Testing private methods (should you?) in JUnit - Build an Automation Script

Verify behavior of a public method that uses a private method internally

Preconditions (1)

Step 1: Create an instance of the class

Step 2: Call the public method with a specific input

Step 3: Observe the output or behavior influenced by the private method

✅ Expected Result: The public method returns the expected result that depends on the private method's logic

Automation Requirements - JUnit 5

Assertions Needed:

Assert the public method returns the expected value influenced by the private method

Best Practices:

Test only public methods, not private methods directly

Use assertions to verify outputs or side effects

Avoid reflection or changing access modifiers to test private methods

Automated Solution

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

class Calculator {
    private int multiplyByTwo(int number) {
        return number * 2;
    }

    public int doubleAndAddThree(int number) {
        return multiplyByTwo(number) + 3;
    }
}

public class CalculatorTest {

    @Test
    void testDoubleAndAddThree() {
        Calculator calculator = new Calculator();
        int input = 5;
        int expected = 13; // (5 * 2) + 3 = 13
        int actual = calculator.doubleAndAddThree(input);
        assertEquals(expected, actual, "The public method should return correct result based on private method logic");
    }
}

This test class CalculatorTest tests the public method doubleAndAddThree of the Calculator class.

The private method multiplyByTwo is not tested directly. Instead, we verify the public method's output, which depends on the private method.

This approach follows best practices by testing behavior through public interfaces, avoiding fragile tests that depend on private implementation details.

Common Mistakes - 3 Pitfalls

Trying to test private methods directly using reflection

Changing private methods to public just to test them

Not asserting expected results and only calling methods

Bonus Challenge

Now add tests for the public method with multiple input values using parameterized tests

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