Java collection

Comparator

The elements in TreeSet are sorted according to specified Comparator. If no Comparator is specified, elements will be placed according to their natural ascending order.

class MyComparator implements Comparator<Student>

{

    @Override

    public int compare(Student s1, Student s2)
    {
   if(s1.id == s2.id)
        {
            return 0;
        }
        else
        {
            return s2.perc_Of_Marks_Obtained - s1.perc_Of_Marks_Obtained;
        }
    }
}

//Instantiating MyComparator

MyComparator comparator = new MyComparator();
//Creating TreeSet with 'MyComparator' as Comparator.
TreeSet<Student> set = new TreeSet<Student>(comparator);

Order Of Elements	HashSet doesn’t maintain any order of elements.	LinkedHashSet maintains insertion order of elements. i.e elements are placed as they are inserted.	TreeSet orders the elements according to supplied Comparator. If no comparator is supplied, elements will be placed in their natural ascending order.
Performance	HashSet gives better performance than the LinkedHashSet and TreeSet.	The performance of LinkedHashSet is between HashSet and TreeSet. It’s performance is almost similar to HashSet. But slightly in the slower side as it also maintains LinkedList internally to maintain the insertion order of elements.	TreeSet gives less performance than the HashSet and LinkedHashSet as it has to sort the elements after each insertion and removal operations.

Collections is an utility class in java.util package. It consists of only static methods which are used to operate on objects of type Collection.

Collections.max()	This method returns maximum element in the specified collection.
Collections.sort()	This method sorts the specified collection.

Collections.copy()	This method copies all elements from one collection to another collection.
Collections.reverse()	This method reverses the order of elements in the specified collection.