@CsvFileSource for external CSV in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of JUnit test using @CsvFileSource with external CSV

Consider this JUnit 5 test method that uses @CsvFileSource to read data from an external CSV file named data.csv located in src/test/resources. What will be the output when the test runs?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.CsvFileSource;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class CalculatorTest {

    @ParameterizedTest
    @CsvFileSource(resources = "/data.csv", numLinesToSkip = 1)
    void testAdd(int a, int b, int expectedSum) {
        assertEquals(expectedSum, a + b);
    }
}

A The test passes for all rows in data.csv except the header line.

B The test fails because the CSV file path is incorrect.

C The test throws a runtime exception due to missing default constructor.

D The test is ignored because @CsvFileSource requires inline CSV data.

💡 Hint

Check how @CsvFileSource loads files from the resources folder and the effect of numLinesToSkip.

The @CsvFileSource annotation loads the CSV file from the classpath resource path. The numLinesToSkip=1 skips the header line. The test method receives each row's values as parameters and asserts the sum. Since the file is correctly located and formatted, the test passes for all data rows.

❓ assertion

intermediate

Correct assertion for @CsvFileSource test with string inputs

Given this parameterized test using @CsvFileSource with string inputs from an external CSV, which assertion correctly verifies that the input string is not empty?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.CsvFileSource;
import static org.junit.jupiter.api.Assertions.*;

public class StringTest {

    @ParameterizedTest
    @CsvFileSource(resources = "/strings.csv")
    void testNotEmpty(String input) {
        // Which assertion is correct here?
    }
}

A assertFalse(input.isEmpty());

B assertTrue(input == null);

C assertEquals(input.length(), 0);

D assertNull(input);

💡 Hint

Think about how to check that a string is not empty.

assertFalse(input.isEmpty()) checks that the string is not empty. The other options either check for null or empty incorrectly.

❓ locator

advanced

Correct resource path for @CsvFileSource external CSV file

You want to load a CSV file named test-data.csv located in src/test/resources/data/ folder using @CsvFileSource. Which resource path is correct?

A "src/test/resources/data/test-data.csv"

B "data/test-data.csv"

C "/data/test-data.csv"

D "/src/test/resources/data/test-data.csv"

💡 Hint

Remember that resource paths are relative to the classpath root and start with a slash.

The resource path in @CsvFileSource must start with a slash and be relative to the classpath root. So "/data/test-data.csv" is correct. The full file system path or missing slash are incorrect.

🔧 Debug

advanced

Identify the cause of failure in @CsvFileSource test

This test fails with java.lang.IllegalArgumentException: CSV file not found. What is the most likely cause?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.CsvFileSource;

public class DebugTest {

    @ParameterizedTest
    @CsvFileSource(resources = "/testdata.csv")
    void testData(int x, int y) {
        // test logic
    }
}

A The test method parameters do not match the CSV columns.

B The CSV file is not located in the classpath root or resources folder.

C The CSV file has a header but numLinesToSkip is not set.

D The test method is missing the @Test annotation.

💡 Hint

Check the file location and classpath setup.

The error indicates the file cannot be found in the classpath. The most common cause is that the CSV file is not placed in the src/test/resources folder or the path is incorrect.

❓ framework

expert

Behavior of @CsvFileSource with empty lines in external CSV

When using @CsvFileSource with an external CSV file that contains empty lines, what happens during test execution?

A JUnit fails to start tests and reports a syntax error in the CSV.

B JUnit skips empty lines automatically and runs tests only on valid rows.

C JUnit treats empty lines as rows with empty strings and passes empty parameters.

D JUnit throws a <code>ParameterResolutionException</code> due to missing parameters.

💡 Hint

Consider how JUnit handles parameterized tests when parameters are missing.

Empty lines in the CSV cause JUnit to receive no parameters for that test invocation, leading to a ParameterResolutionException because the test method expects parameters.

Practice

1. What is the main purpose of using @CsvFileSource in JUnit tests?

easy

A. To run the same test multiple times with different data from a CSV file

B. To write data into a CSV file during test execution

C. To generate random test data automatically

D. To skip tests based on CSV file content

Solution

1. Step 1: Understand the role of @CsvFileSource

   @CsvFileSource provides external CSV data to parameterized tests, allowing multiple runs with different inputs.

2. Step 2: Compare options with this role

   Only To run the same test multiple times with different data from a CSV file correctly describes running tests multiple times with CSV data. Others describe unrelated actions.

3. Final Answer:

   To run the same test multiple times with different data from a CSV file -> Option A

4. Quick Check:

   @CsvFileSource runs tests with CSV data = B [OK]

Hint: Remember: @CsvFileSource feeds test data, not writes or skips [OK]

Common Mistakes:

• Thinking @CsvFileSource writes data instead of reading
• Confusing it with random data generation
• Assuming it controls test skipping

2. Which of the following is the correct syntax to skip the header line in a CSV file when using @CsvFileSource?

easy

A. @CsvFileSource(resources = "/data.csv", skipLines = 1)

B. @CsvFileSource(resources = "/data.csv", skipHeader = true)

C. @CsvFileSource(resources = "/data.csv", ignoreFirstLine = true)

D. @CsvFileSource(resources = "/data.csv", numLinesToSkip = 1)

Solution

1. Step 1: Recall the correct attribute for skipping lines

   The correct attribute to skip lines in @CsvFileSource is numLinesToSkip.

