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HashSet vs. TreeSet in Java

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Explanation

A java.lang.NullPointerException is thrown when trying to insert a null into a TreeSet.
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));
		
		ts.forEach(System.out::println);
	}
}
// Declaring a custom Comparator:
class MyComp implements Comparator<Student>{
	@Override
	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;
		
	}
}

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");
hs.add(10);

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.

Hierarchy:

RELATED TAGS

hash

tree

set

java

data structures

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HashSet	TreeSet
Null values can be added	Nulls aren’t allowed
No fixed iteration order	Iteration over items is in natural order or a given Comparator is used
Heterogenous values are allowed	Only homogenous values are allowed
Time complexity for insertion/deletion/retrieval: `O(1)`	Time complexity for the same operations: `O(log n)`
Implements the Set interface	Implements the NavigableSet interface