Unit testing in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Flutter unit test?

Consider this simple Dart function and its unit test. What will the test output be when run?

Flutter

int add(int a, int b) {
  return a + b;
}

void main() {
  test('add returns correct sum', () {
    expect(add(2, 3), 5);
  });
}

A Test fails because add returns 6 instead of 5.

B Test throws a runtime exception because add is undefined.

C Test fails due to a syntax error in the test code.

D Test passes successfully with no errors.

💡 Hint

Check what the add function returns and what the expect statement checks.

The add function correctly returns the sum of two integers. The test expects add(2, 3) to be 5, which matches the function output, so the test passes.

📝 Syntax

intermediate

Which option contains a syntax error in this Flutter unit test?

Identify the option that will cause a syntax error when running this Flutter unit test code.

Flutter

void main() {
  test('string length test', () {
    String text = 'hello';
    expect(text.length, 5);
  });
}

A

void main() {
  test('string length test', () {
    String text = 'hello'
    expect(text.length, 5);
  });
}

B

void main() {
  test('string length test', () {
    String text = 'hello';
    expect(text.length, 5);
  });
}

C

void main() {
  test('string length test', () {
    String text = 'hello';
    expect(text.length == 5);
  });
}

D

void main() {
  test('string length test', () {
    String text = 'hello';
    expect(text.length, 6);
  });
}

💡 Hint

Look for missing semicolons or wrong expect usage.

Option A is missing a semicolon after String text = 'hello', causing a syntax error. Other options have different issues but no syntax errors.

❓ lifecycle

advanced

What happens if you forget to call setUp() in a Flutter test suite?

In a Flutter test file, you have a setUp() function that initializes some variables before each test. What is the effect of not calling setUp() explicitly?

A setUp() is automatically called before each test, so no effect.

B Tests will fail because variables are not initialized properly.

C Tests will run but with stale or null variables causing unpredictable results.

D Tests will throw a runtime error immediately.

💡 Hint

Think about how the test framework manages setUp functions.

Flutter's test package automatically calls setUp() before each test. You do not call it manually. Forgetting to call it has no effect.

🔧 Debug

advanced

Why does this Flutter unit test fail unexpectedly?

Given this test code, why does the test fail even though the function seems correct? int multiply(int a, int b) { return a * b; } void main() { test('multiply test', () { expect(multiply(2, 3), 5); }); }

A The multiply function has a syntax error causing failure.

B The test framework is not imported, so test() is undefined.

C The expected value 5 is incorrect; multiply(2, 3) returns 6.

D The test fails because multiply returns a string instead of int.

💡 Hint

Check the expected value against the actual function output.

The multiply function returns 6 for inputs 2 and 3, but the test expects 5, causing the test to fail.

🧠 Conceptual

expert

What is the main benefit of using mocks in Flutter unit testing?

Why do developers use mock objects in Flutter unit tests?

A To speed up the app by replacing real code with faster mock code in production.

B To simulate dependencies and isolate the unit under test for reliable testing.

C To automatically generate UI components during tests without manual coding.

D To avoid writing any test code by using prebuilt mock tests.

💡 Hint

Think about how mocks help when your code depends on external services or complex objects.

Mocks simulate dependencies so tests focus only on the unit's logic, making tests reliable and fast.

Practice

1.

What is the main purpose of unit testing in Flutter?

easy

A. To check small parts of code for bugs early

B. To design the user interface

C. To deploy the app to the store

D. To write documentation for the app

Solution

1. Step 1: Understand unit testing purpose and compare options

   Unit testing focuses on testing small parts of code independently to find bugs early. Only To check small parts of code for bugs early matches the purpose of unit testing; others relate to different app development stages.

2. Final Answer:

   To check small parts of code for bugs early -> Option A

3. Quick Check:

   Unit testing = check small code parts early [OK]

Hint: Unit tests check small code parts, not UI or deployment [OK]

Common Mistakes:

• Confusing unit testing with UI design
• Thinking unit tests deploy the app
• Believing unit tests write documentation

2.

