CODEWITHSUNDEEP
javadictionary
hellzone
hellzone

Reputation: 5236

How to sort TreeMap without using Arraylist

I want to sort a TreeMap by ascending order wrt "Student.name". I don't want to create a new ArrayList. Is there any way to do this?

Map studentMap = new TreeMap();
studentMap.put(id, student); //id is double

public class Student{
String name;
int number;
....

}

Upvotes: 0

Views: 119

Answers (4)

ioreskovic
ioreskovic

Reputation: 5699

You can do this in two ways:

  1. Since you can't pass a value-based comparator to TreeMap, you can do this:
    TreeMap sort by value

  2. Making Student implement Comparable interface

    public class Student implements Comparable<Student> {
    @Override
    public int compareTo(Student o) {
        return this.getName().compareTo(o.getName());
    }
}

In first case, you need to call the mentioned method on your collection. In the second one, just add students to your map.

Upvotes: 2

Dmitry Tikhonov
Dmitry Tikhonov

Reputation: 301

TreeMap is sorting its entries according to keys, not values. You cannot provide a Comparator<Student> since you have Double keys.

If you really need a map from Double to Student you cannot sort it in the way you want. If not consider using a TreeSet<Student> with a Comparator<Student> or Comparable<Student>.

Upvotes: 0

Deepak
Deepak

Reputation: 2895

if you are using TreeMap then the record will be already sorted .You need to implement the Comparable interface in Student class and have to override the compareTo method 

Student implements Comparable {

  // override compareTo wrt name
  @Override
  public int compareTo(Student otherStudent) {
     return name.compareTo(otherStudent.name);
  }
}

Upvotes: 1

Rahul
Rahul

Reputation: 45060

You can use a custom comparator for that while creating the TreeMap.

Map studentMap = new TreeMap(new Comparator<Student>() {
    @Override
    public int compare(Student o1, Student o2) {
        // Your logic to sort it based on Student name goes here
        return 0;
    }
});

Upvotes: 1

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