Java HashMap 自动值复制问题

Question

每当我开始设置 HashMap 时，值就会被复制。我相信这是我在初始化HashMap时不理解的一些Java规则。下面是我的 HashMap 的 DTO。

public class ClientsByMonth {
private int pax;
private int folios;
private int totalStays;
private HashMap<String, Integer> byCountry = new HashMap<>();   
private HashMap<String, Integer> groups = new HashMap<>();

下面是我初始化 HashMap 的地方。

public class CMBSetter{ 
private HashMap<Integer, Clients> clients = new HashMap<>();
private HashMap<Integer, ClientsByMonth> clientsBM = new HashMap<>();

 public void preSetterList(){
    // ----     --- COUNTRY SETTER ---      ----
     HashMap<String, Integer> byCountry = new HashMap();

    String[] countrys = {"GB ", "PT ", "ES ", "BE ", "IE ", "FR ", "DE ", "CH ", "IR ", "NL ", "   ", "Others"};
    for(int i = 0; i < 12; i++){
        byCountry.put(countrys[i], 0);

    }
    //  ****    *** GROUPS SETTER ***   ****
    HashMap<String, Integer> groups = new HashMap<>();
    Collection<String> keysGroup = groups.keySet();
    groups.put("test", 0);

    Collection<Integer> keysCleint = clients.keySet();

    for(Integer keyC: keysCleint){            
        String groupNameClient = clients.get(keyC).getGroupName();
        boolean namefound = false;

        for(String keyG: keysGroup){
            if(groupNameClient.equals(keyG)){
                namefound = true;
            }  
        }
        if(!namefound){
        groups.put(groupNameClient, 0);
        } 
    }
     //  _)_)_)_   )_)_  DTO SETTER )_)_   _)_)_)_

    for(int i = 0; i < 12; i++){

        clientsBM.put(i, new ClientsByMonth());
        clientsBM.get(i).setByCountry(byCountry[i]);
        clientsBM.get(i).setGroups(groups);
    }
}

我的问题:

如何初始化 HashMap，以便在设置值时不会复制它们？如何初始化 HashMap 而不发生此问题？

我想做的事情:

I.E.2-我想要国家数组来填写我的 DTO ClientsByMonth 中的 byCountry HashMap。如(“GB”，0)和(“IR”，0)和(“DE”，0)。

I.E.2-我希望 Groups setter 迭代客户端 HashMap 并将 GroupName() 下存在的所有名称存储在我的新 HashMap 中，该新 HashMap 具有带有 HashMap 的 DTO 对象。 HashMap groups(BIT, 0) 和 (BOOKING, 0) 和 (TRVLFAR, 0) 等值。

我首先在 HashMap 中创建(预设)所有“标签/键”，因为当我尝试迭代空的 HashMap 时，我总是遇到空指针错误。

Answer 1

解决方案来自评论

乔普·埃根

One beginner's pitfall in doing something like clientsBM.get(i).setGroups(groups); is that you now are sharing the object held by groups. Any change afterwards to groups will hold for all clientsBM.get(i)

heniv181

"I am first creating (presetting) all the "labels/keys" in the HashMap because I am always getting Null pointer errors when I try to iterate over hash map that is empty". Use an Iterator from the keySet to avoid hitting null.