Java Testing Private methods with reflection: ExampleClass demonstration

Java Unit Test: Exercise-8 with Solution

Write a Java program to explore strategies for testing private methods in a class.

Sample Solution:

Java Code:

// ExampleClass.java

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class ExampleClass {

    private int addTwoNumbers(int a, int b) {
        return a + b;

    // Public method that calls the private method
    public int performAddition(int a, int b) {
        return addTwoNumbers(a, b);

    // Testing private method using reflection
    public static int testPrivateMethod(ExampleClass instance, int a, int b)
            throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        // Obtain the Class object for the class containing the private method
        Class clazz = instance.getClass();

        // Specify the name and parameter types of the private method
        Class[] parameterTypes = {int.class, int.class};
        Method privateMethod = clazz.getDeclaredMethod("addTwoNumbers", parameterTypes);

        // Make the private method accessible for testing

        // Invoke the private method on the provided instance
        return (int) privateMethod.invoke(instance, a, b);

    public static void main(String[] args) {
        ExampleClass example = new ExampleClass();

        // Test the public method that calls the private method
        int result = example.performAddition(3, 5);
        System.out.println("Result from public method: " + result);

        try {
            // Test the private method using reflection
            int privateMethodResult = testPrivateMethod(example, 3, 5);
            System.out.println("Result from private method: " + privateMethodResult);
        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {

Sample Output:

Result from public method: 8
Result from private method: 8


In the exercise above,

  • ExampleClass: Defines a class with a private method (addTwoNumbers) and a public method (performAddition) that calls the private method.
  • testPrivateMethod Method: Uses reflection to test the private method. This method takes an instance of "ExampleClass", the parameters for the private method, and uses reflection to invoke the private method.
  • main Method: Demonstrates how to test the public and private methods. The public method is tested directly, and the private method is tested using the testPrivateMethod utility.


Flowchart: Java Testing Private methods with reflection: ExampleClass demonstration
Flowchart: Java Testing Private methods with reflection: ExampleClass demonstration

Java Code Editor:

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