Testing private methods (should you?) in JUnit - Test Execution Trace

Test Overview

This test checks the behavior of a public method that internally uses a private method. It verifies the final output without directly testing the private method, following best practices.

Test Code - JUnit

JUnit

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

class Calculator {
    private int doubleValue(int x) {
        return x * 2;
    }

    public int tripleDouble(int x) {
        return doubleValue(x) * 3;
    }
}

public class CalculatorTest {
    @Test
    void testTripleDouble() {
        Calculator calc = new Calculator();
        int result = calc.tripleDouble(4);
        assertEquals(24, result, "tripleDouble should return doubleValue times 3");
    }
}

Execution Trace - 8 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	-	PASS
2	CalculatorTest creates Calculator instance	Calculator object ready	-	PASS
3	Calls public method tripleDouble(4)	Inside Calculator.tripleDouble method	-	PASS
4	tripleDouble calls private method doubleValue(4)	Inside Calculator.doubleValue method	-	PASS
5	doubleValue returns 8	Back in tripleDouble method	-	PASS
6	tripleDouble returns 8 * 3 = 24	Result ready for assertion	Check if result equals 24	PASS
7	JUnit asserts result == 24	Test assertion executed	assertEquals(24, result)	PASS
8	Test ends successfully	Test passed	-	PASS

Failure Scenario

Failing Condition: The public method tripleDouble returns incorrect value due to private method logic error

Execution Trace Quiz - 3 Questions

Why does the test call the public method instead of the private method directly?

A Because public methods are faster to test

B Because private methods do not contain any logic

C Because private methods cannot be accessed directly from tests

D Because the test framework only supports public methods

Key Result

Test the public behavior of a class rather than private methods directly. Private methods are implementation details and should be tested indirectly through public methods.