containsKey() question

[Thomas Kennedy]

Given

class Klingon{
private int r;
Klingon(int r){this.r=r;}
public int rank(){return this.r;}
public boolean equals(Object o){
if((o instanceof Klingon)==false){return false;}
if(((Klingon)o).rank() != this.rank()){return false;}
return true;
}
public String toString(){return "Klingon "+ String.valueOf(this.rank());}
}

and this

Hashtable<Klingon,Vogon> h = new Hashtable<Klingon,Vogon>();
Klingon k = new Klingon(5);
h.put(k,new Vogon("Gruntbuggly"));
System.out.println(h.containsKey(k)); // true
System.out.println(h.containsKey(new Klingon(5))); // false

I expected to get true in both cases. Why does the second call to containsKey() return false?

[Henry Wong]

I expected to get true in both cases. Why does the second call to containsKey() return false?

A HashMap, Hashtable, HashSet, etc... basically, all the hashing collections, depends on both the equals() and hashCode() method to determine equality. You forgot to override the hashCode() method.

Henry

[Thomas Kennedy]

Got it, thank you!

[Evelin Priyadarsini]

Originally posted by Thomas Kennedy:
[QB]Given

class Klingon{
Klingon k = new Klingon(5);
System.out.println(h.containsKey(new Klingon(5))); // false

Hi,
i have a doubt, whether instance of Klingon k will be equal to the new instance created i.e(new Klingon(5))
Because the new instance created will reference a different memory right. Can you please clear this one.

Thanks in Advance.
[ June 24, 2008: Message edited by: Evelin Priyadarsini ]

[Rob Spoor]

Originally posted by Evelin Priyadarsini:

i have a doubt, whether instance of Klingon k will be equal to the new instance created i.e(new Klingon(5))
Because the new instance created will reference a different memory right. Can you please clear this one.

Equality is determined by the "public boolean equals(Object)" method. As Thomas has specified in this method that two Klingon objects are equal if their rank is equal, and both have a rank of 5, the objects should be equal.

Of course, if you override equals, you should also override the "public int hashCode()" method, as it is specified in the API for java.lang.Object. And that's exactly what Thomas forgot.