Using TreeSet

TreeSet stores objects in a sorted manner. TreeSet stores its elements in a tree and they are automatically arranged in a sorted order.
TreeSet is not synchronized. If more than one thread wants to access it at the same time then it must be synchronized externally.

import java.util.*;
public class DemoTreeSet {
public static void main(String[] args) {
TreeSet t1 = new TreeSet();
t1.add("ggg");
t1.add("aaa");
t1.add("ddd");
t1.add("bbb");
t1.add("nnn");
t1.add("lll");
System.out.println("Contents of treeset");
Iterator it1 =t1.iterator();
while(it1.hasNext()){
Object o1 = it1.next();
System.out.println(o1);
}
}
}

source: java-tips.org