ConcurrentHashMap - 奇怪的行为

Question

Can anyone let me know what goes wrong in this piece of code? I'm pulling my hair out!

There isn't any problem if I use HashMap instead of ConcurrentHashMap. The code is compiled with JDK 5.0

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;
      }
   }
}

This is the output:

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 method implementation is incorrect, due to that testMap.get(key) always returns null. Try this

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 also needs a little change to be consistent with equals

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

Answer 2

The problem comes from your equals in DummyKey .

When you call DummyValue val = testMap.get(key); , the hashcode function finds a match (both keyname of k1 and key are the same and so are their hashcode). Yet equals returns false because k1.keyname is equal to "A" which is not equal to key itself, which is actually of type DummyValue : you are not comparing properly!

Therefore, you need to modify your equals function:

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    DummyKey other = (DummyKey) obj;
    if (keyName == null) {
        if (other.keyName != null)
            return false;
    } else if (!keyName.equals(other.keyName))
        return false;
    return true;
}

Answer 3

Please note that if you change hashCode(), then you must change equals() as well. Otherwise, you will run into problems. If equals() returns true for two items, then their hashCode() value must be equal! The opposite is not required but preferable for better hashing performance. Here is an implementation of equals() and hashCode().

HINT: if you are using eclipse, you can utilize its source generation capability to create the correct hashCode() and equals() method for you. The only thing you need to do is to pick the instance variables that identify the object. To do so in eclipse, while your source code is open, go to the tabs in the top and choose "source", then choose "Generate hashCode() and equals()..."

  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((keyName == null) ? 0 : keyName.hashCode());

    return result;
  }

  Override
  public boolean equals(Object other) {
     if(this == other) return true; //for optimization
     if(! other instanceof this) return false; //also covers for when other == null
     return this.keyName == null ? other.keyName == null : this.keyName.equals(other.keyName);
  }

Answer 4

As others have pointed, the problem lies in the way you override hashcode and equals. Two options : 1) Just remove the hashcode and equals and it works fine 2) I let eclipse generate the source for hashcode and equals and it works fine. This is what my eclipse belted out for me :

@Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result
                + ((keyName == null) ? 0 : keyName.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        DummyKey other = (DummyKey) obj;
        if (keyName == null) {
            if (other.keyName != null)
                return false;
        } else if (!keyName.equals(other.keyName))
            return false;
        return true;
    }