Why is equals() not called while adding to HashSet and hashCode matches?

Question

When I run this code why only hashCode() is called not equals method while my hashCode() implementation generate same hashCode for both entries to HashSet?

import java.util.HashSet;  public class Test1 {     public static void main(String[] args) {         Student st=new Student(89);         HashSet st1=new HashSet();         st1.add(st);         st1.add(st);         System.out.println("Ho size="+st1.size());     } } class Student{     private int name;     private int ID;     public Student(int iD) {         super();         this.ID = iD;     }     @Override     public int hashCode() {         System.out.println("Hello-hashcode");         return ID;     }     @Override     public boolean equals(Object obj) {         System.out.println("Hello-equals");         if(obj instanceof Student){             if(this.ID==((Student)obj).ID){                 return true;             }             else{                 return false;             }         }         return false;       } } 

The output for this is:

Hello-hashcode Hello-hashcode Ho size=1 

Answer 1

The hash set checks reference equality first, and if that passes, it skips the .equals call. This is an optimization and works because the contract of equals specifies that if a == b then a.equals(b).

I attached the source code below, with this check highlighted.

If you instead add two equal elements that are not the same reference, you get the effect you were expecting:

    HashSet st1=new HashSet();     st1.add(new Student(89));     st1.add(new Student(89));     System.out.println("Ho size="+st1.size()); 

results in

$ java Test1 Hello-hashcode Hello-hashcode Hello-equals Ho size=1 

---

Here's the source code from OpenJDK 7, with equality optimization indicated (from HashMap, the underlying implementation of HashSet):

public V put(K key, V value) {     if (key == null)         return putForNullKey(value);     int hash = hash(key.hashCode());     int i = indexFor(hash, table.length);     for (Entry<K,V> e = table[i]; e != null; e = e.next) {         Object k; //                                         v-- HERE         if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {             V oldValue = e.value;             e.value = value;             e.recordAccess(this);             return oldValue;         }     }      modCount++;     addEntry(hash, key, value, i);     return null; }