I was trying to answer this question in the forum and I found that despite of overriding the equals method in the Employee class, I am still able to add duplicate elements to the TreeSet.
The Javadoc of TreeSet.add(E) method says
Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if the set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.
This essentially means that no 2 equals objects will be inserted into TreeSet and equality is determined solely by equals() method of contained objects.
However, below code is adding 2 elements to the Set even though they are equal
public class Employee implements Comparable<Employee> {
String employeeName;
int employeeId;
public Employee(String name, int id) {
this.employeeName = name;
this.employeeId = id;
}
public int compareTo(Employee emp) {
//return this.employeeName.compareTo(emp.employeeName);
return (this.employeeId - emp.employeeId);
}
@Override
public String toString() {
return ("Name is: " + employeeName + " Emp id is: " + employeeId);
}
@Override
public boolean equals(Object emp) {
if(emp instanceof Employee &&((Employee)emp).employeeName.equals(this.employeeName)){
return true;
}
return false;
}
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Set<Employee> set = new TreeSet<Employee>();
Employee e1 = new Employee("A", 1);
Employee e2 = new Employee("A", 2);
System.out.println(e1.equals(e2));
set.add(e1);
set.add(e2);
System.out.println(set);
}
}
And here is the output
true
[Name is: A Emp id is: 1, Name is: A Emp id is: 2]
Why is TreeSet allowing multiple elements even if they are equal?
Now I changed the compareTo method of Employee like this
public int compareTo(Employee emp) {
return this.employeeName.compareTo(emp.employeeName);
}
And the output is
true
[Name is: A Emp id is: 1]
How the TreeSet working properly after overriding compareTo ?
