HashMap prob

[Debajit Paul]

import java.util.*;
public class HashTest{
int hashCode;
public HashTest(int hashCode){
this.hashCode = hashCode;
}
public boolean equals(Object other){
if(other instanceof HashTest){
return ((HashTest)other).hashCode == this.hashCode;
}
return false;
}
public static void main(String [] args) {
HashTest ob1 = new HashTest(3);
HashTest ob2 = new HashTest(2);
HashTest ob3 = new HashTest(1);
Map mp = new HashMap();
mp.put(ob1,"one");
mp.put(ob2,"two");
mp.put(ob3,"three");
ob1 = new HashTest(1);
System.out.println(mp.get(ob1));
}
}
a) one
b) two
c) three
d) null
e) Compilation fails


Why the answere is null?

[Ulf Dittmer]

Please QuoteYourSources.

[Debajit Paul]

javabeat generic questions.....1.5

[Ulf Dittmer]

It returns null because the object that is used as key in the "get" method does not exist as key in the HashMap. The equals method plays no part in this (it's never called).

If you comment out the "ob1 = new HashTest(1);" line, then it will return "one".

[Karl Prenton]

The get method evaluates the hashCode before evaluating equals.

If you override hashCode e.g.
public int hashCode() { return (hashCode * 17) ; }

the output would be "three"

good read : http://www.javaranch.com/journal/200407/ScjpTipLine-hashCodesUncovered.html

[Gary Kevin]

you can print out the hashcode of ob1 just like this:

   
 
import java.util.*;
public class HashTest{
int hashCode;
public HashTest(int hashCode){
this.hashCode = hashCode;
}
public boolean equals(Object other){
if(other instanceof HashTest){
return ((HashTest)other).hashCode == this.hashCode;
}
return false;
}
public static void main(String [] args) {
HashTest ob1 = new HashTest(3);
HashTest ob2 = new HashTest(2);
HashTest ob3 = new HashTest(1);
 
System.out.println(ob1.hashCode());//obj1#1
 
Map mp = new HashMap();
mp.put(ob1,"one");
mp.put(ob2,"two");
mp.put(ob3,"three");
ob1 = new HashTest(1);//obj1#2
 
System.out.println(ob1.hashCode());
 
System.out.println(mp.get(ob1));
}
}
 

The hashCode is different from the previous hashCode after you assign a new HashTest object.

"The hashcode value of an object is used by some collection classes.Although you can think of it as kind of an object ID number,it isn't necessarily unique.Collections such as HashMap and HashSet use the hashcode value of an object to determine how the object should be stored in the collection,and the hashcode is used again to help locate the object in the collection."


because you don't override the hashCode() method,obj1#1 and obj1#2 are in differnt "buckets", they are not equal.