详解Java8的forEach(...)如何提供index值

  List<String> list = Arrays.asList("Hi", "I", "am", "Henry.Yao");
  // 此处已经用到了泛型，不能算是纯粹的Java2代码，仅作Iterator示范
  for (Iterator<String> it = list.iterator(); it.hasNext();) {
    String item = it.next();
    System.out.println("listItem = " + item);
  }

  List<String> list = Arrays.asList("Hi", "I", "am", "Henry.Yao");
  for(String item : list) {
    System.out.println("listItem = " + item);
  }

  list.forEach(item -> {
    System.out.println("listItem = " + item);
  });

  List<String> list = Arrays.asList("Hi", "I", "am", "Henry.Yao");
  for(int index; index<list.size(); index++) { 
    String item = list.get(i);
    System.out.println("list["+index+"] = "+item);
  }

  list.forEach((item, index) -> {
    System.out.println("listItem = " + item);
  }); // Compile ERROR

  // 工具方法
  public static <T> Consumer<T> consumerWithIndex(BiConsumer<T, Integer> consumer) {
    class Obj {
      int i;
    }
    Obj obj = new Obj();
    return t -> {
      int index = obj.i++;
      consumer.accept(t, index);
    };
  }

  list.forEach(LambdaUtils.consumerWithIndex((item, index) -> {
    System.out.println("list[" + index + "]=" + item);
  }));

  // 工具方法
 public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
 Map<Object, Boolean> seen = new ConcurrentHashMap<>();
 return t -> Objects.isNull(seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE));
 }

  // 业务代码
  // 从人员列表中过滤出一个子集(每个部门选一个人)
  employees.stream().filter(distinctByKey(Employee::getDeptCode)).collect(toList());