Which of the following is the correct syntax to write a simple test in Flutter?

?

easy

A. expect('description', test(() => 1 + 1 == 2));

B. test('description', () { expect(1 + 1, 2); });

C. test 'description' { expect(1 + 1 == 2); }

D. test(description, expect(1 + 1, 2));

Solution

1. Step 1: Recall Flutter test syntax and match options

   The correct syntax uses test('description', () { expect(actual, expected); }); Only test('description', () { expect(1 + 1, 2); }); matches the correct syntax with proper function and expect call.

2. Final Answer:

   test('description', () { expect(1 + 1, 2); }); -> Option B

3. Quick Check:

   Correct test syntax = test('description', () { expect(1 + 1, 2); }); [OK]

Hint: Remember test() takes a description and a function with expect() inside [OK]

Common Mistakes:

• Missing parentheses or curly braces
• Swapping test and expect arguments
• Using wrong function call syntax

3.

What will be the output of this test code?

int add(int a, int b) => a + b;

test('add function test', () {
  expect(add(2, 3), 5);
  expect(add(0, 0), 0);
  expect(add(-1, 1), 0);
});

Choose the correct test result.

medium

A. All tests fail

B. First test fails, others pass

C. Second test fails, others pass

D. All tests pass successfully

Solution

1. Step 1: Evaluate add function calls and check expect statements

   add(2, 3) = 5, add(0, 0) = 0, add(-1, 1) = 0, all match expected values. All expect statements compare actual and expected values correctly, so all pass.

2. Final Answer:

   All tests pass successfully -> Option D

3. Quick Check:

   Correct add results = all tests pass [OK]

Hint: Check function output matches expected values exactly [OK]

Common Mistakes:

• Assuming negative numbers cause failure
• Misreading expected values
• Ignoring zero as valid input

4.

Identify the error in this Flutter unit test code:

test('string length test', () {
  var str = 'hello';
  expect(str.length, '5');
});

medium

A. The test description is missing

B. The variable str is not defined

C. The expected value should be an integer, not a string

D. The expect function is used incorrectly

Solution

1. Step 1: Check expect arguments types and type mismatch effect

   str.length returns an integer (5), but expected value is a string ('5'). Expect compares values strictly, so integer 5 != string '5', causing test failure.

2. Final Answer:

   The expected value should be an integer, not a string -> Option C

3. Quick Check:

   Type mismatch in expect causes failure [OK]

Hint: Match expected value type exactly with actual value type [OK]

Common Mistakes:

• Using string instead of int for length
• Forgetting to define test description
• Misusing expect function parameters

5.

You want to test a function that returns a list of even numbers from a given list. Which test code correctly verifies this behavior?

List<int> filterEven(List<int> nums) => nums.where((n) => n % 2 == 0).toList();

// Which test is correct?

hard

A. test('filterEven test', () { expect(filterEven([1,2,3,4]), [2,4]); });

B. test('filterEven test', () { expect(filterEven([1,2,3,4]), [1,3]); });

C. test('filterEven test', () { expect(filterEven([1,2,3,4]), [2,3]); });

D. test('filterEven test', () { expect(filterEven([1,2,3,4]), [1,2,3,4]); });

Solution

1. Step 1: Understand filterEven function and check expected lists

   It returns a list of even numbers from input list. Only test('filterEven test', () { expect(filterEven([1,2,3,4]), [2,4]); }); expects the correct even numbers [2,4]. Others expect incorrect lists.

2. Final Answer:

   test('filterEven test', () { expect(filterEven([1,2,3,4]), [2,4]); }); -> Option A

3. Quick Check:

   Expected even numbers list = test('filterEven test', () { expect(filterEven([1,2,3,4]), [2,4]); }); [OK]

Hint: Expect only even numbers in the output list [OK]

Common Mistakes:

• Expecting odd numbers instead of even
• Including all numbers without filtering
• Mixing even and odd numbers in expected list