Java TreeSet and HashSet similarities and differences

Java TreeSet and HashSet are implementations of the Set interface. Often you may have wondered, whether to choose HashSet or TreeSet and which is faster. In this article, I will be comparing the HashSet and TreeSet classes in Java.

HashSet and TreeSet Similarities

Duplicates not allowed

Since both HashSet and TreeSet implement the Set interface, they do not allow duplicates. So consider the following code:

Set<String> hashSet = new HashSet<String>();
    hashSet.add("Mango");
    hashSet.add("Mango");

    
    Set<String> treeSet = new TreeSet<String>();
    treeSet.add("Mango");
    treeSet.add("Mango");

    
    System.out.println("Number of elements in Hashset:"+hashSet.size());
    System.out.println("Number of elements in Treeset:"+treeSet.size());

When you execute this code, it will print the following output:

Number of elements in Hashset:1
Number of elements in Treeset:1

 Not Thread-Safe

Both HashSet and TreeSet are not synchronized and are not thread-safe. So you need to write code to explicitly synchronize them.

So you will need to write code similar to the following:

Set<String> synHashSet = Collections.synchronizedSet(hashSet);

Set<String> synTreeSet = Collections.synchronizedSet(treeSet);

HashSet and TreeSet Differences

Ordering

A TreeSet is sorted, whereas a HashSet is not sorted. When you insert elements into a HashSet, no order will be maintained. On the other hand, when you insert elements into a TreeSet, they will be ordered according to their natural order. The following code demonstrates this:

HashSet example

Set<String> hashSet = new HashSet<String>();
hashSet.add("Mango");
hashSet.add("Banana");
hashSet.add("Apple");
hashSet.add("Pear");

hashSet.forEach(str -> System.out.println(str));

The code above creates a HashSet of String values, inserts some data and then prints it. When you execute this code, it will print the following output:

Apple
Pear
Mango
Banana

So you can see that the Strings in the HashSet are not sorted.

TreeSet example

Set<String> treeSet = new TreeSet<String>();
treeSet.add("Mango");
treeSet.add("Banana");
treeSet.add("Apple");
treeSet.add("Pear");


treeSet.forEach(str -> System.out.println(str));

When you execute this code, it will print the following output:

Apple
Banana
Mango
Pear

So you can see that the Strings in the TreeSet are sorted alphabetically.

Allowing Null Objects

A HashSet allows null objects, a TreeSet does not allow null objects. So you can insert a null value in a HashSet, but you cannot do this in a TreeSet. The following code demonstrates this:

hashSet.add("Mango"); 
hashSet.add("Banana"); 
hashSet.add(null); 
hashSet.forEach(str -> System.out.println(str));

The above code will not cause any issues. So when you execute this code, the following will be printed to the console:

null
Mango
Banana

However,  consider the following code:

treeSet.add("Mango"); 
treeSet.add("Banana"); 
treeSet.add(null);
 treeSet.forEach(str -> System.out.println(str));

When you run this code, a NullPointerException will occur as follows:

Exception in thread "main" java.lang.NullPointerException
at java.util.TreeMap.put(Unknown Source)
at java.util.TreeSet.add(Unknown Source)
at learnjava.collections.HashSetTreeSetDemo.main(HashSetTreeSetDemo.java:15)

Internal Data Structure

HashSet internally uses a HashTable. TreeSet on the other hand internally uses a Red-Black Tree.

Performance

Since HashSet internally uses a HashTable, it is faster as compared to a TreeSet. So most operations like add, remove, etc are faster in a HashSet.

When to use which?

If you want to store the data in sorted order and don’t care about the performance then you should use a TreeSet. If performance is critical for your application, you should use a HashSet.

Conclusion

So, in this article, we learned about the similarities and differences between Java TreeSet and HashSet.