Deterministic tests in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the test deterministic by fixing the seed.

JUnit

Random random = new Random([1]);
int value = random.nextInt(100);
assertEquals(42, value);

Drag options to blanks, or click blank then click option'

ASystem.currentTimeMillis()

Bnew Date().getTime()

C0

D12345

Attempts:

3 left

💡 Hint

Common Mistakes

Using time-based seeds like System.currentTimeMillis() causes non-deterministic tests.

Using a fixed seed like 0 ensures the random values are the same every run, making the test deterministic.

2fill in blank

medium

Complete the code to assert the expected value in a deterministic test.

JUnit

@Test
public void testRandomValue() {
    Random random = new Random(0);
    int value = random.nextInt(100);
    assertEquals([1], value);
}

Drag options to blanks, or click blank then click option'

A50

B0

C99

D42

Attempts:

3 left

💡 Hint

Common Mistakes

Guessing the expected value without running the code causes assertion failures.

The fixed seed 0 produces 42 as the first random integer less than 100.

3fill in blank

hard

Fix the error in the test to make it deterministic by replacing the non-deterministic call.

JUnit

@Test
public void testTimestamp() {
    long fixedTimestamp = 1234567890L;
    long timestamp = [1];
    assertEquals(fixedTimestamp, timestamp);
}

Drag options to blanks, or click blank then click option'

Anew Date().getTime()

B1234567890L

CSystem.nanoTime()

DInstant.now().toEpochMilli()

Attempts:

3 left

💡 Hint

Common Mistakes

Using System.currentTimeMillis() or similar methods causes flaky tests.

Using a fixed long value like 1234567890L ensures the test is deterministic and passes consistently.

4fill in blank

hard

Fill both blanks to create a deterministic test that checks a fixed random value.

JUnit

@Test
public void testFixedRandom() {
    Random random = new Random([1]);
    int value = random.nextInt([2]);
    assertEquals(42, value);
}

Drag options to blanks, or click blank then click option'

A0

B100

C50

DSystem.currentTimeMillis()

Attempts:

3 left

💡 Hint

Common Mistakes

Using dynamic values for seed or bound causes non-deterministic tests.

Using seed 0 and nextInt(100) produces a deterministic value 42.

5fill in blank

hard

Fill all three blanks to create a deterministic test that asserts a fixed string length.

JUnit

@Test
public void testStringLength() {
    String text = "hello";
    int length = text.[1]();
    assertEquals([2], length);
    assertTrue(length [3] 0);
}

Drag options to blanks, or click blank then click option'

Alength

B5

C>

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using size() instead of length() causes compilation errors.

Using length() gets the string length, which is 5 for "hello". The assertion checks length > 0.

Practice

1. What does it mean for a JUnit test to be deterministic?

easy

A. It always produces the same result given the same input

B. It runs faster than other tests

C. It uses random data each time it runs

D. It depends on external services to pass

Solution

1. Step 1: Understand the meaning of deterministic

   Deterministic means something behaves the same way every time under the same conditions.

2. Step 2: Apply this to JUnit tests

   A deterministic test always gives the same result when run with the same input and environment.

3. Final Answer:

   It always produces the same result given the same input -> Option A

4. Quick Check:

   Deterministic = Same result every time [OK]

Hint: Deterministic means repeatable results every run [OK]

Common Mistakes:

• Confusing deterministic with fast execution
• Thinking random data makes tests deterministic
• Assuming external services guarantee determinism

2. Which of the following is the correct way to write a deterministic assertion in JUnit 5?

easy

A. assertNull(getRandomObject());

B. assertTrue(random.nextBoolean());

C. assertFalse(externalServiceCall());

D. assertEquals(expected, actual);

Solution

1. Step 1: Identify deterministic assertions

   Deterministic assertions compare known expected values to actual results.

2. Step 2: Check each option

   assertEquals(expected, actual); compares expected and actual values directly, which is deterministic. Options A, B, and D depend on random or external calls, which are not deterministic.

3. Final Answer:

   assertEquals(expected, actual); -> Option D

4. Quick Check:

   Deterministic assertions compare fixed expected and actual values [OK]

Hint: Use fixed expected values in assertions for determinism [OK]

Common Mistakes:

• Using random values inside assertions
• Calling external services in assertions
• Assuming assertNull is always deterministic

3. Consider this JUnit test snippet:

int result = calculator.add(2, 3);
assertEquals(5, result);

What will be the test result if the add method always returns the sum of its inputs?

medium

A. Test will fail because 2 + 3 is not 5

B. Test will pass because the result matches expected

C. Test will fail due to syntax error

D. Test will be flaky and sometimes pass

Solution

1. Step 1: Understand the add method behavior

   The add method returns the sum of inputs, so add(2, 3) returns 5.

2. Step 2: Check the assertion

   The assertion expects 5 and the actual result is 5, so the assertion passes.

3. Final Answer:

   Test will pass because the result matches expected -> Option B

4. Quick Check:

   Expected 5, got 5 = pass [OK]

Hint: If expected equals actual, test passes [OK]

Common Mistakes:

• Thinking 2 + 3 is not 5
• Assuming syntax error without checking code
• Believing test is flaky without randomness

4. This JUnit test sometimes fails:

@Test
void testRandomNumber() {
  int num = new Random().nextInt(10);
  assertTrue(num == 0);
}

What is the main reason this test is not deterministic?

medium

A. The test depends on random values causing different results

B. The test uses a new Random object each time

C. The assertion condition is incorrect

D. The test method is missing @BeforeEach annotation

Solution

1. Step 1: Identify randomness in the test

   The test generates a random number each run, so output varies.

2. Step 2: Understand impact on determinism

   Because the number changes, the test result can vary, making it non-deterministic.

3. Final Answer:

   The test depends on random values causing different results -> Option A

4. Quick Check:

   Random values cause non-determinism [OK]

Hint: Random values cause flaky tests, avoid them [OK]

Common Mistakes:

• Thinking assertion condition is wrong
• Believing new Random() fixes determinism
• Confusing test annotations with determinism

5. You want to make this test deterministic:

@Test
void testCurrentTime() {
  long time = System.currentTimeMillis();
  assertTrue(time > 0);
}

Which approach best ensures determinism?

hard

A. Remove the assertion to avoid failures

B. Add Thread.sleep(1000) before assertion

C. Replace System.currentTimeMillis() with a fixed mock time value

D. Run the test multiple times and average results

Solution

1. Step 1: Identify non-deterministic part

   System.currentTimeMillis() returns changing time, causing test to vary.

2. Step 2: Apply mocking for fixed value

   Replacing it with a fixed mock value makes the test produce the same result every run.

3. Final Answer:

   Replace System.currentTimeMillis() with a fixed mock time value -> Option C

4. Quick Check:

   Fixed mock values ensure determinism [OK]

Hint: Mock changing values to fixed ones for deterministic tests [OK]

Common Mistakes:

• Using sleep does not fix changing values
• Removing assertions hides problems, not fix determinism
• Averaging results does not guarantee same output