What happens if we override only hashCode() in a class and use it in a Set?

Question

This may not be the real world scenario but just curious to know what happens, below is the code.

I am creating a set of object of class UsingSet. According to hashing concept in Java, when I first add object which contains "a", it will create a bucket with hashcode 97 and put the object inside it. Again when it encounters an object with "a", it will call the overridden hashcode method in the class UsingSet and it will get hashcode 97 so what is next?

As I have not overridden equals method, the default implementation will return false. So where will be the Object with value "a" be kept, in the same bucket where the previous object with hashcode 97 kept? or will it create new bucket? anybody know how it will be stored internally?

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

class UsingSet {  

  String value;  

  public UsingSet(String value){  
    this.value = value;  
  }  

  public String toString() {  
    return value;  
  }  

  public int hashCode() {  
    int hash = value.hashCode();  
    System.out.println("hashcode called" + hash);  
    return hash;  
  }  

  public static void main(String args[]) {  

    java.util.Set s = new java.util.HashSet();  

    s.add(new UsingSet("A"));  
    s.add(new UsingSet("b"));  
    s.add(new UsingSet("a"));  
    s.add(new UsingSet("b"));   
    s.add(new UsingSet("a"));  

    s.add(new Integer(1));  
    s.add(new Integer(1));  

    System.out.println("s = " + s); 

  }  
}  

output is:

hashcode called65
hashcode called98
hashcode called97
hashcode called98
hashcode called97
s = [1, b, b, A, a, a]

Answer 1

HashCode & Equals methods

1. Only Override HashCode, Use the default Equals: Only the references to the same object will return true. In other words, those objects you expected to be equal will not be equal by calling the equals method.
2. Only Override Equals, Use the default HashCode: There might be duplicates in the HashMap or HashSet. We write the equals method and expect{"abc", "ABC"} to be equals. However, when using a HashMap, they might appear in different buckets, thus the contains() method will not detect them each other.

Answer 2

James Large answer is incorrect, or rather misleading (and part incorrect as well). I will explain.

If two objects are equal according to their equals() method, they must also have the same hash code. If two objects have the same hash code, they do NOT have to be equal too.

Here is the actual wording from the java.util.Object documentation:

• 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.
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

It is true, that if two objects don't have the same hash then they are not equal. However, hashing is not a way to check equality - so it is wildly incorrect to say that it is a faster way to check equality.

Also, it is also wildly incorrect to say the hashCode function is an efficient way to do anything. This is all up to implementation, but the default implementation for hashCode of a string is very inefficient as the String gets large. It will perform a calculation based on each char of the String, so if you are using large Strings as keys, then this becomes very inefficient; moreso if you have a large number of buckets.

In a Map (HashSet uses a HashMap internally), there are buckets and in each bucket is a linked list. Java uses the hashCode() function to find out which bucket it belongs in (it actually will modify the hash, depending on how many buckets exist). Since two objects may share the same hash, it will iterate through the linked list sequentially next, checking the equals() method to see if the object is a duplicate. Per the java.util.Set documenation:

A collection that contains no duplicate elements.

So, if its hashCode() leads it to a bucket, in which that bucket contains an Object where the .equals() evaluates to true, then the previous Object is overwritten with the new Object. You can probably view here for more information: How does a Java HashMap handle different objects with the same hash code?

Generally speaking though, it is good practice that if you overwrite the hashCode function, you also overwrite the equals function (if I'm not mistaken, this breaks the contract if you choose not to).