Question

我有一张地图，handicapMap，一个整数键，以及一组字符串。地图已经填充了测试数据，我现在希望将各个值添加到字符串集中。我尝试了它，但无法编译它

public class HandicapRecords
{
   private Map<Integer, Set<String>> handicapMap;

   public HandicapRecords()
   {
      handicapMap = new HashMap<>();

   }

   public void handicapMap()
   {
     Set<String> players = new HashSet<>();

     players.add("Michael");
     players.add("Roger"); 
     players.add("Toby");
     handicapMap.put(10, players);

     players = new HashSet<>();
     players.add("Bethany");
     players.add("Martin");
     handicapMap.put(16, players);

     players = new HashSet<>();
     players.add("Megan");
     players.add("Declan");
     handicapMap.put(4, players);
   }

public void addValue(int aKey, String aValue)
{
 handicapMap.put(aKey, players.add(aValue)); \\what I had already tried
 }

Answer 1

Set#add返回boolean，主要取决于该值是否已添加 - 请参阅API。

您想要用来向Map值添加新值的惯用语（即Set<String>），其关键是：

handicapMap.get(aKey).add(aValue);

您可能还应首先检查空Map值：

Set<String> theValue = handicapMap.get(aKey);
if (theValue != null) {
    theValue.add(aValue);
}

Answer 2

试试这个：

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class HandicapRecords {

private Map<Integer, Set<String>> handicapMap;

public HandicapRecords() {
    handicapMap = new HashMap<>();

}
Set<String> players = new HashSet<>();

public void handicapMap() {

    players.add("Michael");
    players.add("Roger");
    players.add("Toby");
    handicapMap.put(10, players);

    players = new HashSet<>();
    players.add("Bethany");
    players.add("Martin");
    handicapMap.put(16, players);

    players = new HashSet<>();
    players.add("Megan");
    players.add("Declan");
    handicapMap.put(4, players);
}

public void addValue(int aKey, String aValue) {
    players.add(aValue);
    handicapMap.put(aKey, players);
}
}

Answer 3

将Set<String> players声明为实例变量或在amethod内部声明，并且不要忘记检查地图是否为null，因为这会导致NPE。

public void addValue(int aKey, String aValue)
{
 Set<String> players = new HashSet<>();
 if(handicapMap!=null){
 handicapMap.put(aKey, players.add(aValue)); 
 }
}

Answer 4

试试这个

   public void addValue(int aKey, String aValue)
{

    if (handicapMap.containsKey(aKey)) {

        handicapMap.get(aKey).add(aValue); // May want to add a check if the key has no values
    }
}

Answer 5

首先，您需要检查是否已存在该密钥的映射。如果是这种情况，您只需要将值添加到现有集合中。否则，您需要先创建集合并将其添加到地图中，然后将新值添加到该集合中。

public void addValue(int aKey, String aValue)
{
    Set<String> players = handicapMap.get(aKey);

    if (players == null) {
        players = new HashSet<>();
        handicapMap.put(aKey, players);
    }

    players.add(aValue);
}

将值添加到地图中的一组字符串

5 个答案: