Line and branch coverage in JUnit - Test Execution Trace

Test Overview

This test checks a simple method that returns the maximum of two numbers. It verifies both line coverage and branch coverage by testing different input cases to cover all decision paths.

Test Code - JUnit 5

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

class MaxUtil {
    public static int max(int a, int b) {
        if (a > b) {
            return a;
        } else {
            return b;
        }
    }
}

class MaxUtilTest {
    @Test
    public void testMaxFirstGreater() {
        assertEquals(5, MaxUtil.max(5, 3));
    }

    @Test
    public void testMaxSecondGreater() {
        assertEquals(7, MaxUtil.max(4, 7));
    }

    @Test
    public void testMaxEqual() {
        assertEquals(6, MaxUtil.max(6, 6));
    }
}

Execution Trace - 8 Steps

Step	Action	System State	Assertion	Result
1	Test runner starts and loads MaxUtilTest class	JUnit test environment initialized	—	PASS
2	Runs testMaxFirstGreater: calls MaxUtil.max(5, 3)	Inside max method with a=5, b=3	Check if a > b (5 > 3) is true	PASS
3	Returns 5 from max method	max method returns 5	Assert returned value equals 5	PASS
4	Runs testMaxSecondGreater: calls MaxUtil.max(4, 7)	Inside max method with a=4, b=7	Check if a > b (4 > 7) is false	PASS
5	Returns 7 from max method	max method returns 7	Assert returned value equals 7	PASS
6	Runs testMaxEqual: calls MaxUtil.max(6, 6)	Inside max method with a=6, b=6	Check if a > b (6 > 6) is false	PASS
7	Returns 6 from max method	max method returns 6	Assert returned value equals 6	PASS
8	Test runner finishes all tests	All tests executed	All assertions passed	PASS

Failure Scenario

Failing Condition: The max method returns incorrect value for a test case

Execution Trace Quiz - 3 Questions

Which test case covers the branch where 'a > b' is true?

A testMaxSecondGreater

B testMaxEqual

C testMaxFirstGreater

D None of the above

Key Result

To achieve full branch coverage, tests must include cases for all decision outcomes, including equal values when comparing.

Practice

1. What does line coverage measure in JUnit testing?

easy

A. Whether the program runs without errors

B. Whether all possible input values are tested

C. Whether the test cases run in parallel

D. Whether each line of code is executed during tests

Solution

1. Step 1: Understand line coverage concept

   Line coverage measures if every line of code is executed at least once during testing.

2. Step 2: Compare with other options

   Options A, B, and D describe different testing aspects, not line coverage.

3. Final Answer:

   Whether each line of code is executed during tests -> Option D

4. Quick Check:

   Line coverage = executed lines [OK]

Hint: Line coverage = Did each line run? Check executed lines [OK]

Common Mistakes:

• Confusing line coverage with branch coverage
• Thinking line coverage checks input values
• Assuming line coverage means no errors

2. Which of the following is the correct JUnit annotation to mark a test method for coverage measurement?

easy

A. @Test

B. @Coverage

C. @RunCoverage

D. @Measure

Solution

1. Step 1: Recall JUnit test annotation

   The standard annotation to mark a test method in JUnit is @Test.

2. Step 2: Verify other options

   Annotations like @Coverage, @RunCoverage, and @Measure do not exist in JUnit.

3. Final Answer:

   @Test -> Option A

4. Quick Check:

   JUnit test method = @Test [OK]

Hint: JUnit test methods always use @Test annotation [OK]

Common Mistakes:

• Assuming coverage requires special annotation
• Using non-existent annotations
• Confusing coverage tools with JUnit annotations

3. Consider this Java method tested with JUnit:

public int max(int a, int b) {
  if (a > b) {
    return a;
  } else {
    return b;
  }
}

If your tests only call max(5, 3), what coverage is achieved?

medium

A. 100% line coverage, 100% branch coverage

B. 100% line coverage, 50% branch coverage

C. 50% line coverage, 100% branch coverage

D. 50% line coverage, 50% branch coverage

Solution

1. Step 1: Analyze lines executed by max(5, 3)

   Calling max(5, 3) executes the if condition and return a;, as well as the else block's closing brace, so all lines are executed, resulting in 100% line coverage.

2. Step 2: Analyze branches covered

   Only the 'if' branch (a > b true) is taken; the 'else' branch is not tested, so 50% branch coverage.

3. Final Answer:

   100% line coverage, 50% branch coverage -> Option B

4. Quick Check:

   One path = full line coverage, partial branch coverage [OK]

Hint: Test all if-else paths for full branch coverage [OK]

Common Mistakes:

• Thinking one test achieves 100% line coverage
• Confusing line coverage with branch coverage
• Ignoring else branch execution

4. You wrote tests for this method:

public boolean isPositive(int n) {
  if (n > 0) {
    return true;
  }
  return false;
}

But coverage report shows branch coverage less than 100%. What is the likely cause?

medium

A. Tests never call with n ≤ 0, missing else branch coverage

B. Tests have syntax errors causing incomplete runs

C. JUnit does not support branch coverage

D. Method has no branches to cover

Solution

1. Step 1: Identify branches in method

   The method has two branches: if (n > 0) true, else false.

2. Step 2: Understand branch coverage less than 100%

   If tests never call with n ≤ 0, the else branch is not executed, lowering branch coverage.

3. Final Answer:

   Tests never call with n ≤ 0, missing else branch coverage -> Option A

4. Quick Check:

   Branch coverage needs both if and else tested [OK]

Hint: Test both true and false conditions for full branch coverage [OK]

Common Mistakes:

• Assuming syntax errors cause coverage issues
• Believing JUnit lacks branch coverage support
• Ignoring else branch execution

5. Given this method:

public int classify(int x) {
  if (x > 0) {
    if (x % 2 == 0) {
      return 1;
    } else {
      return 2;
    }
  } else {
    return 3;
  }
}

What is the minimum number of JUnit test cases needed to achieve 100% line and branch coverage?

hard

A. 4

B. 2

C. 3

D. 5

Solution

1. Step 1: Identify branches

   There are three branches: outer if (x > 0) true and false, and inner if (x % 2 == 0) true and false.

2. Step 2: Determine test cases to cover all branches

   Tests needed: one with x > 0 and even (return 1), one with x > 0 and odd (return 2), and one with x ≤ 0 (return 3).

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Three tests cover all branches and lines [OK]

Hint: Cover all if-else paths with minimal tests [OK]

Common Mistakes:

• Thinking two tests cover all branches
• Ignoring nested if branches
• Overestimating tests needed