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.