stream-API:用流替换for-each循环(内部使用昂贵的操作调用)

Question

我目前正在努力提高我的Java8技能,并偶然发现了这种情况.我的目标是用方法foo中的一些Java8流替换方法中的现有循环fooStream.

问题是,我在循环中调用了一个非常昂贵的操作,并希望在此调用的结果满足我的期望时退出它.我的流实现现在总是进入一次非常昂贵的操作(在map-call中),而不是我的老式循环实现.

我的流实现可以改进以防止这个额外的呼叫吗？

public class Foo {
  public static void main(final String[] args) {
    final List<String> someList = Arrays.asList(null, null, "xxx", null, null, "foo");
    foo(someList);
    fooStream(someList);
  }

  public static String foo(final List<String> someList) {
    for (final String foo : someList) {
      final String result = extremelyExpensiveOperation(foo, "loop");
      if (result != null) {
        return result;
      }
    }

    return null;
  }

  public static String fooStream(final List<String> someList) {
    return someList //
        .stream() //
        .filter(p -> extremelyExpensiveOperation(p, "streamFilter") != null) //
        .map(p -> extremelyExpensiveOperation(p, "streamMap")) //
        .findFirst().orElse(null);
  }

  private static String extremelyExpensiveOperation(final String foo, final String operation) {
    System.out.println("entered extremelyExpensiveOperation: " + operation);
    // do some extremely expensive operation
    // but in our show and tell case just return foo
    return foo;
  }

}

Answer 1

这应该很简单:

return someList //
            .stream() //
            .map(p -> extremelyExpensiveOperation(p, "streamFilter"))
            .filter(Objects::nonNull)
            .findFirst()
            .orElse(null);