Compare Two TreeSet in Java Example (equals method)

This example shows how to compare two TreeSet objects in Java. This example also shows how to compare two TreeSet objects using the TreeSet equals method.

How to compare two TreeSet objects in Java using the equals method?

We can check if two TreeSet objects are equal or not using the equals method. The TreeSet class inherits the equals method from the parent class AbstractSet.

1	public boolean equals(Object o)

The equals method of the TreeSet compares the specified object with this TreeSet object for equality. It returns true if the specified object is a set, both have the same number of elements (having the same size), and every element of the specified set object is also contained in this TreeSet object (using the containsAll method).

In other words, the equals method of the TreeSet class compares two TreeSet objects by elements. If both have the same elements, it returns true, false otherwise.

import java.util.TreeSet;

public class CompareTreeSetExample {

public static void main(String[] args) {

//first TreeSet

TreeSet<String> tSetCharacters1 = new TreeSet<String>();

tSetCharacters1.add("c");

tSetCharacters1.add("d");

tSetCharacters1.add("b");

tSetCharacters1.add("a");

//second TreeSet

TreeSet<String> tSetCharacters2 = new TreeSet<String>();

tSetCharacters2.add("c");

tSetCharacters2.add("d");

tSetCharacters2.add("b");

tSetCharacters2.add("a");

* To compare two TreeSet objects, use

* the equals method

//this will return true as both are equal

System.out.println(tSetCharacters1.equals(tSetCharacters2));

//remove an element from second TreeSet

tSetCharacters2.remove("d");

//this will return false as both are not equal now

System.out.println(tSetCharacters1.equals(tSetCharacters2));

}

Output

1 2	true false

How to compare TreeSet objects having elements of a custom class?

Let’s first see an example of that.

import java.util.TreeSet;

class Document{

private int documentId;

public Document(int documentId){

this.documentId = documentId;

}

public int getDocumentId(){

return this.documentId;

}

public String toString(){

return "Document - " + getDocumentId();

}

public class CompareTreeSetExample {

public static void main(String[] args) {

TreeSet<Document> tSetDocuments1 = new TreeSet<Document>();

tSetDocuments1.add(new Document(1));

tSetDocuments1.add(new Document(2));

TreeSet<Document> tSetDocuments2 = new TreeSet<Document>();

tSetDocuments2.add(new Document(1));

tSetDocuments2.add(new Document(2));

System.out.println( tSetDocuments1.equals(tSetDocuments2) );

}

Output

Exception in thread "main" java.lang.ClassCastException: com.javacodeexamples.collections.treeset.Document cannot be cast to java.lang.Comparable

at java.util.TreeMap.compare(Unknown Source)

at java.util.TreeMap.put(Unknown Source)

at java.util.TreeSet.add(Unknown Source)

As we can see from the output, the code threw java.lang.ClassCastException exception. The reason is, all elements of the TreeSet either must implement the Comparable interface and define the compareTo method or a custom Comparator must be provided in the TreeSet constructor.

Let’s implement the Comparable interface in the Document class and try again.

import java.util.TreeSet;

//implement the Comparable interface

class Document implements Comparable<Document>{

private int documentId;

public Document(int documentId){

this.documentId = documentId;

}

public int getDocumentId(){

return this.documentId;

}

public String toString(){

return "Document - " + getDocumentId();

}

//override the compareTo method

public int compareTo(Document otherDocument) {

return this.getDocumentId() - otherDocument.getDocumentId();

}

public class CompareTreeSetExample {

public static void main(String[] args) {

TreeSet<Document> tSetDocuments1 = new TreeSet<Document>();

tSetDocuments1.add(new Document(1));

tSetDocuments1.add(new Document(2));

TreeSet<Document> tSetDocuments2 = new TreeSet<Document>();

tSetDocuments2.add(new Document(1));

tSetDocuments2.add(new Document(2));

System.out.println( tSetDocuments1.equals(tSetDocuments2) );

}

Output

true

It worked as expected this time.

Tip: Always override the toString, equals, hashCode methods in your custom class.

This example is a part of the TreeSet in Java Tutorial with Examples.

Please let me know your views in the comments section below.

References:
Java 8 TreeSet

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Rahim

I have a master's degree in computer science and over 18 years of experience designing and developing Java applications. I have worked with many fortune 500 companies as an eCommerce Architect. Follow me on LinkedIn and Facebook.