java - 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;
}