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Question

我正在试图反转我的集合，它由两个ArrayList (具有相同的对象)组成。它分别反转两个ArrayList，如下所示：

赛迪

卡尔

赛迪

卡尔

但我想把它们放在一起像：

赛迪

赛迪

卡尔

卡尔

我尝试了两次添加Collections.reverse (就像删除一样)，但没有起作用。这是可能的还是我应该放弃？以下是我的当前代码：

    List<People> peopleList = new ArrayList<People>();
    Student student1 = new Student("Diana", "Carter", new Date("09/15/1992"), 111222333);
    peopleList.add(student1);
    Faculty faculty1 = new Faculty("Clark", "Kent", new Date("05/22/1990"), 199242003,
            323232);
    peopleList.add(faculty1);
    Staff staff1 = new Staff("Bruce", "Wayne", new Date("01/01/1993"),  161257235,
            100000);
    peopleList.add(staff1);
    Collections.addAll(peopleList);

    ArrayList<People> peopleListClone = new ArrayList<People>();
    peopleListClone.addAll(peopleList);
    peopleListClone.addAll(peopleList);

    Collections.addAll(peopleListClone);
    DisplayPeople(peopleListClone, "////////////////// People list clone     initialized.");

    peopleListClone.remove(student1);
    peopleListClone.remove(student1);
    DisplayPeople(peopleListClone, "////////////////// People list after student elements removed.");

    Collections.reverse(peopleListClone);
    Collections.reverse(peopleListClone);
    DisplayPeople(peopleListClone, "////////////////// People list clone sorted in reverse.");

Answer 1

尝尝这个。

List<People> peopleListClone = peopleList.stream()
    .flatMap(s -> Stream.of(s, s))
    .collect(Collectors.toList());

而不是

ArrayList<People> peopleListClone = new ArrayList<People>();
peopleListClone.addAll(peopleList);
peopleListClone.addAll(peopleList);

Answer 2

我仍然不太明白你想用你写出来的代码实现什么。如果这能帮助您理解克隆和逆转ArrayLists的方式，请告诉我。

List<String> list = new ArrayList<String>();
list.add("apple");
list.add("ball");
list.add("cat");
list.add("dog");            // [apple, ball, cat, dog]  
Collections.reverse(list);
System.out.println(list);   // [dog, cat, ball, apple]

List<String> list2 = new ArrayList<String>();
list2.addAll(list);         // MAKING A HARD COPY

Collections.reverse(list2); // REVERSING THE HARD COPY
System.out.println(list2);  // [apple, ball, cat, dog]  REVERSED HARD COPY
System.out.println(list);   // [dog, cat, ball, apple] STILL THE SAME ORIGINAL