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equals, hashCode Q

 
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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?


Select 1 correct option.
a This is an invalid implementation of hashCode() method with respect to the given equals() method.


b Only one of the Info objects will be stored in the HashMap.


c Both the objects will be stored in the HashMap.


d An exception will be thrown at run time at line //2.

Answer: a

I can't understand the reason for the output. Plz explain.
 
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It is possible for two Info objects to be equal under equals() but have different results under hashCode(). This violates the contract for hashCode().

If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.


Consider:
j1 = new Info( "aaa", "bbb", "ccc" );
j2 = new Info( "aa", "abbb", "ccc" );
 
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Too Good Mike
 
With a little knowledge, a cast iron skillet is non-stick and lasts a lifetime.
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