java - 如何访问 java 8 排序流中的前一项？

Question

在排序流中，我想访问前一个元素。

我正在寻找这样的东西:

class Main {
  public static void main(String[] args) {
    List<String> stringList = Arrays.asList("1: First", "2: Second", "3: Third", "4: Fourth");

    List<String> resultList = stringList.stream()
      .sorted()
      .filter(s -> s.contains("Second") && s.previous().contains("First"))
                                           ^^^^^^^^^^^^ doesn't compile
      .collect(Collectors.toList());

    System.out.println(resultList);
  }
}

这种情况下的预期输出是

Second

重要提示:我想分解出 filter() 方法中的 lambda，以便在不同的地方重用它。

Answer 1

您可以保存每一步的上一个:

class Main {
    public static void main(String[] args) {
        List<String> stringList = Arrays.asList("1: First", "2: Second", "3: Third", "4: Fourth");
        List<String> resultList = stringList.stream()
                .sorted()
                .filter(new Predicate<String>() {
                    private String prev = null;

                    @Override
                    public boolean test(String s) {
                        boolean result = s.contains("Second") && prev != null && prev.contains("First");
                        prev = s;
                        return result;
                    }
                })
                .collect(Collectors.toList());
        System.out.println(resultList);
    }
}

并且您可以定义单独的自定义 Predicate 类以重用:

class Main {
    public static void main(String[] args) {
        List<String> stringList = Arrays.asList("1: First", "2: Second", "3: Third", "4: Fourth");
        List<String> resultList = stringList.stream()
                .sorted()
                .filter(new WithPrevPredicate() {
                    @Override
                    public boolean test(String prev, String current) {
                        return current.contains("Second") && prev != null && prev.contains("First");
                    }
                })
                .collect(Collectors.toList());
        System.out.println(resultList);
    }

    private static abstract class WithPrevPredicate<T> implements Predicate<T> {
        private T prev = null;

        @Override
        public boolean test(T current) {
            boolean test = test(prev, current);
            prev = current;
            return test;
        }

        public abstract boolean test(T prev, T current);
    }
}