Here's the Main Class called Question :
package com.test;
import java.util.HashSet;
public class Question {
public static void main(String[] args) {
HashSet uniqueObjects = new HashSet();
TestObject obj1= new TestObject();
obj1.setId(1);
TestObject obj2= new TestObject();
obj2.setId(1);
uniqueObjects.add(obj1);
uniqueObjects.add(obj2);
System.out.println(uniqueObjects.size());
}
}
Here's The TestObject Class :
package com.test;
public class TestObject {
private int id ;
@Override
public boolean equals(Object other)
{
if (other == null)
return false;
if (other == this)
return true;
if (!(other instanceof TestObject))
return false;
TestObject otherObject = (TestObject)other;
if(this.id == otherObject.getId())
return true;
return false;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
}
The program storing unique objects in the hash set , based on the id .
Can you figure out what is wrong with this program and why ?
Solution :
The problem with this code is , hash set is supposed to store only unique values by using equals method which we have already overridden , but it does not work as expected , so even if you enter two object with same id , size of hash set come as 2 instead of 1 .
The reason behind this is , to find out two object are equal or not , hash set first compares the hash code of two objects . If the hash codes are not equal , objects will not be considered equal , and it will not even use the equals method . In our case we have not overridden the hashCode method , so it will use the default hashcode method of object class , which returns memory address of objects in hex , since these two are different objects , so hashCode value will not be matched , even if equals method returns true . So we need to implement hashcode method also .
