Difference between HashSet and TreeSet in java

By
Rakesh
-

In this post, we will see the difference between HashSet and TreeSet in Java. First, we will see HashSet vs TreeSet in brief later we will see each point in details.

HashSetTreeSet
1. HashSet does not maintain any order.1. TreeSet maintains ascending order..
2. Hashset allows null value.2. In case of TreeSet if you add null value it will not give any compile time error but when you will try to iterate the elements it will throw NullPointerException.
3. Hashset is faster than TreeSet.3. TreeSet is slower than HashSet because it internally perform sorting operation.
4. Hashset internally uses HashMap4. TreeSet internally uses NavigableMap.
5. If we define generic type Object, in case of HashSet we can have different type of element.5. In case of TreeSet , we can't have different type of elements.

 

Let’s see all points in detail.

HashSet does not maintain any order whereas Treeset maintains ascending order.

Example of HashSet –

import java.util.HashSet;
import java.util.Set;
 
public class HashSetExample {
	public static void main(String[] args) {
		Set<String> setObject = new HashSet<>();
 
		setObject.add("ram");
		setObject.add("mohan");
		setObject.add("shyam");
		setObject.add("mohan");
 
		// duplicates are not allowed but it will not give
		// any compilation error
		setObject.add("ram");
		System.out.println("set object -- " + setObject);
 
	}
}

Output is –

        set object — [shyam, mohan, ram]

Example of TreeSet –

package treeset;
 
import java.util.Set;
import java.util.TreeSet;
 
public class TreeSetExample {
	public static void main(String[] args) {
		Set<String> treeSetObj = new TreeSet<>();
		treeSetObj.add("ram");
		treeSetObj.add("mohan");
		treeSetObj.add("sohan");
		treeSetObj.add("rohan");
		treeSetObj.add("ram");
		System.out.println(treeSetObj);
 
	}
}

Output is –

[mohan, ram, rohan, sohan]

In the above example, we can see TreeSet is maintaining ascending order whereas HashSet is unordered.

HashSet allows null value, TreeSet will throw NullPointerException.

 

package hashsetvstreeset;
import java.util.*;
public class HashSetVsTreeSetGet {
public static void main(String[] args) {
	
		Set<String> setObject = new HashSet<>();
		
		setObject.add("ram");
		setObject.add("mohan");
		setObject.add("shyam");
		setObject.add("mohan");
		//we can have null in HashSet
		setObject.add(null);
		
		for (String str : setObject) {
			System.out.println(str);
		}
		
		Set<String> treeSetObj = new TreeSet<>();
		treeSetObj.add("ram");
		treeSetObj.add("mohan");
		treeSetObj.add("sohan");
		treeSetObj.add("rohan");
		//TreeSet doen't allow null.
		//At Compile time there is no problem
		//While running it will throw NullPointerException
		treeSetObj.add(null);
		
		
		for (String str : treeSetObj) {
			System.out.println(str);
		}
		
		
		
	}
}

Output is –

null
shyam
mohan
ram
Exception in thread “main” java.lang.NullPointerException
at java.util.TreeMap.put(Unknown Source)
at java.util.TreeSet.add(Unknown Source)
at hashsetvstreesetperformance.HashSetVsTreeSetGet.main(HashSetVsTreeSetGet.java:29)

 

 

HashSet internally uses HashMap whereas TreeSet internally uses NavigableMap.

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;

    private transient HashMap<E,Object> map;

    private static final Object PRESENT = new Object();

  
    public HashSet() {
        map = new HashMap<>();
    }
}

 

public class TreeSet<E> extends AbstractSet<E>
    implements NavigableSet<E>, Cloneable, java.io.Serializable
{
 
    private transient NavigableMap<E,Object> m;

  
    private static final Object PRESENT = new Object();

  
    TreeSet(NavigableMap<E,Object> m) {
        this.m = m;
    }
}

 

 

HashSet we can have different type of elements, if we try to add a different type of elements in TreeSet will throw ClassCastException. TreeSet internally compares two elements, if we have different types of elements it will not able to compare.

package addingtime;

import java.util.*;

public class Example2 {
public static void main(String[] args) {
	Set<Object> setObject = new HashSet<>();
	 
	setObject.add("ram");
	setObject.add("mohan");
	setObject.add("shyam");
	setObject.add(3);
	setObject.add(3.97f);
	System.out.println("HashSet can have different types of elements-- " + setObject);
	
	Set<Object> treeSetObj = new TreeSet<>();
	treeSetObj.add("ram");
	treeSetObj.add("mohan");
	treeSetObj.add("sohan");
	treeSetObj.add(3);
	treeSetObj.add(3.97f);
	System.out.println(treeSetObj);
}
}

 

Output is –

HashSet can have different types of elements– [3, shyam, 3.97, mohan, ram]
Exception in thread “main” java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
at java.lang.Integer.compareTo(Unknown Source)
at java.util.TreeMap.put(Unknown Source)
at java.util.TreeSet.add(Unknown Source)
at addingtime.Example2.main(Example2.java:20)

 

 

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