In this tutorial, we will learn how to write a JUnit test to use Assertions.assertNotEquals() method with message.

The assertNotEquals() method asserts that two objects are not equals. If they are, an AssertionError without a message is thrown. If unexpected and actual are null, they are considered equal.

Let's first create Book, BookService classes, and then we will write JUnit test cases to use assertEquals() methods.

Create Book Class

package junit5.bookstore.model;

public class Book {
	
	private String bookId;
	private String title;
	private String publisher;
	
	public Book(String bookId, String title, String publisher) {
		this.bookId = bookId;
		this.title = title;
		this.publisher = publisher;
	}

	public String getBookId() {
		return bookId;
	}

	public void setBookId(String bookId) {
		this.bookId = bookId;
	}

	public String getTitle() {
		return title;
	}

	public void setTitle(String title) {
		this.title = title;
	}

	public String getPublisher() {
		return publisher;
	}

	public void setPublisher(String publisher) {
		this.publisher = publisher;
	}
	
}

Create BookService Class

package junit5.bookstore.service;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import junit5.bookstore.model.Book;

public class BookService {
	
	private List<Book> listOfBooks = new ArrayList<>();
	
	public void addBook(Book book) {
		listOfBooks.add(book);
	}	
	
	public List<Book> books(){
		return Collections.unmodifiableList(listOfBooks);
	}
	
	public Book getBookById(String bookId) {
		for(Book book : listOfBooks) {
			if(bookId.equals(book.getBookId())) {
				return book;
			}
		}
		return null;
	}
	
	public String[] getBookIdsByPublisher(String publisher) {
		List<String> bookIds = new ArrayList<>();
		for(Book book : listOfBooks) {
			if(publisher.equals(book.getPublisher())) {
				bookIds.add(book.getBookId());
			}
		}
		return bookIds.toArray(new String[bookIds.size()]);
	}
	
	public List<String> getBookTitlesByPublisher(String publisher) {
		List<String> bookTitles = new ArrayList<>();
		for(Book book : listOfBooks) {
			if(publisher.equals(book.getPublisher())) {
				bookTitles.add(book.getTitle());
			}
		}
		return bookTitles;
	}
}

Write JUnit test to use assertEquals() method

The below Java program demonstrates the usage of all overloaded assertNotEquals() static method with a message:

package junit5.assertions.assertNotEquals;

import static org.junit.jupiter.api.Assertions.assertNotEquals;

import org.junit.jupiter.api.Test;

import junit5.bookstore.model.Book;
import junit5.bookstore.service.BookService;

public class AssertNotEqualsDemo {
	
	@Test
	public void assertNotEqualsWithMessage() {
		BookService bookService = new BookService();
		
		Book headFirstJavaBook = new Book("1", "Head First Java", "Wrox");
		Book headFirstDesignPatternBook = new Book("2", "Head First Design Pattern", "Packt");
		
		bookService.addBook(headFirstJavaBook);
		bookService.addBook(headFirstDesignPatternBook);
		
		Book actualBook = bookService.getBookById("1");
		
		assertNotEquals("6", actualBook.getBookId(), "BookId matches to expected value!");
	}
	
	@Test
	public void assertNotEqualsWithMessageSupplier() {
		BookService bookService = new BookService();
		
		Book headFirstJavaBook = new Book("1", "Head First Java", "Wrox");
		Book headFirstDesignPatternBook = new Book("2", "Head First Design Pattern", "Packt");
		
		bookService.addBook(headFirstJavaBook);
		bookService.addBook(headFirstDesignPatternBook);
		
		Book actualBook = bookService.getBookById("1");
		
		assertNotEquals("5", actualBook.getBookId(), () -> "BookId matches to expected value!");
	}
	
}