JUnit assertNotEquals Method

The assertNotEquals method in JUnit is used to test if two values are not equal. JUnit provides several overloaded versions of this method to handle different data types and to provide custom messages for test failures. This guide covers the basics of using the assertNotEquals method, including its syntax and examples of its different overloads.

Table of Contents

1. Introduction
2. assertNotEquals Method Syntax
3. Examples
   • Basic Usage
   • Comparing Strings
   • Comparing Arrays
   • Providing a Custom Message
   • Handling Floating Point Numbers
4. Real-World Use Case
5. Conclusion

Introduction

The assertNotEquals method in JUnit is an assertion method used to verify that two values are not equal. If the values are equal, the assertion fails, and the test is marked as failed. This method is fundamental for writing unit tests that check the correctness of code by ensuring that certain values are not the same.

assertNotEquals Method Syntax

Here is the basic syntax of the assertNotEquals method:

assertNotEquals(expected, actual);
assertNotEquals(message, expected, actual);
assertNotEquals(expected, actual, delta);
assertNotEquals(message, expected, actual, delta);

Parameters:

• expected: The value that is expected to be different from the actual value.
• actual: The actual value produced by the code.
• message: Optional. A custom message to display if the assertion fails.
• delta: Used for comparing floating-point numbers to account for precision errors.

Returns:

• Nothing. The method throws an assertion error if the values are equal.

Examples

Basic Usage

Compare two integers to ensure they are not equal.

Example

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

public class CalculatorTest {
    @Test
    void testAddition() {
        int unexpected = 4;
        int actual = 2 + 3;
        assertNotEquals(unexpected, actual);
    }
}

Comparing Strings

Compare two strings to ensure they are not equal.

Example

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

public class StringTest {
    @Test
    void testStringInequality() {
        String unexpected = "Hello";
        String actual = "World";
        assertNotEquals(unexpected, actual);
    }
}

Comparing Arrays

To compare two arrays, use assertNotEquals indirectly by comparing their string representations.

Example

import static org.junit.jupiter.api.Assertions.assertNotEquals;
import org.junit.jupiter.api.Test;
import java.util.Arrays;

public class ArrayTest {
    @Test
    void testArrayInequality() {
        int[] unexpected = {1, 2, 3};
        int[] actual = {4, 5, 6};
        assertNotEquals(Arrays.toString(unexpected), Arrays.toString(actual));
    }
}

Providing a Custom Message

Include a custom message to display if the assertion fails.

Example

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

public class CustomMessageTest {
    @Test
    void testWithCustomMessage() {
        int unexpected = 5;
        int actual = 2 + 2;
        assertNotEquals(unexpected, actual, "Custom Message: Unexpected value matched actual value");
    }
}

Handling Floating Point Numbers

Compare two floating-point numbers with a delta to account for precision errors.

Example

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

public class FloatingPointTest {
    @Test
    void testFloatingPointInequality() {
        double unexpected = 5.0;
        double actual = 4.99;
        assertNotEquals(unexpected, actual, 0.001);
    }
}

Real-World Use Case

Testing a BookService Class

A common use case for assertNotEquals is testing the methods of a BookService class to ensure that certain operations do not produce the same results.

Class Under Test

import java.util.HashMap;
import java.util.Map;

public class BookService {
    private Map<String, String> books = new HashMap<>();

    public boolean addBook(String isbn, String title) {
        if (books.containsKey(isbn)) {
            return false;
        }
        books.put(isbn, title);
        return true;
    }

    public boolean updateBook(String isbn, String title) {
        if (!books.containsKey(isbn)) {
            return false;
        }
        books.put(isbn, title);
        return true;
    }

    public boolean deleteBook(String isbn) {
        if (!books.containsKey(isbn)) {
            return false;
        }
        books.remove(isbn);
        return true;
    }

    public String getBook(String isbn) {
        return books.get(isbn);
    }
}

Test Class

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

public class BookServiceTest {
    private final BookService bookService = new BookService();

    @Test
    void testAddBook() {
        bookService.addBook("123456", "JUnit 5 Guide");
        assertNotEquals("Different Book", bookService.getBook("123456"));
    }

    @Test
    void testUpdateBook() {
        bookService.addBook("123456", "JUnit 5 Guide");
        bookService.updateBook("123456", "JUnit 5 Advanced Guide");
        assertNotEquals("JUnit 5 Guide", bookService.getBook("123456"));
    }

    @Test
    void testDeleteBook() {
        bookService.addBook("123456", "JUnit 5 Guide");
        bookService.deleteBook("123456");
        assertNotEquals("JUnit 5 Guide", bookService.getBook("123456"));
    }
}

In this example, the BookServiceTest class tests the BookService methods using assertNotEquals. It includes tests for adding, updating, and deleting books to ensure that the results are not the same when expected.

Conclusion

The assertNotEquals method in JUnit is used for verifying that two values are not equal in your tests. By using assertNotEquals and its overloaded methods, you can compare various data types, provide custom messages for test failures, and handle floating-point precision issues. Understanding and using the assertNotEquals method effectively is crucial for developing robust and maintainable Java applications.