How do I test a class that has private methods, fields or inner classes ?

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 Testing private methods, fields, or inner classes can be challenging because they are not accessible from outside the class. There are a few approaches you can take to test these elements of your code.


Testing private methods, fields, and inner classes can be challenging because they are not meant to be accessed from outside of the class. However, there are some techniques you can use to test them:


Reflection: You can use the Java Reflection API to access private methods and fields. However, this is not a recommended practice as it can make your tests fragile and reduce the maintainability of your code.

// Example code for accessing private method using reflection in Java


import java.lang.reflect.Method;

public class PrivateMethodTester {

  public static void main(String[] args) throws Exception {

    MyClass obj = new MyClass();

    Class cls = obj.getClass();

    Method method = cls.getDeclaredMethod("myPrivateMethod", new Class[] { int.class });

    method.setAccessible(true);

    int result = (int) method.invoke(obj, new Object[] { 42 });

    System.out.println(result);

  }

}

class MyClass {

  private int myPrivateMethod(int value) {

    return value + 1;

  }

}

Make them package-private: You can change the visibility of the private methods and fields to package-private (no keyword), which makes them accessible from other classes in the same package. This way, you can write test classes in the same package and access the package-private methods and fields.

Use inner test classes: You can write test classes as inner classes of the class being tested and use them to access the private methods and fields. This way, the inner test classes can access the private methods and fields as if they were package-private.

Extract the private logic into a separate class: If the private methods or fields contain complex logic that needs to be tested, you can extract this logic into a separate, publicly accessible class and test it independently.

Use powermock or other similar testing frameworks: PowerMock is a testing framework that allows you to test private methods and fields, among other things. However, it's important to use these types of frameworks sparingly and only when necessary, as they can make your tests more complex and harder to maintain.

In general, it's best to test only the public interface of a class, and if the private methods or fields are complex enough to require testing, it might indicate that they should be refactored into their own class.

  1. Wrapper class: Another option is to create a wrapper class that wraps the private elements you want to test. You can then write tests against the public methods of the wrapper class, which in turn exercise the private elements.
// Example code for testing private method using wrapper class in Java

public class PrivateMethodTester {

  public static void main(String[] args) {
    MyWrapper wrapper = new MyWrapper(new MyClass());
    int result = wrapper.callPrivateMethod(42);
    System.out.println(result);
  }
}

class MyWrapper {
  private MyClass obj;

  public MyWrapper(MyClass obj) {
    this.obj = obj;
  }

  public int callPrivateMethod(int value) {
    return obj.myPrivateMethod(value);
  }
}

class MyClass {
  private int myPrivateMethod(int value) {
    return value + 1;
  }
}

  1. PowerMock or similar frameworks: Another option is to use a testing framework that provides support for mocking and testing private elements, such as PowerMock. These frameworks allow you to override the behavior of private elements and test them in isolation, without the need for reflection or wrapper classes.
// Example code for testing private method using PowerMock in Java import org.junit.Test; import org.junit.runner.RunWith; import org.powermock.api.mockito.PowerMockito; import org.powermock.core.classloader.annotations.PrepareForTest; import org.powermock.modules.junit4.PowerMockRunner; @RunWith(PowerMockRunner.class) @PrepareForTest(MyClass.class) public class PrivateMethodTester { @Test public void testPrivateMethod() throws Exception { MyClass obj = PowerMockito.spy(new MyClass()); PowerMockito.when(obj, "myPrivateMethod", 42).thenReturn




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John Justin is a Software Engineer since last 9 years,Developing applications in Java and related technologies,blogger,writer and social worker.

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Hi, my name is John Justin. I'm a software engineer living in Bentonville. This blog is to provide you with daily outfit ideas and share my personal style.This is my personal blog. Opinions expressed in my posts may or may not coincide with those of my employer.

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