Argument conversion in JUnit - Build an Automation Script

Test argument conversion in JUnit parameterized test

Preconditions (2)

Step 1: Create a parameterized test method that accepts a String argument

Step 2: Use @ConvertWith annotation to convert the String argument to a custom type

Step 3: Provide test data as strings

Step 4: Run the test method with converted arguments

Step 5: Verify the converted argument is correctly transformed inside the test

✅ Expected Result: The test runs successfully with the argument converted from String to the custom type, and assertions pass confirming correct conversion

Automation Requirements - JUnit 5

Assertions Needed:

Assert that the converted argument matches expected values

Best Practices:

Use @ParameterizedTest with @ValueSource for input strings

Implement a custom ArgumentConverter class

Use @ConvertWith annotation on the test method parameter

Keep test methods clear and focused

Use assertions from org.junit.jupiter.api.Assertions

Automated Solution

JUnit

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.converter.ArgumentConversionException;
import org.junit.jupiter.params.converter.ArgumentConverter;
import org.junit.jupiter.params.converter.ConvertWith;
import org.junit.jupiter.params.provider.ValueSource;

class Temperature {
    private final double celsius;

    public Temperature(double celsius) {
        this.celsius = celsius;
    }

    public double getCelsius() {
        return celsius;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Temperature other = (Temperature) obj;
        return Double.compare(other.celsius, celsius) == 0;
    }

    @Override
    public int hashCode() {
        return Double.hashCode(celsius);
    }
}

class TemperatureConverter implements ArgumentConverter {
    @Override
    public Object convert(Object source, java.lang.reflect.Parameter context) throws ArgumentConversionException {
        if (!(source instanceof String)) {
            throw new ArgumentConversionException("Can only convert from String");
        }
        String str = (String) source;
        try {
            // Assume input like "36.5C" - remove 'C' and parse double
            if (!str.endsWith("C")) {
                throw new ArgumentConversionException("String must end with 'C'");
            }
            double value = Double.parseDouble(str.substring(0, str.length() - 1));
            return new Temperature(value);
        } catch (NumberFormatException e) {
            throw new ArgumentConversionException("Invalid temperature format", e);
        }
    }
}

public class ArgumentConversionTest {

    @ParameterizedTest
    @ValueSource(strings = {"36.5C", "0C", "100C"})
    void testTemperatureConversion(@ConvertWith(TemperatureConverter.class) Temperature temp) {
        Assertions.assertNotNull(temp);
        // Check that the celsius value matches expected values
        String input = temp.getCelsius() + "C";
        Assertions.assertTrue(input.endsWith("C"));
    }
}

This test uses JUnit 5's parameterized test feature with @ValueSource to provide string inputs representing temperatures.

The TemperatureConverter class implements ArgumentConverter to convert the input string (like "36.5C") into a Temperature object by parsing the numeric part.

The test method testTemperatureConversion uses @ConvertWith to apply this converter to the parameter. Inside the test, assertions check that the converted object is not null and that the value ends with 'C', confirming correct conversion.

This approach keeps the test clean and focused on verifying argument conversion works as expected.

Common Mistakes - 4 Pitfalls

{'mistake': 'Not implementing ArgumentConverter interface correctly', 'why_bad': "The converter won't be recognized or will throw errors during test execution", 'correct_approach': 'Implement the convert method properly and handle exceptions to provide clear error messages'}

Using raw strings in test without conversion

Not validating input format inside converter

Hardcoding assertions without considering converted object properties

Bonus Challenge

Now add data-driven testing with 3 different temperature string inputs including edge cases like negative and zero values

Show Hint

Practice

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

easy

A. To generate random test data automatically

B. To skip certain test cases during execution

C. To convert input arguments to the required test method parameter types

D. To log test execution details

Solution

1. Step 1: Understand the role of argument conversion

   Argument conversion changes input data into the type needed by the test method parameters.

2. Step 2: Identify the purpose of @ConvertWith

   The @ConvertWith annotation tells JUnit which converter class to use to transform the input argument.

