@ValueSource for simple values in JUnit

Introduction

@ValueSource helps run the same test many times with different simple inputs. It saves time and avoids repeating code.

You want to check a method with many numbers or strings quickly.

You need to test if a function works for several fixed values.

You want to avoid writing many similar test methods for different inputs.

You want to catch errors that happen only for some specific inputs.

You want to keep your test code clean and easy to read.

Syntax

JUnit

@ParameterizedTest
@ValueSource(ints = {1, 2, 3})
void testWithInts(int number) {
    // test code using number
}

@ValueSource supports simple types like ints, longs, strings, doubles, and chars.

The parameter in the test method must match the type of values in @ValueSource.

Examples

This test runs 3 times, once for each fruit string.

JUnit

@ParameterizedTest
@ValueSource(strings = {"apple", "banana", "cherry"})
void testWithStrings(String fruit) {
    assertNotNull(fruit);
}

This test checks that each number is positive.

JUnit

@ParameterizedTest
@ValueSource(ints = {10, 20, 30})
void testWithInts(int number) {
    assertTrue(number > 0);
}

This test runs for each character and checks it is a letter.

JUnit

@ParameterizedTest
@ValueSource(chars = {'a', 'b', 'c'})
void testWithChars(char letter) {
    assertTrue(Character.isLetter(letter));
}

Sample Program

This test runs 3 times with different words. It checks each word is not empty.

JUnit

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

public class SimpleValueSourceTest {

    @ParameterizedTest
    @ValueSource(strings = {"hello", "world", "JUnit"})
    void testStringLength(String word) {
        assertTrue(word.length() > 0);
    }
}

Important Notes

Use @ValueSource only for simple, single-parameter values.

For multiple parameters, use other providers like @CsvSource.

Keep your test method parameter type matching the @ValueSource type exactly.

Summary

@ValueSource runs one test multiple times with simple values.

It helps test many inputs without repeating code.

Works well for ints, strings, chars, doubles, and longs.

Practice

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

easy

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

B. To create a new test class automatically

C. To skip tests based on conditions

D. To generate random test data during runtime

Solution

1. Step 1: Understand the role of @ValueSource

   @ValueSource provides a list of simple values to run a test repeatedly with each value.

2. Step 2: Compare options with this role

   Only To run the same test multiple times with different simple input values correctly describes running the same test multiple times with different inputs.

3. Final Answer:

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

4. Quick Check:

   @ValueSource = run test multiple times [OK]

Hint: Remember: @ValueSource repeats tests with simple inputs [OK]

Common Mistakes:

• Thinking @ValueSource creates new test classes
• Confusing @ValueSource with conditional test skipping
• Assuming it generates random data

2. Which of the following is the correct syntax to use @ValueSource with integers in JUnit 5?

easy

A. @ValueSource(numbers = {1, 2, 3})

B. @ValueSource(values = {1, 2, 3})

C. @ValueSource(int = {1, 2, 3})

D. @ValueSource(ints = {1, 2, 3})

Solution

1. Step 1: Recall correct attribute name for integers

   The attribute for integer arrays in @ValueSource is ints.

2. Step 2: Check syntax correctness

   @ValueSource(ints = {1, 2, 3}) uses @ValueSource(ints = {1, 2, 3}), which is the correct syntax.

3. Final Answer:

   @ValueSource(ints = {1, 2, 3}) -> Option D

4. Quick Check:

   ints attribute = correct syntax [OK]

Hint: Use 'ints' attribute for integer arrays in @ValueSource [OK]

Common Mistakes:

• Using wrong attribute names like 'values' or 'numbers'
• Missing 's' in 'ints'
• Using singular 'int' instead of 'ints'

3. Given the following test method, what will be the output when run?

@ParameterizedTest
@ValueSource(strings = {"apple", "banana"})
void testLength(String fruit) {
    System.out.println(fruit.length());
}

medium

A. 5 and 6 printed on separate lines

B. 6 and 5 printed on separate lines

C. Only 5 printed once

D. Compilation error due to missing annotation

Solution

1. Step 1: Identify input values from @ValueSource

   The strings are "apple" (length 5) and "banana" (length 6).

2. Step 2: Understand test execution

   The test runs twice, printing the length of each string in order: 5 then 6.

3. Final Answer:

   5 and 6 printed on separate lines -> Option A

4. Quick Check:

   "apple"=5, "banana"=6 [OK]

Hint: Check string lengths in order given by @ValueSource [OK]

Common Mistakes:

• Mixing order of outputs
• Assuming only one run instead of multiple
• Forgetting @ParameterizedTest annotation

4. Identify the error in this JUnit 5 test using @ValueSource:

@ParameterizedTest
@ValueSource(strings = {"test", null})
void testNotNull(String input) {
    assertNotNull(input);
}

medium

A. Missing @Test annotation

B. assertNotNull is not a valid assertion

C. Using null in @ValueSource causes a runtime error

D. Strings array must not contain more than one value

Solution

1. Step 1: Check @ValueSource limitations

   @ValueSource does not support null values; including null causes errors.

2. Step 2: Verify other parts

   assertNotNull is valid, @ParameterizedTest is present, and multiple strings are allowed.

3. Final Answer:

   Using null in @ValueSource causes a runtime error -> Option C

4. Quick Check:

   null not allowed in @ValueSource [OK]

Hint: Never put null in @ValueSource arrays [OK]

Common Mistakes:

• Thinking null is allowed in @ValueSource
• Confusing @ParameterizedTest with @Test
• Believing assertNotNull is invalid

5. How can you use @ValueSource to test a method that accepts both int and long values?

hard

A. Use @ValueSource with ints only and cast to long inside the test

B. Use two separate test methods each with @ValueSource for ints and longs

C. Use @ValueSource with longs only and cast to int inside the test

D. Use @ValueSource with mixed int and long arrays in one method

Solution

1. Step 1: Understand @ValueSource type restrictions

   @ValueSource attributes accept only one primitive type array per annotation (ints or longs, not mixed).

2. Step 2: Choose approach to test both types

   To test both int and long inputs, create two test methods each with its own @ValueSource for the correct type.

3. Final Answer:

   Use two separate test methods each with @ValueSource for ints and longs -> Option B

4. Quick Check:

   Separate methods for int and long [OK]

Hint: Use separate tests for different primitive types [OK]

Common Mistakes:

• Trying to mix int and long in one @ValueSource
• Casting inside test instead of separate methods
• Assuming @ValueSource supports mixed types