Using a comparator class with priority queue.

import java.util.Comparator;
import java.util.PriorityQueue;

public class UsingComparator {
	public static void main(String [] args) {
		PriorityQueue<String> stringsByLength = new PriorityQueue<String>(new CompareByLength());
		stringsByLength.add("hello");
		stringsByLength.add("hi");
		stringsByLength.add("greetings");
		stringsByLength.add("bye");
		
		while(!stringsByLength.isEmpty()) {
			System.out.println(stringsByLength.poll());
		}
	}
	
}

class CompareByLength implements Comparator<String> {

	@Override
	public int compare(String s1, String s2) {			
		return s1.length() - s2.length();
	}
	
}

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