2. Step 2: Match options with correct attribute

   Only @CsvFileSource(resources = "/data.csv", numLinesToSkip = 1) uses numLinesToSkip = 1 which skips the first line (header).

3. Final Answer:

   @CsvFileSource(resources = "/data.csv", numLinesToSkip = 1) -> Option D

4. Quick Check:

   Skip header line = numLinesToSkip = 1 [OK]

Hint: Use numLinesToSkip to skip headers, not skipHeader or ignoreFirstLine [OK]

Common Mistakes:

• Using non-existent attributes like skipHeader or ignoreFirstLine
• Confusing skipLines with numLinesToSkip
• Forgetting to skip header causing test failures

3. Given the CSV file data.csv with content:

name,age
Alice,30
Bob,25

and the test method:

@ParameterizedTest
@CsvFileSource(resources = "/data.csv", numLinesToSkip = 1)
void testUser(String name, int age) {
    System.out.println(name + "-" + age);
}

What will be the printed output when the test runs?

medium

A. name-age
Alice-30
Bob-25

B. Alice-30
Bob-25

C. Alice-30

D. Test fails due to header line

Solution

1. Step 1: Understand numLinesToSkip effect

   numLinesToSkip=1 skips the header line "name,age" so only data lines remain.

2. Step 2: Identify printed lines

   Test runs twice, printing "Alice-30" and "Bob-25" each on separate lines.

3. Final Answer:

   Alice-30 Bob-25 -> Option B

4. Quick Check:

   Header skipped, prints data lines only = D [OK]

Hint: Skip header to avoid printing it as data [OK]

Common Mistakes:

• Not skipping header causing it to print as data
• Expecting header line to print
• Confusing output format with CSV file content

4. Consider this test snippet:

@ParameterizedTest
@CsvFileSource(resources = "/users.csv")
void testUser(String name, int age) {
    assertTrue(age > 0);
}

The CSV file users.csv has a header line. What is the likely problem when running this test?

medium

A. Test will fail because header line is treated as data causing NumberFormatException

B. Test will pass because header is ignored automatically

C. Test will fail due to missing @Test annotation

D. Test will skip all lines and run zero times

Solution

1. Step 1: Check if header is skipped

   No numLinesToSkip is set, so header line is treated as data.

2. Step 2: Understand impact on parsing

   Trying to convert header "age" to int causes NumberFormatException, failing the test.

3. Final Answer:

   Test will fail because header line is treated as data causing NumberFormatException -> Option A

4. Quick Check:

   Header not skipped causes parse error = A [OK]

Hint: Always skip header lines to avoid parsing errors [OK]

Common Mistakes:

• Assuming header is skipped automatically
• Forgetting to add numLinesToSkip=1
• Confusing @ParameterizedTest with @Test

5. You want to test a method that calculates discounts using data from a CSV file with columns: product,price,discountRate. The CSV has a header line. Which is the best way to write the parameterized test method signature and annotation to correctly read and use this data?

hard

A. @ParameterizedTest
@CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1)
void testDiscount(String product, String price, String discountRate) { ... }

B. @ParameterizedTest
@CsvFileSource(resources = "/discounts.csv")
void testDiscount(String product, int price, int discountRate) { ... }

C. @ParameterizedTest
@CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1)
void testDiscount(String product, double price, double discountRate) { ... }

D. @ParameterizedTest
@CsvFileSource(resources = "/discounts.csv", skipHeader = true)
void testDiscount(String product, double price, double discountRate) { ... }

Solution

1. Step 1: Identify header skipping attribute

   Use numLinesToSkip = 1 to skip the CSV header line.

2. Step 2: Match method parameters to CSV columns and types

   Product is String, price and discountRate are decimal numbers, so use double for price and discountRate.

3. Step 3: Check options for correctness

   @ParameterizedTest @CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1) void testDiscount(String product, double price, double discountRate) { ... } correctly uses numLinesToSkip=1 and matching parameter types. @ParameterizedTest @CsvFileSource(resources = "/discounts.csv") void testDiscount(String product, int price, int discountRate) { ... } misses skipping header and uses int instead of double. @ParameterizedTest @CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1) void testDiscount(String product, String price, String discountRate) { ... } uses String for numeric fields. @ParameterizedTest @CsvFileSource(resources = "/discounts.csv", skipHeader = true) void testDiscount(String product, double price, double discountRate) { ... } uses invalid attribute skipHeader.

4. Final Answer:

   @ParameterizedTest @CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1) void testDiscount(String product, double price, double discountRate) { ... } -> Option C

5. Quick Check:

   Skip header with numLinesToSkip=1 and match types = C [OK]

Hint: Use numLinesToSkip=1 and match parameter types to CSV columns [OK]

Common Mistakes:

• Using skipHeader instead of numLinesToSkip
• Not skipping header causing parse errors
• Mismatching parameter types with CSV data