TreeSet constructor with Comparator<?> parameter

java generics comparator treeset
TheRapture87 picture TheRapture87 · Apr 17, 2015 · Viewed 51.6k times · Source

In Java’s documentation for its class TreeSet one of the constructors is shown to have the following header:

TreeSet(Comparator<? super E> c)

Can someone help explain why there is a constructor for TreeSet which takes a comparator object as its argument? I have no clue why this is done.

Answer

Eran picture Eran · Apr 17, 2015

The elements in a TreeSet are kept sorted.

If you use a constructor that has no Comparator, the natural ordering of the element class (defined by the implementation of Comparable) would be used to sort the elements of the TreeSet.

If you want a different ordering, you supply a Comparator in the constructor.

Related questions

Making a generic comparator class

I'm trying to make a comparator that can take any type of an element to compare. I'm unsure about how to create the class. I just want it to compare two elements of the same type (But whatever type the …

Read More
Comparable and Comparator contract with regards to null

Comparable contract specifies that e.compareTo(null) must throw NullPointerException. From the API: Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false. On the …

Read More
How come generic type parameter says "extends" Comparable not "implements"?

I tried to write generic function that remove the duplicate elements from array. public static <E extends Comparable<E>> ArrayList<E> removeDuplicate(E[] arr) { //do quicksort Arrays.sort(arr); ArrayList<E> list = …

Read More