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);
}

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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.