@CsvSource for inline CSV data in JUnit

Introduction

@CsvSource lets you run the same test many times with different data. It saves time and avoids repeating code.

You want to test a method with many input values quickly.

You have small sets of test data that fit well in your test code.

You want to check different combinations of inputs and expected outputs.

You want to keep test data close to the test logic for easy reading.

You want to avoid creating external files for simple test data.

Syntax

JUnit

@ParameterizedTest
@CsvSource({"value1, value2, expectedResult", "value3, value4, expectedResult2"})
void testMethod(type param1, type param2, type expected) {
    // test code
}

Each string inside @CsvSource represents one test case with comma-separated values.

Values are passed as parameters to the test method in the same order.

Examples

Tests addition with two sets of numbers.

JUnit

@ParameterizedTest
@CsvSource({"1, 2, 3", "4, 5, 9"})
void addTest(int a, int b, int expected) {
    assertEquals(expected, a + b);
}

Tests string length for different words.

JUnit

@ParameterizedTest
@CsvSource({"apple, 5", "banana, 6"})
void lengthTest(String word, int length) {
    assertEquals(length, word.length());
}

Tests boolean to uppercase string conversion.

JUnit

@ParameterizedTest
@CsvSource({"true, TRUE", "false, FALSE"})
void caseTest(boolean flag, String text) {
    assertEquals(text, String.valueOf(flag).toUpperCase());
}

Sample Program

This test runs three times with different numbers. It checks if adding two numbers equals the expected result.

JUnit

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

public class CalculatorTest {

    @ParameterizedTest
    @CsvSource({"1, 2, 3", "10, 20, 30", "5, 7, 12"})
    void testAdd(int a, int b, int expected) {
        assertEquals(expected, a + b);
    }
}

Important Notes

Use quotes around values if they contain spaces or special characters.

Empty strings can be represented by double quotes "" inside the CSV data.

Keep the number of CSV values and method parameters the same to avoid errors.

Summary

@CsvSource helps run one test many times with different data.

It keeps test data inside the test code for easy reading.

Use it for small, simple sets of test data without external files.

Practice

1.

What is the main purpose of using @CsvSource in JUnit tests?

easy

A. To run the same test multiple times with different input data

B. To connect to a database and fetch test data

C. To generate random test data automatically

D. To create a graphical user interface for tests

Solution

1. Step 1: Understand the role of @CsvSource

   @CsvSource allows specifying multiple sets of input data inline for a single test method.

2. Step 2: Identify the purpose in testing

   This lets the test run repeatedly with each data set, checking behavior for different inputs.

3. Final Answer:

   To run the same test multiple times with different input data -> Option A

4. Quick Check:

   @CsvSource runs tests with multiple inputs [OK]

Hint: Remember: @CsvSource repeats tests with different data [OK]

Common Mistakes:

• Thinking @CsvSource connects to databases
• Confusing it with random data generation
• Assuming it creates UI elements

2.

Which of the following is the correct syntax to use @CsvSource for a test method with two parameters int a and int b?

@ParameterizedTest
@CsvSource({"1, 2", "3, 4"})
void testSum(int a, int b) { ... }

easy

A. @CsvSource({"1; 2", "3; 4"})

B. @CsvSource({"1, 2", "3, 4"})

C. @CsvSource({"1 2", "3 4"})

D. @CsvSource({"1|2", "3|4"})

Solution

1. Step 1: Check the delimiter used in @CsvSource

   The default delimiter for @CsvSource is a comma (,), so values should be separated by commas.

2. Step 2: Match the syntax with parameters

   Since the method expects two int parameters, each CSV line must have two comma-separated values.

3. Final Answer:

   @CsvSource({"1, 2", "3, 4"}) -> Option B

4. Quick Check:

   Comma separates values in @CsvSource [OK]

Hint: Use commas to separate values inside @CsvSource strings [OK]

Common Mistakes:

• Using semicolons or spaces instead of commas
• Using wrong delimiters like |
• Not matching parameter count with CSV values

3.

What will be the output of the following JUnit test when run with @CsvSource?

@ParameterizedTest
@CsvSource({"2, 3, 5", "4, 5, 9", "1, 1, 3"})
void testAdd(int a, int b, int expected) {
    assertEquals(expected, a + b);
}

medium

A. All tests fail

B. All tests pass

C. The first test fails

D. The last test fails

Solution

1. Step 1: Evaluate each test case

   Test 1: 2 + 3 = 5 matches expected 5 (pass) Test 2: 4 + 5 = 9 matches expected 9 (pass) Test 3: 1 + 1 = 2 does not match expected 3 (fail)

2. Step 2: Determine overall result

   Since one test fails, the test suite reports failure for that case.

3. Final Answer:

   The last test fails -> Option D

4. Quick Check:

   Check sums against expected values [OK]

Hint: Add inputs and compare to expected for each CSV line [OK]

Common Mistakes:

• Assuming all tests pass without checking expected values
• Ignoring the last test case
• Confusing parameter order

4.

Identify the error in this JUnit test using @CsvSource:

@ParameterizedTest
@CsvSource({"apple, 1", "banana"})
void testFruitCount(String fruit, int count) {
    assertTrue(count > 0);
}

medium

A. assertTrue is used incorrectly

B. Method parameters do not match test data types

C. Missing second value in second CSV line causes error

D. @CsvSource cannot have more than one line

Solution

1. Step 1: Check CSV lines against method parameters

   The method expects two parameters: a String and an int. The first CSV line has two values, but the second line has only one.

2. Step 2: Understand the impact of missing value

   Missing the second value in the second line causes a runtime error because the test cannot map parameters properly.

3. Final Answer:

   Missing second value in second CSV line causes error -> Option C

4. Quick Check:

   Each CSV line must have values matching parameters [OK]

Hint: Count CSV values to match method parameters exactly [OK]

Common Mistakes:

• Ignoring missing values in CSV lines
• Assuming assertTrue usage causes error
• Thinking @CsvSource limits lines to one

5.

You want to test a method isAdult(int age) that returns true if age is 18 or more, false otherwise. Which @CsvSource usage correctly tests ages 17, 18, and 20 with expected results?

hard

A. @CsvSource({"17, false", "18, true", "20, true"})

B. @CsvSource({"17; false", "18; true", "20; true"})

C. @CsvSource({"17 true", "18 true", "20 true"})

D. @CsvSource({"17|false", "18|true", "20|true"})

Solution

1. Step 1: Use correct delimiter for @CsvSource

   The default delimiter is a comma, so values must be comma-separated.

2. Step 2: Match test data with method parameters

   Each line has an int age and a boolean expected result, correctly formatted with commas.

3. Final Answer:

   @CsvSource({"17, false", "18, true", "20, true"}) -> Option A

4. Quick Check:

   Comma-separated values match parameters and expected results [OK]

Hint: Use commas to separate all values in @CsvSource [OK]

Common Mistakes:

• Using semicolons or spaces instead of commas
• Using wrong delimiters like |
• Not matching parameter count or types