TreeSet

In this chapter you will learn:

What is TreeSet
How to create TreeSet
Create a TreeSet with custom Comparator
Create TreeSet from a List

TreeSet class

TreeSet extends AbstractSet and implements the NavigableSet interface. It creates a collection that uses a tree for storage. Objects are stored in sorted, ascending order.

TreeSet is a generic class that has this declaration:

class TreeSet<E>

E specifies the type of objects that the set will hold. A demonstration of a tree set with String elements sorted according to their natural ordering.

import java.util.Set;
import java.util.TreeSet;
/*from  j av a 2s. c  om*/
public class Main {
  public static void main(String[] args) {
    Set<String> ss = new TreeSet<String>();
    String[] fruits = { "apples", "pears", "grapes", "bananas", "kiwis" };
    for (String fruit : fruits){
      ss.add(fruit);
    }
    for (String s : ss){
      System.out.print(s + " ");
    }
  }
}

Output:

apples bananas grapes kiwis pears

Create TreeSet

TreeSet() Creates a tree set, sorted according to the natural ordering of its elements.
TreeSet(Collection<? extends E> c) Creates a tree set containing the elements in the specified collection, sorted according to the natural ordering of its elements.
TreeSet(Comparator<? super E> comparator) Creates a tree set, sorted according to the specified comparator.
TreeSet(SortedSet<E> s) Creates a tree set from SortedSet.

The first form constructs an empty tree set that will be sorted in ascending order according to the natural order of its elements. The second form builds a tree set that contains the elements of c.

The third form constructs an empty tree set that will be sorted according to the comparator specified by comp. The fourth form builds a tree set that contains the elements of ss.

The following code creates a TreeSet from List

import java.util.Arrays;
import java.util.TreeSet;
//  j  ava2 s .co m
public class Main {
  public static void main(String args[]) throws Exception {

    String elements[] = { "java2s.com", "C", "D", "G", "F" };
    TreeSet<String> set = new TreeSet<String>(Arrays.asList(elements));

    System.out.println(set.headSet("D"));
    System.out.println(set.tailSet(""));
  }
}

Create a TreeSet with custom Comparator

The following code creates a Comparator first and then use it to create a TreeSet.

import java.util.Comparator;
import java.util.TreeSet;
//from j  a v  a2s.co m
class MyComparator implements Comparator<String> {
    public int compare(String a, String b) {
        String aStr, bStr;
        aStr = a;
        bStr = b;
        return bStr.compareTo(aStr);
    }
}

public class Main{
    public static void main(String args[]) {
        TreeSet<String> ts = new TreeSet<String>(new MyComparator());

        ts.add("java2s.com");
        ts.add("A");
        ts.add("B");
        ts.add("E");
        ts.add("F");
        ts.add("D");

        for (String element : ts)
            System.out.print(element + " ");

        System.out.println();
    }
}

The output:

Create TreeSet from a List

The following code creates a TreeSet from a List. And The list is created from an array.

import java.util.Arrays;
import java.util.TreeSet;
//from j a v a 2 s.c o  m
public class Main {
  public static void main(String args[]) throws Exception {

    String elements[] = { "java2s.com", "C", "D", "G", "F" };
    TreeSet<String> set = new TreeSet<String>(Arrays.asList(elements));

    System.out.println(set.headSet("D"));
    System.out.println(set.tailSet(""));
  }
}

The output:

Next chapter...

What you will learn in the next chapter:

How to add elements to a TreeSet

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