How to Test Non-Public Methods in Java Using JUnit?

How can I effectively test non-public (private or protected) methods in my Java classes using JUnit?

Testing non-public methods in Java can be challenging, especially when using JUnit, which primarily focuses on testing public methods. However, there are several strategies you can employ to effectively test these non-public methods without compromising the encapsulation principles of object-oriented programming. Below, we explore various techniques for achieving this.

import org.junit.jupiter.api.Test;
import java.lang.reflect.Method;

public class MyClassTest {
    @Test
    public void testPrivateMethod() throws Exception {
        MyClass myClass = new MyClass();
        Method method = MyClass.class.getDeclaredMethod("myPrivateMethod");
        method.setAccessible(true);
        String result = (String) method.invoke(myClass);
        assertEquals("expectedResult", result);
    }
}

Causes

• Encapsulation limits the visibility of methods and fields, making direct testing difficult.
• JUnit focuses on public interfaces by default, which can leave critical code untested.

Solutions

• Utilize reflection to access private methods within your test classes, although this approach should be used sparingly as it breaks encapsulation principles.
• Refactor the non-public methods into a new class with a public interface. This promotes better testability and design.
• Create package-private methods that your test classes in the same package can access if possible, allowing for direct testing while maintaining some level of encapsulation.
• Use frameworks like Mockito or PowerMock that allow for more advanced testing techniques, such as mocking and spying on private methods.

Mistake: Using reflection too frequently can lead to maintenance challenges in your codebase.

Solution: Reserve reflection for simple use cases where no other approach is feasible, such as legacy code.

Mistake: Testing private methods can encourage poor design by making developers focus on implementation details instead of behavior.

Solution: Prioritize testing public methods—if private methods need testing, consider if they should be extracted to public interfaces.