assertThrows for exceptions in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the result of this JUnit test using assertThrows?

Consider the following JUnit 5 test method. What will be the test execution result?

JUnit

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

class Calculator {
    int divide(int a, int b) {
        return a / b;
    }
}

public class CalculatorTest {
    @Test
    void testDivideByZero() {
        Calculator calc = new Calculator();
        assertThrows(ArithmeticException.class, () -> calc.divide(10, 0));
    }
}

A Test passes because ArithmeticException is thrown as expected.

B Test fails because no exception is thrown.

C Test fails because NullPointerException is thrown instead.

D Test fails due to a compilation error in the test code.

💡 Hint

Think about what happens when dividing by zero in Java.

Dividing an integer by zero in Java throws an ArithmeticException. The assertThrows method expects this exception, so the test passes.

❓ assertion

intermediate

Which assertThrows usage correctly verifies NullPointerException?

You want to test that calling method process(null) throws NullPointerException. Which assertThrows statement is correct?

JUnit

class Processor {
    void process(String input) {
        if (input == null) throw new NullPointerException();
    }
}

A assertThrows(NullPointerException.class, () -> processor.process("text"));

B assertThrows(Exception.class, () -> processor.process(null));

C assertThrows(IllegalArgumentException.class, () -> processor.process(null));

D assertThrows(NullPointerException.class, () -> processor.process(null));

💡 Hint

The exception type must match exactly the one thrown.

Only option D expects NullPointerException and calls process with null, which triggers the exception.

🔧 Debug

advanced

Why does this assertThrows test fail unexpectedly?

Examine the test below. It is supposed to check that getItem(-1) throws IndexOutOfBoundsException, but the test fails. Why?

JUnit

import static org.junit.jupiter.api.Assertions.assertThrows;
import org.junit.jupiter.api.Test;
import java.util.List;

public class ListTest {
    @Test
    void testGetItem() {
        List<String> list = List.of("a", "b", "c");
        assertThrows(IndexOutOfBoundsException.class, () -> list.get(1));
    }
}

A The test fails because List.of creates a mutable list that throws a different exception.

B The test fails because list.get(1) returns "b" and does not throw an exception.

C The test fails because IndexOutOfBoundsException is not the correct exception type.

D The test fails because assertThrows requires a try-catch block around the lambda.

💡 Hint

Check the index used in list.get and the list size.

Index 1 is valid for the list of size 3, so no exception is thrown and assertThrows fails.

🧠 Conceptual

advanced

What does assertThrows return and how can it be used?

In JUnit 5, assertThrows returns the caught exception. How can this feature be used in tests?

A To automatically retry the test if the exception occurs.

B To suppress the exception so the test continues without failure.

C To verify the exception message or properties after catching it.

D To convert checked exceptions into unchecked exceptions.

💡 Hint

Think about what you can do with the exception object after catching it.

assertThrows returns the exception object, allowing assertions on its message or fields.

❓ framework

expert

How to test a method that throws a checked exception using assertThrows?

Given a method that throws IOException, which assertThrows usage correctly tests it?

JUnit

import java.io.IOException;

class FileReader {
    void readFile(String path) throws IOException {
        if (path == null) throw new IOException("Path is null");
    }
}

A assertThrows(IOException.class, () -> { fileReader.readFile(null); });

B assertThrows(Exception.class, () -> fileReader.readFile(null));

C assertThrows(IOException.class, () -> fileReader.readFile(null));

D assertThrows(IOException.class, () -> fileReader.readFile("file.txt"));

💡 Hint

Checked exceptions require handling or declaration in lambda.

Option A uses a block lambda that allows throwing checked IOException, matching the method signature.