3. Final Answer:

   To convert input arguments to the required test method parameter types -> Option C

4. Quick Check:

   Argument conversion = convert input arguments [OK]

Hint: Remember: @ConvertWith changes input type for tests [OK]

Common Mistakes:

• Thinking @ConvertWith skips tests
• Confusing conversion with logging
• Assuming it generates test data

2. Which of the following is the correct way to declare a custom argument converter class in JUnit?

easy

A. public class MyConverter extends SimpleArgumentConverter { ... }

B. public class MyConverter implements ConverterInterface { ... }

C. public class MyConverter extends ConverterBase { ... }

D. public class MyConverter implements ArgumentConverter { ... }

Solution

1. Step 1: Recall JUnit's recommended base class for converters

   JUnit provides SimpleArgumentConverter as a base class to extend for custom converters.

2. Step 2: Match the correct class declaration

   The correct syntax is to extend SimpleArgumentConverter, not implement interfaces or extend unknown classes.

3. Final Answer:

   public class MyConverter extends SimpleArgumentConverter { ... } -> Option A

4. Quick Check:

   Extend SimpleArgumentConverter = correct converter class [OK]

Hint: Custom converters extend SimpleArgumentConverter class [OK]

Common Mistakes:

• Implementing ArgumentConverter interface directly
• Extending non-existent base classes
• Using wrong interface names

3. Given the following code snippet, what will be the output when the test runs?

@ParameterizedTest
@ValueSource(strings = {"42"})
void testWithConversion(@ConvertWith(MyConverter.class) Integer number) {
    System.out.println(number + 8);
}

public class MyConverter extends SimpleArgumentConverter {
    @Override
    protected Object convert(Object source, Class targetType) {
        return Integer.parseInt(source.toString());
    }
}

medium

A. 50

B. 42

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand the conversion process

   The string "42" is converted to Integer 42 by MyConverter using Integer.parseInt.

2. Step 2: Calculate the printed output

   The test method adds 8 to the converted Integer 42, resulting in 50 printed.

3. Final Answer:

   50 -> Option A

4. Quick Check:

   42 + 8 = 50 [OK]

Hint: Convert string to int, then add 8 for output [OK]

Common Mistakes:

• Assuming no conversion happens
• Expecting string concatenation instead of addition
• Thinking code causes errors

4. Identify the error in this custom converter code:

public class MyConverter extends SimpleArgumentConverter {
    @Override
    protected Object convert(Object source, Class targetType) {
        if (targetType != Integer.class) {
            throw new IllegalArgumentException("Conversion only supports Integer");
        }
        return source;
    }
}

medium

A. The method should return Integer, not Object

B. The source is returned without converting to Integer

C. The targetType check should use equals() method

D. The class should implement ArgumentConverter interface

Solution

1. Step 1: Analyze the convert method's return value

   The method returns source directly, which is a String, not converted to Integer.

2. Step 2: Identify the missing conversion

   It should parse the source string to Integer before returning, e.g., Integer.parseInt(source.toString()).

3. Final Answer:

   The source is returned without converting to Integer -> Option B

4. Quick Check:

   Return value must be converted type [OK]

Hint: Always convert source to target type before returning [OK]

Common Mistakes:

• Returning source without conversion
• Using != instead of equals() for class check (less critical)
• Confusing return type requirements

5. You want to write a parameterized test that accepts a CSV string like "John,25,true" and converts it into a custom Person object with fields name (String), age (int), and active (boolean). Which approach correctly uses argument conversion in JUnit?

hard

A. Use @ValueSource(strings = {"John,25,true"}) and parse CSV inside the test method manually

B. Use @CsvSource with three separate parameters and no converter

C. Create a converter implementing ArgumentProvider and use @ArgumentsSource annotation

D. Create a converter extending SimpleArgumentConverter that splits the CSV and constructs Person; annotate parameter with @ConvertWith using this converter

Solution

1. Step 1: Understand the need for custom conversion

   The input is a single CSV string that must be converted into a Person object before the test runs.

2. Step 2: Identify the correct use of @ConvertWith

   Creating a converter extending SimpleArgumentConverter that parses the CSV and returns a Person object, then annotating the test parameter with @ConvertWith, is the proper approach.

3. Final Answer:

   Create a converter extending SimpleArgumentConverter that splits the CSV and constructs Person; annotate parameter with @ConvertWith using this converter -> Option D

4. Quick Check:

   Custom converter + @ConvertWith = correct approach [OK]

Hint: Use @ConvertWith with a converter that builds Person from CSV [OK]

Common Mistakes:

• Parsing CSV inside test method instead of converter
• Using wrong annotation like @ArgumentsSource
• Passing multiple parameters without conversion