assertNotEquals in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ assertion

intermediate

Understanding assertNotEquals behavior

What will happen when the following JUnit assertion runs?

assertNotEquals(5, 3 + 2);

JUnit

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

public class TestClass {
    @org.junit.jupiter.api.Test
    void testValues() {
        assertNotEquals(5, 3 + 2);
    }
}

A The test will pass because 5 does not equal 3 + 2

B The test will fail because 5 equals 3 + 2

C The test will fail due to a compilation error

D The test will pass because assertNotEquals always passes

💡 Hint

Remember assertNotEquals expects the two values to be different for the test to pass.

assertNotEquals fails the test if the two values are equal. Since 3 + 2 equals 5, the assertion fails.

❓ assertion

intermediate

Using assertNotEquals with objects

Given the following code, what is the result of the assertion?

String a = new String("test");
String b = new String("test");
assertNotEquals(a, b);

JUnit

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

public class TestClass {
    @org.junit.jupiter.api.Test
    void testStrings() {
        String a = new String("test");
        String b = new String("test");
        assertNotEquals(a, b);
    }
}

A The test will fail because assertNotEquals cannot compare objects

B The test will pass because assertNotEquals compares references

C The test will fail because a and b have the same content

D The test will pass because a and b are different objects

💡 Hint

assertNotEquals uses equals() method to compare objects, not references.

assertNotEquals compares objects using their equals() method. Since both strings have the same content, equals() returns true, so the assertion fails.

❓ Predict Output

advanced

Output of assertNotEquals with arrays

What happens when this test runs?

int[] arr1 = {1, 2, 3};
int[] arr2 = {1, 2, 3};
assertNotEquals(arr1, arr2);

JUnit

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

public class TestClass {
    @org.junit.jupiter.api.Test
    void testArrays() {
        int[] arr1 = {1, 2, 3};
        int[] arr2 = {1, 2, 3};
        assertNotEquals(arr1, arr2);
    }
}

A The test will fail due to a NullPointerException

B The test will fail because arr1 and arr2 have the same contents

C The test will pass because assertNotEquals compares array contents

D The test will pass because arr1 and arr2 are different array objects

💡 Hint

Arrays do not override equals(), so equals() compares references.

assertNotEquals uses equals() which for arrays compares references. Since arr1 and arr2 are different objects, the assertion passes.

🔧 Debug

advanced

Debugging assertNotEquals failure with custom objects

A test fails with assertNotEquals on two custom objects even though their fields differ. What is the likely cause?

class Person {
  String name;
  Person(String name) { this.name = name; }
}

Person p1 = new Person("Alice");
Person p2 = new Person("Bob");
assertNotEquals(p1, p2);

JUnit

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

class Person {
  String name;
  Person(String name) { this.name = name; }
}

public class TestClass {
  @org.junit.jupiter.api.Test
  void testPersons() {
    Person p1 = new Person("Alice");
    Person p2 = new Person("Bob");
    assertNotEquals(p1, p2);
  }
}

A Person class does not override equals(), so default reference equality is used

B Person class overrides equals() incorrectly causing failure

C The test fails because p1 and p2 have the same name

D assertNotEquals cannot compare custom objects

💡 Hint

Check if equals() is overridden correctly to compare fields.

The test fails (meaning equals() returns true) despite different fields, so the likely cause is that Person overrides equals() incorrectly, not properly comparing the fields.

🧠 Conceptual

expert

Choosing assertNotEquals vs assertEquals in test design

Which scenario best justifies using assertNotEquals instead of assertEquals in a test case?

A Verifying that a method changes a value from its initial state

B Checking that two objects have identical content

C Confirming that a method returns a specific expected value

D Ensuring that two variables reference the exact same object

💡 Hint

assertNotEquals is used to confirm values are different, not equal.

assertNotEquals is useful to check that a value has changed from an initial state, ensuring the method under test modifies data.

Practice

1. What does the JUnit method assertNotEquals check in a test?

easy

A. That two values are not the same

B. That two values are equal

C. That a value is true

D. That a value is null

Solution

1. Step 1: Understand the purpose of assertNotEquals

   This method is used to verify that two values are different, meaning they should not be equal.

2. Step 2: Compare with other assertions

   Unlike assertEquals which checks equality, assertNotEquals ensures values differ.

