自定义通用类作为哈希映射问题的关键

Question

I have this following test code:

public static final String[] list = {
    "apple","ball","cat","dog","egg","fan","girl","hat","igloo","jerk"  
};

...

HashMap<DoubleKey<Integer, Integer>, String> hm = new HashMap<DoubleKey<Integer, Integer>, String>();
Set<DoubleKey<Integer, Integer>> s = new TreeSet<DoubleKey<Integer, Integer>>();
Random g = new Random();
for(int i=0; i<10; i++){
    int first = g.nextInt(9999) + 1000; 
    int second = g.nextInt(9999) + 1000; 
    DoubleKey<Integer, Integer> k1 = new DoubleKey<Integer, Integer>(first, second);
    DoubleKey<Integer, Integer> k2 = new DoubleKey<Integer, Integer>(first, second);
    s.add(k1);
    hm.put(k2, list[i]);
}

Set<DoubleKey<Integer, Integer>> ts = hm.keySet();
Iterator<DoubleKey<Integer, Integer>> itr = ts.iterator();
while(itr.hasNext()){
    DoubleKey<Integer, Integer> k = itr.next(); 
    System.out.println(k.getFirstKey().toString() + " + " + k.getSecondKey().toString() + " -> " + hm.get(k).toString());
}

System.out.println("----");
Iterator<DoubleKey<Integer, Integer>> sItr = s.iterator();
while(sItr.hasNext()){
    DoubleKey<Integer, Integer> k = sItr.next();
    String currStr = hm.get(k);
    System.out.println(k.getFirstKey().toString() + " + " + k.getSecondKey().toString() + " -> " + currStr);
}

What I did is to create a Custom Generic Class DoubleKey<K, J> to contain a key having two parts. As you can see, the Set s and the keys of HashMap hm are have the same components, but was instantiated differently ( k1 = k2 ). When I try to get a value using the keys on s to hm , it returns null , though at the first printing it shows the correct mapping.

Sample Output:

3922 + 2544 -> girl
9267 + 3750 -> hat
3107 + 10929 -> apple
5162 + 8834 -> fan
8786 + 1125 -> cat
10650 + 4078 -> egg
3808 + 7363 -> jerk
1364 + 7657 -> dog
1364 + 4412 -> ball
1583 + 1460 -> igloo
----
10650 + 4078 -> null
1364 + 4412 -> null
1364 + 7657 -> null
1583 + 1460 -> null
3107 + 10929 -> null
3808 + 7363 -> null
3922 + 2544 -> null
5162 + 8834 -> null
8786 + 1125 -> null
9267 + 3750 -> null

This is my DoubleKey implemention:

public class DoubleKey<K extends Comparable<K>,J extends Comparable<J>> implements Comparable<DoubleKey<K,J>>{

    private K key1;
    private J key2;

    public DoubleKey(K key1, J key2){
        this.key1 = key1;
        this.key2 = key2;
    } 

    public K getFirstKey(){
        return this.key1;
    }

    public J getSecondKey(){
        return this.key2;
    }

    // need for Comparable interface
    public int compareTo(DoubleKey<K,J> aThat){
        // NOTE: check for nulls
        return (this.key1.toString() + this.key2.toString()).compareTo(aThat.key1.toString() + aThat.key2.toString());
    }

    public boolean equals(DoubleKey<K,J> aThat){
        return (this.key1.toString() + this.key2.toString()).equals(aThat.key1.toString() + aThat.key2.toString());
    }

}

How did it happened? Can two objecst (in this case from a custom generic) be different eve3n if they have instantiated with 2 same values? How can I correct this? I hope someone can help me here. Thanks!

Answer 1

Additionally to .hashCode() , you should have an implementation of equals(Object) , not (only) equals(DoubleKey<...>) , since otherwise you'll have two independent methods here (and only the first one is actually called by the HashMap). Here is a proposal:

public boolean equals(Object other) {
    if(this == other)
       return true;
    if(!(other instanceof DoubleKey))
       return false;
    DoubleKey that = (DoubleKey)other;
    return (this.key1 == null ? that.key1 == null : this.key1.equals(that.key1)) &&
           (this.key2 == null ? that.key2 == null : this.key2.equals(that.key2));
}

The hashCode method should be made to fit this, too, for example like this:

public int hashCode() {
    return key1.hashCode() * 3 + key2.hashCode() * 5;
}

Your key1.toString()+key2.toString() comparison is a bit dangerous, as it lets (1, 21).equals((12,1)) be true, which is usually not intended. The same is true for your compareTo method - compare the components using their compareTo method, not the concatenated String.

Answer 2

Learn this lesson now: If you override the equals method (as you have done), then you MUST override the hashcode method too. That method is used for various things, including looking up items in HashMaps.

Answer 3

Where is DoubleKey class's hashCode method override? I don't think that it will work as a proper key unless you implement this because otherwise your two objects will be considered different.