sorting a hashset based on its value

[kalai sundar]

Could not sort a hashset based on its value . In the below code i have hashset
which contains multiple student information .So want to sort the haset based on the student id.
Below code is not working.Can anyone help me.

public  class student{

public  long studentid;
public Long getStudentId() {
		return studentid;
	}

	public void setStudId(Long studentid) {
		this.studentid= studentid;
	}

}


public class ttt {
public static void main(String[] s) {

	Student d = new  Student ();
	d.setStudId(3);
	
	Student d1 = new  Student ();
	d.setStudId(2);
	
	Student d2 = new  Student ();
	d.setStudId(1);
	
     Set ss = new HashSet();
     ss.add(d);
     ss.add(d1);
     ss.add(d2);
     System.out.println("before sorting "+ss);

	 Collections.sort (new java.util.ArrayList(ss), new StudentSorting());
	 System.out.println("after sorting "+ss);
	
}
}





public class StudentSorting implements Comparator<Student >{

	 public int compare(Student o1, Student o2) { 
		 
		 return Long.valueOf(o1.getStudentId()).compareTo(Long.valueOf((o2.getStudentId())));    
		 }

}

[Christophe Verré]

HashSets have no notion of order. You are sorting an ArrayList anonymous instance. This won't affect the HashMap.

(next time, UseCodeTags when you post some code)

[Dinakar Kas]

Use TreeSet and supply the Comparator argument to its constructor. I will solve your problem.

Dinakar

[Dinakar Kas]

sorry. *It* will solve yor problem.

[kalai sundar]

I am not suppose to change its as Treeset .I have to sort the student information
inside the HashSet based on student id.

[Christophe Verré]

kalai sundar wrote:I am not suppose to change its as Treeset .I have to sort the student information
inside the HashSet based on student id.

You can't. You might have misunderstood the requirements of your assignment.

[fred rosenberger]

The whole point of a hashset is to make it easy/quick to find/insert something. You pass it a key, and a hash value is computed. This is used to determine where to store the actual data.

If you then sort the values inside the hashset, that would move where stuff is. That then means you can't easily find it.

It's kind of like saying "i'm going to put stuff into a sorted list, then randomize it". if you do that, you no longer have a sorted list. If you re-arrange stuff in the hashset, it is no longer a hashset.