Top 10 Key Differences Between HashSet and TreeSet in Java

1. Overview of HashSet and TreeSet

Before diving into the differences, let’s briefly review what HashSet and TreeSet are.

1.1 What is HashSet?

A HashSet is a collection that uses a hash table for storage. It implements the Set interface, meaning it does not allow duplicate elements. The elements are unordered and unsorted, making HashSet suitable for scenarios where you need fast lookup, insertion, and deletion.

1.2 What is TreeSet?

A TreeSet is a collection that implements the NavigableSet interface. It uses a Red-Black tree for storage, meaning the elements are stored in a sorted and ordered manner. TreeSet does not allow duplicate elements either, but it is ideal for situations where you need to maintain a natural ordering of elements.

2. Key Differences Between HashSet and TreeSet

2.1 Ordering

• HashSet: Does not maintain any order of elements. The order in which elements are added does not correlate with the order they are stored.
• TreeSet : Automatically orders the elements based on their natural ordering or a specified comparator.

2.2 Performance

• HashSet: Offers constant time complexity O(1) for basic operations like add, remove, and contains, making it much faster when order is not a concern.
• TreeSet : Offers log(n) time complexity for basic operations, as the elements are stored in a tree structure, which takes more time than a hash-based structure.

2.3 Internal Storage Mechanism

HashSet: Uses a hash table internally. Each element’s hash code is used to determine its storage location. If two elements have the same hash code, a technique called chaining or probing is used to handle collisions.

Example Code:

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

TreeSet : Uses a Red-Black tree internally. Each element is placed according to its natural order or a provided comparator, ensuring that the tree remains balanced.

Example Code:

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

2.4 Null Elements

• HashSet: Allows one null element, as it can hash the null value.
• TreeSet : Does not allow null elements because it needs to compare elements to sort them, and comparing null to any object throws a NullPointerException.

2.5 Synchronization

• HashSet: Not synchronized by default, but can be synchronized using Collections.synchronizedSet.
• TreeSet : Also not synchronized by default, but can be synchronized in the same manner.

2.6 Duplicate Elements

Both HashSet and TreeSet do not allow duplicate elements. However, the method of detecting duplicates differs. HashSet uses the hashCode() and equals() methods, while TreeSet uses the compareTo() or a Comparator.

2.7 Memory Usage

• HashSet: Generally requires more memory due to the underlying hash table and the potential for linked lists to handle collisions.
• TreeSet : Uses less memory since it uses a tree structure but has more overhead in maintaining order.

2.8 Comparison with LinkedHashSet

HashSet vs. LinkedHashSet: While HashSet does not guarantee any order, LinkedHashSet maintains the insertion order. TreeSet, on the other hand, sorts elements naturally or by a custom comparator.

2.9 Use Cases

• HashSet: Best used when the focus is on fast access time and order is not important.
• TreeSet : Ideal for scenarios where elements need to be accessed in a sorted order.

2.10 Demo Result: Iteration Order

Running the below code snippets demonstrates the difference in iteration order:

// HashSet Example
Set<String> hashSet = new HashSet<>();
hashSet.add("Zebra");
hashSet.add("Apple");
hashSet.add("Mango");
System.out.println("HashSet: " + hashSet); 
// Output may be unordered, e.g., [Apple, Mango, Zebra]

// TreeSet Example
Set<String> treeSet = new TreeSet<>();
treeSet.add("Zebra");
treeSet.add("Apple");
treeSet.add("Mango");
System.out.println("TreeSet: " + treeSet); 
// Output will be sorted, e.g., [Apple, Mango, Zebra]

3. Conclusion

Choosing between HashSet and TreeSet boils down to your specific needs:

• Use HashSet when you require a high-performance set with no concern for the order of elements.
• Use TreeSet when you need elements sorted naturally or by a custom order.

Have any questions? Feel free to drop a comment below!