什么是LINQ Join的Java 8 Stream API等价物？

在C#/ .Net中,可以使用扩展方法Enumerable.Join以SQL’JOIN … ON’方式连接IEnumerable序列.

Java 8(Stream API)中有类似的东西吗？或者模拟Enumerable.Join的最佳方法是什么？

看到：
https://msdn.microsoft.com/en-us/library/bb534675%28v=vs.100%29.aspx

解决方法

public static <Outer,Inner,Key,Result> Stream<Result> join(
        Stream<Outer> outer,Stream<Inner> inner,Function<Outer,Key> outerKeyFunc,Function<Inner,Key> innerKeyFunc,BiFunction<Outer,Result> resultFunc) {

    //Collect the Inner values into a list as we'll need them repeatedly
    List<Inner> innerList = inner.collect(Collectors.toList());

    //matches will store the matches between inner and outer
    final Map<Outer,List<Inner>> matches = new HashMap<>();

    //results will be used to collect the results in
    final List<Result> results = new ArrayList<>();


    outer.forEach(o -> innerList
            .stream()
            //Filter to get those Inners for which the Key equals the Key of this Outer
            .filter(i -> innerKeyFunc.apply(i).equals(outerKeyFunc.apply(o)))
            .forEach(i -> {
                if (matches.containsKey(o)) {
                    //This Outer already had matches,so add this Inner to the List
                    matches.get(o).add(i);
                } else {
                    //This is the first Inner to match this Outer,so create a List
                    List<Inner> list = new ArrayList<>();
                    list.add(i);
                    matches.put(o,list);
                }
            }));

    matches.forEach((out,in) -> in.stream()
            //Map each (Outer,Inner) pair to the appropriate Result...
            .map(i -> resultFunc.apply(out,i))
            //...and collect them
            .forEach(res -> results.add(res)));

    //Return the result as a Stream,like the .NET method does (IEnumerable)
    return results.stream();
}

我只使用以下输入对代码进行了简短测试：

public static void main(String[] args) {
    Stream<String> strings = Arrays.asList("a","b","c","e","f","d").stream();
    Stream<Integer> ints = Arrays.asList(1,2,3,6,5,4).stream();
    Stream<String> results = join(strings,ints,Function.identity(),str    -> Integer.parseInt(str,16) - 9,(o,i) -> "Outer: " + o + ",Inner: " + i);
    results.forEach(r -> System.out.println(r));
}

> int是他们自己的键,所以没有转换
>字符串根据其十六进制值映射到整数 – 9
>(如果int值相等,则元素匹配,默认情况下)
>匹配对被放入String中

打印以下(正确)结果：

Outer: a,Inner: 1
Outer: b,Inner: 2
Outer: c,Inner: 3
Outer: d,Inner: 4
Outer: e,Inner: 5
Outer: f,Inner: 6

当然,还需要进行更深入的测试,但我认为这种实施是正确的.它可能也更有效率,我愿意接受建议.

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