Sorting Keys(Value Objects) in HashMap

[Aman Mohd]

Hi,

I have one HashMap with Student Objects. And i want to sort this map with student id, student name or any other field in student value object. How can I achieve this?

Student is a POJO Class with student id, student name, student age and some other fields with their respective setters and getters.

My requirement is such that I have to sort the hashmap based on any field of the Student class. This student class is the key in the hashmap

E.g.

           Student student1 =  new Student();
		student1.setstudentId(11);
		student1.setName("Dave");
                student1.setAge(19);
		
		Student student2 =  new Student();
		student2.setstudentId(22);
		student2.setName("Chappell");
                student2.setAge(20);

                HashMap hm = new HashMap();
                //Inserted keys as the Value objects of Student
                hm.put(student1, null);
		hm.put(student2, null);

        	int i = student1.compareTo(student2);

I have made my Student class implement Comparable interface and overidden the compareTo method. In this i have just compared the Ids of the students.

                public int compareTo(Object obj) {

			Student comparePlayer = (Student)obj;
	
			
			if (this.getstudentId() > comparePlayer.getstudentId()) {
				return -1;
			} else if (this.getstudentId() < comparePlayer.getstudentId()) {
				return 1;
			} else {
				return 0;
			}
		}

I want to sort this map using the student name field or the student age field or any other field in the student object. How can i accomplish this? This HashMap will have hundreds of Student objects. All of them have to be sorted by any sort parameter which would be stored in the key part of the hashmap.


Thanks.

Amaan

[Rob Spoor]

Please UseCodeTags. You can use the edit button to add them.

A HashMap is always unsorted. If you need sorting check out TreeMap.

[Aman Mohd]

Well i know that... i guess i cudnt explain wat i needed... i need to sort the hashmap key attributes.

[salvin francis]

Maybe you could research on TreeMap instead of HashMap..

Hope that helps.

[salvin francis]

by the way there is a difference between sorting based on keys and sorting based on values.

[Jesper de Jong]

Aman Mohd wrote:Well i know that... i guess i cudnt explain wat i needed... i need to sort the hashmap key attributes.

You could do this: (1) put all the keys of the HashMap in a List, (2) sort the list however you like.

As Rob says, a HashMap is unordered - there is no way that you can put the keys into the HashMap itself in a particular order. Think of a HashMap as a bag with key-value pairs which are not in any particular order.

[salvin francis]

Jesper Young wrote:

Aman Mohd wrote:Well i know that... i guess i cudnt explain wat i needed... i need to sort the hashmap key attributes.

You could do this: (1) put all the keys of the HashMap in a List, (2) sort the list however you like.

As Rob says, a HashMap is unordered - there is no way that you can put the keys into the HashMap itself in a particular order. Think of a HashMap as a bag with key-value pairs which are not in any particular order.

@Jesper,
how about the TreeMap solution ? (Besides the fact that the OP actually mentioned HashMap)

[Jesper de Jong]

You could also do it with a TreeMap. A TreeMap orders its elements either by the "natural ordering" of the elements (in that case, the element class needs to implement the Comparable interface), or you can use the constructor of TreeMap that takes a Comparator object that determines the ordering.

But the original poster specifically asked about ordering a HashMap - that you can't do, because a HashMap isn't an ordered collection.