Dinesh on Java

Several difference between HashSet and TreeSet are similar to what we discussed as difference between TreeMap and HashMap. Anyway Set and Map are two completely different interface so we will revisit those differences here. Probably most important difference between HashSet and TreeSet is the performance. HashSet is faster than TreeSet which means if you need performance use HashSet but HashSet doesn’t provide any kind of ordering so if you need ordering then you need to switch to TreeSet which provides sorting of keys. Sorting can be natural order defined by Comparable interface or any particular order defined by Comparator interface in Java. Apart from differences between HashSet and TreeSet there are some common things between them. let’s see what is common between HashSet and TreeSet in Java.
1) First major difference between HashSet and TreeSet is performance. HashSet is faster than TreeSet and should be preferred choice if sorting of element is not required.

2) Second difference between HashSet and TreeSet is that HashSet allows null object but TreeSet doesn’t allow null Object and throw NullPointerException, Why, because TreeSet uses compareTo() method to compare keys and compareTo() will throw java.lang.NullPointerException as shown in below 
example :

import java.util.*;

public class TreeSetHashSet {

   public static void main(String args[]) {
      HashSet<String> hashSet = new HashSet<String>(); 
      hashSet.add("dineshonjava"); 
      hashSet.add(null); 
        
      TreeSet<String> treeSet = new TreeSet<String>(); 
      treeSet.add("dineshonjava"); 
      treeSet.add(null); //Java.lang.NullPointerException 
   }
}

output:

3) Another significant difference between HashSet and TreeSet is that , HashSet is backed by HashMap while TreeSet is backed by TreeMap in Java.

4) One more difference between HashSet and TreeSet which is worth remembering is that HashSet uses equals() method to compare two object in Set and for detecting duplicates while TreeSet uses compareTo() method for same purpose. if equals() and compareTo() are not consistent, i.e. for two equal object equals should return true while compareTo() should return zero, than it will break contract of Set interface and will allow duplicates in Set implementations like TreeSet

5) Now most important difference between HashSet and TreeSet is ordering. HashSet doesn’t guaranteed any order while TreeSet maintains objects in Sorted order defined by either Comparable or Comparator method in Java.
What is common in HashSet and TreeSet in Java
As I said there are lot of things which are common between HashSet and TreeSet in Java, let’s 
have a look :

1)Both HashSet and TreeSet implements java.util.Set interface which means they follow contract of Set interface and doesn’t allow any duplicates.

2)Both HashSet and TreeSet are not thread-safe and not synchronized. Though you can make them synchronized by using Collections.synchronizedSet() method.

3) Third similarity between TreeSet and HashSet is that, Iterator of both classes are fail-fast in nature. They will throw ConcurrentModificationException if Iterator is modified once Iterator is created. this is not guaranteed and application code should not rely on this code but Java makes best effort to fail as soon as it detects structural change in underlying Set.