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How Do You Make a Private Method Visible in a Test Class?

Published in Testing 2 mins read

You can't directly make a private method visible in a test class. Private methods are designed to be internal to the class and inaccessible from outside. However, you can achieve the desired functionality through various techniques:

1. Reflection

Reflection allows you to access and manipulate private members of a class at runtime. Using the java.lang.reflect package, you can get the method object and invoke it.

Example:

import java.lang.reflect.Method;

public class TestClass {

    public void testPrivateMethod() throws Exception {
        MyClass myClass = new MyClass();
        Method method = MyClass.class.getDeclaredMethod("privateMethod");
        method.setAccessible(true);
        method.invoke(myClass);
    }
}

Note: Reflection is a powerful tool, but it should be used with caution as it can impact performance and break encapsulation.

2. Test-Specific Accessor Methods

Create public accessor methods in the class under test, specifically designed for testing purposes. These methods call the private methods, making them accessible from your test class.

Example:

public class MyClass {

    private void privateMethod() {
        // ...
    }

    public void testPrivateMethod() {
        privateMethod();
    }
}

3. Inheritance

If you can extend the class under test, you can override the private method in the subclass, making it accessible. However, this approach might not be always feasible or desirable.

Example:

public class TestClass extends MyClass {

    @Override
    protected void privateMethod() {
        // ...
    }
}

4. Mocking Frameworks

Mocking frameworks like Mockito or JMockit allow you to create mock objects that mimic the behavior of the class under test. You can then configure the mock object to call the private method directly.

Example:

import org.mockito.Mockito;

public class TestClass {

    @Test
    public void testPrivateMethod() {
        MyClass myClass = Mockito.mock(MyClass.class);
        Mockito.doCallRealMethod().when(myClass).privateMethod();
        myClass.privateMethod();
    }
}

Choosing the best approach depends on your specific needs and the context of your testing. Consider the trade-offs and choose the method that best balances testability and code maintainability.

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