java - ConcurrentHashMap - 奇怪的行为

Question

谁能告诉我这段代码出了什么问题？我要拔头发了!

如果我使用 HashMap 而不是 ConcurrentHashMap 则没有任何问题。代码使用JDK 5.0编译

public class MapTest {
    public Map<DummyKey, DummyValue> testMap = new ConcurrentHashMap<DummyKey, DummyValue>();

  public MapTest() {
      DummyKey k1 = new DummyKey("A");
      DummyValue v1 = new DummyValue("1");
      DummyKey k2 = new DummyKey("B");
      DummyValue v2 = new DummyValue("2");

      testMap.put(k1, v1);
      testMap.put(k2, v2);
  }

  public void printMap() {
      for(DummyKey key : testMap.keySet()){
          System.out.println(key.getKeyName());
          DummyValue val = testMap.get(key);
          System.out.println(val.getValue());
      }
  }

  public static void main(String[] args){
      MapTest main = new MapTest();
      main.printMap();
  }


  private static class DummyKey {
      private String keyName = "";

      public DummyKey(String keyName){
        this.keyName = keyName;
      }

      public String getKeyName() {
        return keyName;
      }

      @Override
      public int hashCode() {
        return keyName.hashCode();
      }

  @Override
      public boolean equals(Object o) {
         return keyName.equals(o);
      }
  }

  private static class DummyValue {
      private String value = "";

      public DummyValue(String value){
         this.value = value;
      }

      public String getValue() {
        return value;
      }
   }
}

这是输出:

B
Exception in thread "main" java.lang.NullPointerException
at test.MapTest.printMap(MapTest.java:27)
at test.MapTest.main(MapTest.java:34)

Answer 1

DummyKey.equals 方法实现不正确，因为 testMap.get(key) 总是返回 null。试试这个

public boolean equals(Object o) {
    if (o instanceof DummyKey) {
        DummyKey other = (DummyKey) o;
        return keyName == null ? other.keyName == null : keyName.equals(other.keyName);
    }
    return false;
}

hashCode 也需要一点改动来和 equals 保持一致

public int hashCode() {
    return keyName == null ? 0 : keyName.hashCode();
}