Question

Suppose I have the below class.

class S{

String txt = null;

S(String i){
txt=i;
}

public static void main(String args []){
S s1 = new S("a");
S s2 = new S("b");
S s3 = new S("a"); 

Map m = new HashMap ();
m.put(s1, "v11");
m.put(s2, "v22");
m.put(s3, "v33");

System.out.println(m.size());   
}

//just a plain implementation
 public boolean equals(Object o)
 {
      S cc = (S) o;
      if (this.i.equals(cc.i))
      {    
         return true;
      }
      else
      {
         return false;
      }    
   }

   public int hashCode()
   {
      return 222;
   }  

}

This will return size as 2 when running above. Its totally fine. If we comment the hashCode() it return 3 which is also correct. But if we comment the equals and keep the hashCode it should return 2 right? instead it returns 3. When putting objects to hashmap map will check the hash code of an object and if its same it will replace the previous value of the map to the new one right?

Thank You.

Answer 1

But if we comment the equals and keep the hashCode it should return 2
right? instead it returns 3.

3 items is the correct behaviour. 3 objects will be hashed to the same bucket, but because all 3 are different this bucket will contain a chain of values (linked list for HashMap in Java) with the same hash code but not equal to each other.

When putting objects to hashmap map will check the hash code of an object and if its same
it will replace the previous value of the map to the new one right?

If they are hashed to the same bucket it doesn’t mean that one value will replace another. Then these values will be compared for equality. If they are equal then old value will be replaced, if they are not – new value will be added to the tail of the linked list (for this bucket).