HashSet

[Akhil Maharaj]

All ,

My understanding is that we can not add duplicate objects to HashSets.
In the following code Dog class is provided with equals and hashcode methods so that duplicate dogs should not get added to HashSet . But I am able to add equal objects to HashSet ? What is that I am missing ?

 import java.util.* ;
class myHashTest{
 

public static void main(String[] args) {
 

Dog d1 = new Dog(10,100);
Dog d2 = new Dog(20,200);
Dog d3 = new Dog(10,300);
 

System.out.println(d1.equals(d2)+"  "+d1.equals(d3));
System.out.println(d1.hashcode()+"  "+d2.hashcode()+"   "+d3.hashcode());
 
HashSet<Dog> hs = new HashSet<Dog>();
 
LinkedHashSet<Dog> lhs = new LinkedHashSet<Dog>();
 
LinkedHashSet<String> lhs2 = new LinkedHashSet<String>();
 
System.out.println(" ");
 
System.out.println(hs.add(d1));
System.out.println(hs.add(d2));
System.out.println(hs.add(d3));
System.out.println(hs.size()+"  "+hs);
 
System.out.println(" ");
 
System.out.println(lhs.add(d1));
System.out.println(lhs.add(d2));
System.out.println(lhs.add(d3));
System.out.println(lhs.size()+"   "+lhs);
 
System.out.println(" ");
 
System.out.println(lhs2.add("Hello"));
System.out.println(lhs2.add("ABC"));
System.out.println(lhs2.add("ABC"));
System.out.println(lhs2.size()+"   "+lhs2);
 
System.out.println(" ");
 

}
 

}
 
class Dog{
 
int h;
int w;
 
Dog(int x, int y) {   h=x;w=y;}
Dog() {  }
 
public boolean equals(Object d) {
return  d instanceof Dog ?(this.h==((Dog)d).h) :false ;
}
 
public int hashcode() { return h;}
 

 

 
}   

[Keith Lynn]

Be careful to have the right capitalization in the name of the hashCode method.

[Akhil Maharaj]

Keith ,

Thanks for pointing out the mistake on hashCode / hashcode .
Because of the typo ,Effectively in my code there was no hashCode .
But there was equals method.

In such scenario , How the duplicates are identified . Are the duplicates identified based on equals method or based on default hashCode method ?

[ahmed yehia]

The Contract says: "If the equals() method returns true, the hashCode() of the two objects must be the same"

Two objects are considered duplicates if they return true using equals() And have the same hashCode.