Deterministic tests in JUnit - Build an Automation Script

Verify that the random number generator produces the same sequence with a fixed seed

Preconditions (2)

Step 1: Create a Random object with seed 12345

Step 2: Generate 5 random integers using nextInt()

Step 3: Record the generated numbers

Step 4: Create another Random object with the same seed 12345

Step 5: Generate 5 random integers again

Step 6: Compare both sequences of numbers

✅ Expected Result: Both sequences of 5 random integers are exactly the same, proving the test is deterministic

Automation Requirements - JUnit 5

Assertions Needed:

Assert that the two sequences of random numbers are equal

Best Practices:

Use fixed seed for Random to ensure deterministic output

Use assertArrayEquals or assertEquals for comparing sequences

Keep test isolated and repeatable

Use descriptive test method names

Automated Solution

JUnit

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

public class DeterministicRandomTest {

    @Test
    void testRandomSequenceIsDeterministic() {
        int seed = 12345;
        Random random1 = new Random(seed);
        int[] sequence1 = new int[5];
        for (int i = 0; i < 5; i++) {
            sequence1[i] = random1.nextInt();
        }

        Random random2 = new Random(seed);
        int[] sequence2 = new int[5];
        for (int i = 0; i < 5; i++) {
            sequence2[i] = random2.nextInt();
        }

        assertArrayEquals(sequence1, sequence2, "Random sequences should be identical with the same seed");
    }
}

This test creates two Random objects with the same fixed seed (12345). Because the seed is fixed, the sequence of numbers generated by both Random instances will be the same.

We generate 5 integers from each Random object and store them in arrays. Then we use assertArrayEquals to verify both arrays are identical.

This ensures the test is deterministic, meaning it will always produce the same result every time it runs, which is important for reliable automated tests.

The test method is annotated with @Test from JUnit 5, and the code uses clear variable names and comments to keep it easy to understand.

Common Mistakes - 3 Pitfalls

Not using a fixed seed for the Random object

Comparing random numbers one by one with separate assertions

Using System.out.println for verification instead of assertions

Bonus Challenge

Now add data-driven testing with 3 different seeds to verify deterministic sequences for each seed

Show Hint

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