please explain the output?

[Sonali Sehgal]

This is an enthuware question.

Consider the following class:

 

import java.util.*;
public class Info
{
	String s1, s2, s3;
	public Info(String a, String b, String c)
	{
		s1=a; s2=b; s3=c;
	}
	public boolean equals(Object obj)
	{
		if(! (obj instanceof Info) ) return false;
		Info i = (Info) obj;
		return (s1+s2+s3).equals(i.s1+i.s2+i.s3);
	}
	public int hashCode()
	{
		return s1.hashCode();
	}
	public static void main(String[] args)
	{
		HashMap map = new HashMap();
		Info i1 = new Info("aaa", "aaa", "aaa");
		Info i2 = new Info("aaa", "bbb", "ccc");
		map.put(i1, "hello"); //1
		map.put(i2, "world"); //2
	}
}


   

Which of the following statements are correct regarding the above class?
Option 1: This is an invalid implementation of hashCode() method with respect to the given equals() method.
Option 2: Only one of the Info objects will be stored in the HashMap.
Option 3:Both the objects will be stored in the HashMap however retrieving them using the Info objects as keys will not be possible.
Option 4:An exception will be thrown at run time at line //2.


Please explain what is the right answer for this question???

I am not able to understand this code of hashcode???

[Sebastian Janisch]

Option 1 should be the correct answer. Please check the other post for explanation.

[Balraj Momi]

Question 1 is correct is because hashCode() and equals() method should use same instance variables. And in this question, only one String s1 is being used in hashCode() method. This implementation will not fail, It will still work. But It is not in line with equals method.

Option 2: Wrong - Both object will be stored because both objects are not equal.
Option 3: Wrong - It will be possible to retrieve values from this map by using either of these objects as Key
Option 4: Wrong - No Exception thrown

Regards
Balraj

[shivendra tripathi]

The equal contract says, if equals() return for two object then hashCode() for these two object must be same. In the current implementation suppose I have two objects with filed values "aa" , "bb", "cc" and "bb", "aa", "cc". For this example equals() will return true but hashCode for these two objects won't be same.
I guess by now you must have got your ans. You can use a thumb rule that in proper hashCode implementation you should use all the information used in your equals() method implementation.

[Jyoti B.Shah]

Shivendra,

equals method will return false for two objects with filed values "aa" , "bb", "cc" and "bb", "aa", "cc" and hashcode for them will return different values, which is perfectly legal. I am not sure how the implementation of hashcode() method is invalid. It would be great if someone could provide a example where equals() will return true but hascode() for those two objects will return different values.

Output of following code returns -

false
hello
world

  import java.util.*;  
 public class Info  
 {  
     String s1, s2, s3;  
     public Info(String a, String b, String c)  
     {  
         s1=a; s2=b; s3=c;  
     }  
     public boolean equals(Object obj)  
     {  
         if(! (obj instanceof Info) ) return false;  
         Info i = (Info) obj;  
         return (s1+s2+s3).equals(i.s1+i.s2+i.s3);  
     }  
     public int hashCode()  
     {  
         return s1.hashCode();  
     }  
     public static void main(String[] args)  
     {  
         HashMap map = new HashMap();  
         Info i1 = new Info("aa", "bb", "cc");  
         Info i2 = new Info("bb", "aa", "cc");  
	 System.out.println(i1.equals(i2));
         map.put(i1, "hello"); //1  
         map.put(i2, "world"); //2 
	 System.out.println(map.get(i1));
	  System.out.println(map.get(i2));
     }  
 }   

[Jyoti B.Shah]

Got answer to my question. If we have two info objects as {"aa","bb","cc"} and {"a","abb","cc"}, although they will be equal(s1+s2+s3 is same for both objects) but their hascode will be different which voilates the hascode() contract.

[Sebastian Janisch]

Watch out with the example you just came up with.

{"aa","bb","cc"}, {"a","abb","cc"} and your implementation of equals means that two objects that are actually meaningfully different will pass the equals test. You should put some sort of separator into the check, like this..

  
        public boolean equals(Object obj)
	{
		if(! (obj instanceof Info) ) return false;
		Info i = (Info) obj;
		return (s1+"|"+s2+"|"+s3).equals(s1+"|"+s2+"|"+s3);
	}

  

or write different if statements that check for each variable on its own ...

[shivendra tripathi]

Hi Jyothi,

Good catch, I wanted to explain the scenario you mentioned "aa" , "bb", "cc" and "a", "abb", "cc" but somehow i put it in wrong way...

[Sonali Sehgal]

Hi,

Thanks to all of you for explaining me this topic of hashcode I think I am able to understand it better now.