为什么清除hashmap方法清除数组列表中添加的map

Question

我正在尝试重用相同的HashMap,例如下面的例子来填充列表.首先,我在地图中添加了一些值,将地图添加到列表中,然后清除地图,以便再次添加新值并在列表中添加第二组值,依此类推......

但是,似乎clear()方法还删除了先前在列表中添加的值,如果我不使用clear()方法,则先前在列表中添加的每组值都会被新的值集覆盖,以便最终在这个特殊的例子我将在列表中有4个相同的值集.

我做错了什么？

    List<HashMap<String, String>>dataList = new ArrayList<HashMap<String, String>>();
    HashMap<String, String> map = new HashMap<String, String>();

    map.put(Answer.ID, "0");
    map.put(Answer.IMAGE, "color_icon_awesome");
    map.put(Answer.TITLE, firstOption);
    dataList.add(map);
    map.clear();             

    map.put(Answer.ID, "1");
    map.put(Answer.IMAGE, "color_icon_awesome");
    map.put(Answer.TITLE, secondOption);
    dataList.add(map);
    map.clear();

    map.put(Answer.ID, "2");
    map.put(Answer.IMAGE, "color_icon_awesome");
    map.put(Answer.TITLE, thirdOption);
    dataList.add(map);
    map.clear();

    map.put(Answer.ID, "3");
    map.put(Answer.IMAGE, "color_icon_awesome");
    map.put(Answer.TITLE, fourthOption);
    dataList.add(map);
    map.clear();

Answer 1

dataList.add(map)将map在列表中引用,所以它不是副本.然后当你再做时map.clear(),它也会删除列表中地图的内容,因为它是完全相同的对象.不要dataList.add(map.clone())代替或(最好)做map = new HashMap<>();之后.

map.put(Answer.ID, "0");
map.put(Answer.IMAGE, "color_icon_awesome");
map.put(Answer.TITLE, firstOption);
dataList.add(map);
map = new HashMap<>();

旁注:您的代码看起来像是可以使用对象而不是地图:

class AnswerObject {

  private String id;
  private String image;
  private String title;

  public AnswerObject(String id, String image, String title) {
      this.id = id;
      this.image = image;
      this.title = title;
  }

  // some getters and setters and some other usefull code

}

这应该使您的代码更好,更可读

List<AnswerObject> dataList = new ArrayList<>();
dataList.add(new AnswerObject("0", "color_icon_awesome", firstOption));
dataList.add(new AnswerObject("1", "color_icon_awesome", secondOption));
dataList.add(new AnswerObject("2", "color_icon_awesome", thirdOption));
dataList.add(new AnswerObject("3", "color_icon_awesome", fourthOption));

但随意忽略;-)