Question

How to inject method (using reflection) into super class without extending tested class?
I need to replace getExternalData() to test the logic inside someMethod(). I cannot change tested classes – because of that I was thinking about injecting method.

I have flowing classes:

public class B extends A{
  public String someMethod(){
    String someString = super.getExternalData();
    //logic to be tested - manipulating on someString
    return someString;
  }
}

public class A{
  public String getExternalData(){
    //some Logic that generates "externalData" string
    return externalData;
  }
}

And test:

public class CesTest{
  @Test
  public void someMethodTest()(
    B instance = new B();
    //...
    //...
    assertEquals("expectedData", instance.someMethod());
  }
}

Answer 1

Simple answer is you can’t without doing some really odd stuff that would result in your not testing the class under test but some odd mocked-out version of the class under test. You need to do whatever setup is required to have getExternalData return the appropriate value. That being said, class A should be written in such a was that a mock could be injected to provide this external data without hitting an external resource.