Java Hashset.contains() produces mysterious result

Question

I don\'t usually code in Java, but recently I started not having a choice. I might have some major misunderstanding of how to properly use HashSet. So it might be possible s

Answer 1

When you are adding objects to a set it internally calls equals and hashCode methods. You have to override these two methods. For example I have taken one bean class with name,id,designation, then created and added an employee object.

HashSet set = new HashSet();
Employee employee = new Employee();
employee.setId(1);
employee.setName("jagadeesh");
employee.setDesignation("programmer");
set.add(employee);
Employee employee2 = new Employee();
employee2.setId(1);
employee2.setName("jagadeesh");
employee2.setDesignation("programmer");
set.add(employee2);

set.add() calls internally the equals and hashCode methods. So you have to override these two methods in your bean class.

@Override
public int hashCode(){
    StringBuffer buffer = new StringBuffer();
    buffer.append(this.name);
    buffer.append(this.id);
    buffer.append(this.designation);
    return buffer.toString().hashCode();
}
@Override
public boolean equals(Object object){
    if (object == null) return false;
    if (object == this) return true;
    if (this.getClass() != object.getClass())return false;
    Employee employee = (Employee)object;
    if(this.hashCode()== employee.hashCode())return true;
   return false;
}   

Here we are overriding equals() and hashCode(). When you add an object to the HashSet method it internally iterates all objects and calls the equals method. Hence we overrid hashCode, it compares every objects hashCode with its current hashCode and returns true if both are equal, else it returns false.