Understanding equals() with hashcode()

[jaffar hussain]

I am pasting you two code snippets.

1. Implementing hashcode() and equals() with HashSet:

package others;

import java.util.HashSet;
import java.util.Iterator;

public class HashCodeTest_HashSet
{
private String name;

public HashCodeTest_HashSet(String name)
{
this.name = name;

}

public boolean equals ( Object o )
{
if ( !(o instanceof HashCodeTest_HashSet) ) {
return false;
}

if ( name == null ) {
return false;
}

HashCodeTest_HashSet obj = (HashCodeTest_HashSet) o;
return name.equals(obj.name);
}

public int hashCode()
{
return name.hashCode();
}

public String toString()
{
return name;
}

public static void main(String args[])
{

HashCodeTest_HashSet obj1 = new HashCodeTest_HashSet("ABC");
HashCodeTest_HashSet obj2 = new HashCodeTest_HashSet("PQR");
HashCodeTest_HashSet obj3 = new HashCodeTest_HashSet("ABC");

HashSet<HashCodeTest_HashSet> hs = new HashSet<HashCodeTest_HashSet>();

hs.add(obj1);
System.out.println(obj1.hashCode());

hs.add(obj2);
System.out.println(obj2.hashCode());

hs.add(obj3);
System.out.println(obj3.hashCode());

Iterator<HashCodeTest_HashSet> itr = hs.iterator();

while(itr.hasNext())
System.out.println("Elements are : "+itr.next());

}
}

2. Implementing hashcode() and equals() with HashMap:

package others;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map.Entry;
import java.util.Set;



public class HashCodeTest_HashMap
{

private String name;

public HashCodeTest_HashMap(String name)
{

this.name = name;

}

public boolean equals ( Object o )
{
if ( !(o instanceof HashCodeTest_HashMap) ) {
return false;
}

if ( name == null ) {
return false;
}

HashCodeTest_HashMap obj = (HashCodeTest_HashMap) o;
return name.equals(obj.name);
}

public int hashCode()
{
return name.hashCode();
}

public String toString()
{
return name;
}

public static void main(String args[])
{

HashCodeTest_HashMap obj1 = new HashCodeTest_HashMap("ABC");
HashCodeTest_HashMap obj2 = new HashCodeTest_HashMap("PQR");
HashCodeTest_HashMap obj3 = new HashCodeTest_HashMap("XYZ");

HashMap<Integer, HashCodeTest_HashMap> hm = new HashMap<Integer, HashCodeTest_HashMap>();

hm.put(1,obj1);
System.out.println(obj1.hashCode());

hm.put(2,obj2);
System.out.println(obj2.hashCode());

hm.put(1,obj3);
System.out.println(obj3.hashCode());

Set<Entry<Integer, HashCodeTest_HashMap>> set = hm.entrySet(); //doubt

Iterator<Entry<Integer, HashCodeTest_HashMap>> itr = set.iterator();//doubt

while(itr.hasNext())
System.out.println("Elements are : "+itr.next());

System.out.println("For Key 2 Value is : "+hm.get(2));
}
}
// other methods

My Question is :

While debugging with HashSet line by line I am able to find the flow entering into equals method when its get matched with two equal hashcodes.

But while I am debugging with HashMap line by line I am not able to find the flow entering into equals method when its get matched with two equal hashcodes.


So I am unable to understand how and when exactly equals gets called and why its not getting called with HashMap please somebody explain me this?

Or any link or reference to understand this...

Thanks in advance...

[Matthew Brown]

Using HashMap, it's the keys that are compared, not the values. It's quite happy to have lots of entries with the same value, as long as the keys are unique. And you're using a HashMap<Integer, HashCodeTest_HashMap>.

So in your second code the HashMap doesn't care about the equals() and hashCode() methods of HashCodeTest_HashMap, as your keys are all Integers.

[jaffar hussain]

Matthew Brown wrote:Using HashMap, it's the keys that are compared, not the values. It's quite happy to have lots of entries with the same value, as long as the keys are unique. And you're using a HashMap<Integer, HashCodeTest_HashMap>.

So in your second code the HashMap doesn't care about the equals() and hashCode() methods of HashCodeTest_HashMap, as your keys are all Integers.

Thank you I tried obj1,obj2,obj3 as keys it worked equals got called thank you once again

[Matthew Brown]

You're welcome!