3. Final Answer:

   That two values are not the same -> Option A

4. Quick Check:

   assertNotEquals means values differ [OK]

Hint: Remember: 'NotEquals' means values must differ [OK]

Common Mistakes:

• Confusing assertNotEquals with assertEquals
• Thinking it checks for null values
• Assuming it checks boolean true

2. Which of the following is the correct syntax to assert that two integers are not equal in JUnit with a custom error message?

easy

A. assertNotEquals(5, 10);

B. assertNotEquals(5 == 10);

C. assertNotEquals(5, 10, "Error");

D. assertNotEquals(5 != 10);

Solution

1. Step 1: Review method signatures

   JUnit's assertNotEquals can take two values and an optional message as the last parameter.

2. Step 2: Check each option

   assertNotEquals(5, 10); is valid but lacks a message; assertNotEquals(5, 10, "Error"); is correct syntax with message; Options B and C are invalid because they misuse boolean expressions.

3. Final Answer:

   assertNotEquals(5, 10, "Error"); -> Option C

4. Quick Check:

   Correct syntax includes values and optional message [OK]

Hint: Use assertNotEquals(value1, value2, message) for clarity [OK]

Common Mistakes:

• Passing boolean expressions instead of values
• Omitting required parameters
• Using incorrect parameter order

3. What will happen when the following JUnit test runs?

int a = 3;
int b = 3;
assertNotEquals(a, b);

medium

A. Compilation error due to wrong syntax

B. Test passes because a and b are equal

C. Test passes because a and b are not equal

D. Test fails because a and b are equal

Solution

1. Step 1: Analyze variable values

   Both a and b are assigned the value 3, so they are equal.

2. Step 2: Understand assertNotEquals behavior

   assertNotEquals expects the two values to be different. Since they are equal, the assertion fails and the test fails.

3. Final Answer:

   Test fails because a and b are equal -> Option D

4. Quick Check:

   Equal values cause assertNotEquals to fail [OK]

Hint: assertNotEquals fails if values are the same [OK]

Common Mistakes:

• Thinking test passes when values are equal
• Confusing assertNotEquals with assertEquals
• Assuming syntax error without checking code

4. Identify the error in this JUnit test snippet:

String expected = "hello";
String actual = "hello";
assertNotEquals(expected, actual);

medium

A. Test fails because expected and actual are equal

B. No error; test passes

C. Syntax error due to missing message

D. NullPointerException at runtime

Solution

1. Step 1: Check values of variables

   Both expected and actual hold the string "hello", so they are equal.

2. Step 2: Understand assertNotEquals effect

   The assertion expects different values. Since they are equal, the test fails but no syntax or runtime error occurs.

3. Final Answer:

   Test fails because expected and actual are equal -> Option A

4. Quick Check:

   Equal strings cause assertNotEquals to fail [OK]

Hint: assertNotEquals fails if strings match exactly [OK]

Common Mistakes:

• Assuming test passes when strings are equal
• Thinking message parameter is mandatory
• Expecting runtime exceptions incorrectly

5. You want to test that a method generateId() never returns the value 0. Which JUnit assertion using assertNotEquals correctly tests this with 0 as the first argument and a custom message?

hard

A. assertNotEquals(0, 0, "ID should not be zero");

B. assertNotEquals(0, generateId(), "ID should not be zero");

C. assertNotEquals(generateId(), 0);

D. assertNotEquals("0", generateId());

Solution

1. Step 1: Identify correct parameter order and message

   The expected forbidden value is 0, so it should be the first parameter. The actual value is from generateId(). Adding a message helps explain failure.

2. Step 2: Evaluate options for correctness

   assertNotEquals(0, generateId(), "ID should not be zero"); correctly uses assertNotEquals(0, generateId(), "ID should not be zero"). assertNotEquals(generateId(), 0); misses the message and reverses parameters. assertNotEquals(0, 0, "ID should not be zero"); compares 0 to 0, which always fails. assertNotEquals("0", generateId()); compares string "0" to int, which is incorrect.

3. Final Answer:

   assertNotEquals(0, generateId(), "ID should not be zero"); -> Option B

4. Quick Check:

   Use expected value first, actual second, with message [OK]

Hint: Put expected value first, actual second, add message [OK]

Common Mistakes:

• Swapping expected and actual parameters
• Comparing different data types
• Omitting helpful failure message