📘 Lesson · Lesson 41

HashSet vs TreeSet in Java

In this lesson: HashSet vs TreeSet compared — ordering, speed, null handling — with examples and comparison table. In Hindi & English.

Two kinds of Set

HashSet and TreeSet are both Sets — they store only unique items — but the big difference is order. HashSet keeps no order and is faster; TreeSet keeps items automatically sorted but is a little slower. You pick based on whether you need sorting.

💡 HashSet = unique, no order, fast. TreeSet = unique, always sorted, a bit slower.

HashSet

import java.util.HashSet;

HashSet<Integer> set = new HashSet<>();
set.add(30); set.add(10); set.add(20);
set.add(10);            // duplicate ignored
System.out.println(set);   // order not guaranteed

[20, 10, 30]

HashSet stores unique items with no guaranteed order, which makes it very fast for add, remove and contains. Use it when you only care about uniqueness and speed.

TreeSet

import java.util.TreeSet;

TreeSet<Integer> set = new TreeSet<>();
set.add(30); set.add(10); set.add(20);
System.out.println(set);   // sorted automatically

[10, 20, 30]

TreeSet keeps items in sorted order automatically. Adding 30, 10, 20 gives them back as 10, 20, 30. It is the choice when you need a set that stays sorted.

Order is the big difference

HashSet — no order; items may appear in any arrangement.
TreeSet — natural sorted order (numbers ascending, strings A-Z).
LinkedHashSet — a third option that keeps insertion order.

If order does not matter, HashSet is best. If you need sorting, use TreeSet.

Performance and nulls

Speed: HashSet is faster (near constant time); TreeSet is slower (keeps sorting).
Null: HashSet allows one null; TreeSet allows none (it must compare to sort).
Requirement: TreeSet elements must be comparable.

Comparison

Point	HashSet	TreeSet
Unique items	Yes	Yes
Order	None	Sorted
Speed	Faster	Slower
Null allowed	One	None
Best for	Speed, uniqueness	Sorted unique data

Common mistakes

Expecting order from HashSet. It has none — use TreeSet or LinkedHashSet.
Adding null to TreeSet. It throws NullPointerException.
Using TreeSet when order is not needed. HashSet is faster.
Non-comparable elements in TreeSet. They must be comparable, or supply a Comparator.

Summary

Both store only unique items.
HashSet has no order and is faster.
TreeSet keeps items sorted but is slower.
HashSet allows one null; TreeSet allows none.
Use HashSet for speed, TreeSet for sorted data, LinkedHashSet for insertion order.

Next, revise sets in the Collections Framework, or learn sorting rules with Comparable vs Comparator.

Frequently Asked Questions

What is the difference between HashSet and TreeSet?

Both store only unique items, but HashSet does not keep any order and is faster, while TreeSet keeps items sorted automatically and is a bit slower. Choose HashSet for speed, TreeSet when you need sorted order.

Does HashSet keep insertion order?

No. HashSet stores items in an unpredictable order for speed. If you need insertion order, use LinkedHashSet; if you need sorted order, use TreeSet.

How does TreeSet sort its items?

TreeSet keeps items in natural sorted order (numbers ascending, strings alphabetically) using their compareTo method, or a Comparator you provide. Every element must be comparable, or it throws an error.

Which is faster, HashSet or TreeSet?

HashSet is faster for add, remove and contains (roughly constant time). TreeSet is slower because it keeps everything sorted. Use HashSet unless you specifically need ordering.

Can HashSet and TreeSet store null?

HashSet allows a single null element. TreeSet does not allow null because it must compare elements to sort them, and comparing null causes a NullPointerException.

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