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HashSet vs. TreeSet in Java

Educative Answers Team

HashSets and TreeSets are both sets, therefore, they do not allow duplicates. Also, neither of them is thread-safe, i.e., they require manual synchronization when multiple threads try to access them. However, since both of them are separate classes in Java, there are some differences among them:



  • Null values can be added

  • No fixed iteration order

  • Heterogenous values are allowed

  • Time complexity for insertion/deletion/retrieval: O(1)

  • Implements the Set interface


  • Nulls aren’t allowed[1]

  • Iteration over items is in natural order or a given Comparator is used[2]

  • Only homogenous values are allowed[3]

  • Time complexity for the same operations: O(log n)

  • Implements the NavigableMapinterface[4]


  1. A java.lang.NullPointerException is thrown when trying to insert a null into a TreeSet.
  2. By default, a TreeSet sorts values in natural and ascending order. However, strings are sorted in their alphabetical order (“10” comes before “2” because “1” comes before “2”). A custom Comparator<T> can be passed to the constructor of the TreeSet. This is usually done to sort a user-defined type (e.g., Student), or to change how an existing type (e.g., String, Integer, etc.) is sorted. An example is shown below:
import java.util.TreeSet;
import java.util.Comparator;

public class TreeComparator {

	public static void main(String[] args) {
		TreeSet<Student> ts = new TreeSet<>(new MyComp());
		ts.add(new Student("Tom", 45));
		ts.add(new Student("Jerry", 50));
		ts.add(new Student("Spike", 40));

// Declaring a custom Comparator:
class MyComp implements Comparator<Student>{

	public int compare(Student o1, Student o2) {
		// Sort by marks:
		return o1.getMarks().compareTo(o2.getMarks());

class Student{
	String name;
	Integer marks;
	Student(String n, int m){
		name = n;
		marks = m;
	String getName() {
		return name;
	Integer getMarks() {
		return marks;
	// Overriding toString() to use println:
	public String toString() {
		return name + ": " + marks;
  1. A Collection in Java (e.g., HashSet, TreeSet, ArrayList) can be declared without specifying its type in <>. This provides the flexibility to add elements of different data types in a heterogeneous Collection:

Declaring a Collection in the following way will be considered as a warning; nevertheless, the program will compile and execute:

HashSet hs = new HashSet();
hs.add("a string");

      On the other hand, a TreeSet is a homogeneous Collection. Consequently, the       first element we add to it will be considered as its only allowed data-type;       otherwise, java.lang.ClassCastException will be thrown upon execution.

TreeSet ts = new TreeSet();
ts.add(10);  // Type fixed as Integer.
ts.add("a string");  // Throws exception.
  1. Hierarchy:
svg viewer
Class diagram showing a part of the hierarchy


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