Problem with Hashtable key.

Hi all,

I need a hashtable with multi-dimensional keys (e.g. 3 keys per value).

My test harness looks like this:

My HashIndex class for 1 key-, 2 keys- and 3 keys-per-value indexing:

Code:

---

public class HashIndex extends Object
{
  public HashIndex(Object key1) {
    m_keys.add(key1);
  }

  public HashIndex(Object key1, Object key2) {
    m_keys.add(key1);
    m_keys.add(key2);
  }

  public HashIndex(Object key1, Object key2, Object key3) {
    m_keys.add(key1);
    m_keys.add(key2);
    m_keys.add(key3);
  }

  public boolean equals(HashIndex idx)
  {
          System.out.println("got here");
          return m_keys.equals(idx.m_keys);
  }

  protected Vector<Object> m_keys = new Vector<Object>();
}

---

And an example of the HashIndex in use:

Code:

---

Hashtable<HashIndex, String> ht = new Hashtable<HashIndex, String>();
ht.put(new HashIndex("a", "b", "c"), new String("The quick brown fox jumps over the lazy dog."));
System.out.println(ht.get(new HashIndex("a", "b", "c")));

---

I would expect the output from this to be:
got here
true

But what I am actually getting is just:
null

Therefore, the Hashtable is not using the HashIndex's equals method.

Any ideas? :confused::confused::confused:

You need to override the hashcode and equals methods in your HashIndex class.

Hmmmm ....

Ok, I've put a method that looks like this into the HashIndex class, just to test it:

Code:

---

public int hashCode()
{
  return 1;
}

---

I think that should make the hashtable think that all HashIndexes are equal.

But ... I still get the same result as before!

You didn't override anything, you created an overloaded equals( ... ) method that never is called. The signature of the equals( ... ) method has to be:

Code:

---

public boolean equals(Object that) { ... }

---

kind regards,

Jos

Ah! Now it works. Many thanks!

Isn't HashTable legacy code?

Quote:

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Originally Posted by al_Marshy_1981

Isn't HashTable legacy code?

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Doesn't appear to be.

Quote:

---

Originally Posted by al_Marshy_1981

Isn't HashTable legacy code?

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Nope, a Hashtable (lowercase tee) is to a HashMap as a Vector is to an ArrayList; the older classes have synchronized methods and are retrofitted to match the Collections framework. StringBuffer and StringBuilder have a similar relationship.

kind regards,

Jos