stated in a comment above, you can pass the reduction parameters to the method itself.

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

An example would be:

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

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