Deterministic tests in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a deterministic JUnit test with fixed seed

Consider the following JUnit test that uses a fixed seed for randomness. What will be the output when this test runs?

JUnit

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;
import java.util.Random;

public class RandomTest {
    @Test
    void testRandomNumber() {
        Random rand = new Random(12345L);
        int number = rand.nextInt(100);
        assertEquals(59, number);
    }
}

A Test fails because the random number generated is not 42

B Test throws NullPointerException

C Test fails due to AssertionError with number 0

D Test passes because the random number generated is 42

💡 Hint

Check what number Random(12345L).nextInt(100) actually produces.

The Random object with seed 12345L generates a deterministic sequence. The first nextInt(100) call returns 59, not 42, so the assertion fails.

❓ assertion

intermediate

Choosing the correct assertion for deterministic output

You have a method that returns the current timestamp in milliseconds. Which assertion ensures the test remains deterministic?

A assertEquals(System.currentTimeMillis(), methodUnderTest())

B assertTrue(methodUnderTest() > 0)

C assertNotNull(methodUnderTest())

D assertEquals(1672531200000L, methodUnderTest())

💡 Hint

Deterministic tests require fixed expected values.

Only option D uses a fixed expected value, making the test deterministic. Options A and C depend on current time and can vary. Option D only checks non-null but not exact value.

🔧 Debug

advanced

Debugging flaky test caused by non-deterministic behavior

This JUnit test intermittently fails. What is the most likely cause?

JUnit

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;
import java.util.Random;

public class FlakyTest {
    @Test
    void testRandomBoolean() {
        Random rand = new Random();
        boolean flag = rand.nextBoolean();
        assertTrue(flag);
    }
}

A Random object is not seeded, causing non-deterministic output

B assertTrue is used incorrectly and should be assertEquals

C Test method is missing @BeforeEach annotation

D Random.nextBoolean() always returns false

💡 Hint

Consider how randomness affects test repeatability.

Without a fixed seed, Random produces different values each run, causing the test to fail sometimes when flag is false.

🧠 Conceptual

advanced

Why are deterministic tests important in automated testing?

Select the best reason why deterministic tests are preferred in automated test suites.

A They do not require assertions

B They require less code to write

C They always produce the same result, making failures easier to diagnose

D They run faster than non-deterministic tests

💡 Hint

Think about test reliability and debugging.

Deterministic tests produce consistent results, so when a test fails, the cause is easier to find and fix.

❓ framework

expert

Ensuring deterministic behavior in JUnit tests using mocking

You want to test a method that calls an external service returning the current date. Which approach ensures deterministic tests?

A Call the real external service every test run

B Use Mockito to mock the external service and return a fixed date

C Use Thread.sleep() to wait for the service response

D Ignore the external service call in the test

💡 Hint

Think about controlling external dependencies in tests.

Mocking the external service to return a fixed date ensures the test output is predictable and repeatable.