Argument conversion in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of JUnit parameterized test with argument conversion

Consider the following JUnit 5 parameterized test using a custom argument converter. What will be the output when the test runs?

JUnit

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 static org.junit.jupiter.api.Assertions.*;

class Temperature {
    private final double celsius;
    Temperature(double celsius) { this.celsius = celsius; }
    double getCelsius() { return celsius; }
}

class TemperatureConverter implements ArgumentConverter {
    @Override
    public Object convert(Object source, java.lang.reflect.Parameter context) throws ArgumentConversionException {
        if (source instanceof String s) {
            try {
                double f = Double.parseDouble(s);
                return new Temperature((f - 32) * 5 / 9);
            } catch (NumberFormatException e) {
                throw new ArgumentConversionException("Invalid temperature format");
            }
        }
        throw new ArgumentConversionException("Unsupported source type");
    }
}

public class TempTest {
    @ParameterizedTest
    @org.junit.jupiter.params.provider.ValueSource(strings = {"32", "212", "100"})
    void testFreezingAndBoiling(@ConvertWith(TemperatureConverter.class) Temperature temp) {
        assertTrue(temp.getCelsius() >= 0 && temp.getCelsius() <= 100);
    }
}

A Test fails with NumberFormatException.

B Test fails with AssertionError for input "100".

C Test throws ArgumentConversionException for input "100".

D All tests pass successfully.

💡 Hint

Think about the conversion formula and the assertion range.

The converter converts Fahrenheit strings to Celsius correctly. Inputs "32", "212", and "100" Fahrenheit convert to 0, 100, and 37.777... Celsius respectively, all within the asserted range 0 to 100, so all tests pass.

❓ assertion

intermediate

Correct assertion for converted argument in JUnit test

Given a JUnit parameterized test that converts a String to an Integer using a custom converter, which assertion correctly verifies the integer is positive?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.converter.ConvertWith;
import static org.junit.jupiter.api.Assertions.*;

class PositiveIntegerConverter implements org.junit.jupiter.params.converter.ArgumentConverter {
    @Override
    public Object convert(Object source, java.lang.reflect.Parameter context) {
        return Integer.parseInt(source.toString());
    }
}

public class NumberTest {
    @ParameterizedTest
    @org.junit.jupiter.params.provider.ValueSource(strings = {"5", "10", "-3"})
    void testPositive(@ConvertWith(PositiveIntegerConverter.class) Integer number) {
        // Which assertion is correct here?
    }
}

A assertTrue(number > 0);

B assertFalse(number <= 0);

C assertEquals(true, number > 0);

D assertNotNull(number);

💡 Hint

Choose the assertion that directly checks positivity clearly and simply.

assertTrue(number > 0); is the clearest and most direct assertion to verify the number is positive. Others are less clear or do not check positivity explicitly.

🔧 Debug

advanced

Identify the cause of ArgumentConversionException in JUnit test

This JUnit parameterized test throws ArgumentConversionException at runtime. What is the most likely cause?

JUnit

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.converter.ArgumentConverter;
import org.junit.jupiter.params.converter.ArgumentConversionException;
import org.junit.jupiter.params.converter.ConvertWith;

class BooleanStringConverter implements ArgumentConverter {
    @Override
    public Object convert(Object source, java.lang.reflect.Parameter context) throws ArgumentConversionException {
        if (source instanceof String s) {
            if (s.equalsIgnoreCase("true")) return Boolean.TRUE;
            if (s.equalsIgnoreCase("false")) return Boolean.FALSE;
            throw new ArgumentConversionException("Invalid boolean string");
        }
        throw new ArgumentConversionException("Unsupported source type");
    }
}

public class BoolTest {
    @ParameterizedTest
    @org.junit.jupiter.params.provider.ValueSource(strings = {"true", "false", "yes"})
    void testBoolean(@ConvertWith(BooleanStringConverter.class) Boolean value) {
        assertNotNull(value);
    }
}

A The converter does not support Boolean type as target.

B The test method signature is missing @Test annotation.

C The input "yes" is not handled by the converter, causing ArgumentConversionException.

D The assertion assertNotNull(value) fails for input "false".

💡 Hint

Check which input strings the converter accepts.

The converter only accepts "true" or "false" strings (case-insensitive). The input "yes" is not recognized and causes ArgumentConversionException.

❓ framework

advanced

Best practice for argument conversion in JUnit 5 parameterized tests

Which practice is recommended when implementing a custom ArgumentConverter in JUnit 5?

A Return null silently if conversion input is invalid.

B Throw ArgumentConversionException with a clear message when conversion fails.

C Catch all exceptions and return a default value without notifying the test.

D Use System.exit() to stop tests on conversion failure.

💡 Hint

Think about how JUnit reports conversion errors.

Throwing ArgumentConversionException with a clear message helps JUnit report conversion failures properly and helps debugging. Returning null or default values silently hides errors.

🧠 Conceptual

expert

Effect of argument conversion failure on JUnit 5 parameterized test execution

If a custom ArgumentConverter throws ArgumentConversionException for one parameter value in a JUnit 5 parameterized test, what happens to the test execution?

A Only the test case with the failing parameter is skipped; other parameter values are tested.

B JUnit retries the conversion automatically until it succeeds or times out.

C The test passes but logs a warning about the conversion failure.

D The test run fails immediately and no further parameter values are tested.

💡 Hint

Consider how JUnit handles parameterized test failures per argument.

JUnit reports the test case with the failing argument conversion as failed or aborted but continues running other parameterized test cases with valid conversions.