Java Unit Test Mock, target class new() method requires depencies mock

Question

I want to test A class.

In A class constructor method, a private final dependency d is created. I want to mock d. Here is the code

public class A {
    public A () throws FileNotFoundException {}
    private final D d = new D();
}

public class D { 
    public D () throws FileNotFoundException {
        Reader reader = new InputStreamReader(new FileInputStream("filePath"));
        // other code...
    }
}

Since the filePath is a special path fitting in the docker config, me and my mentor decided to use MOCK to return a d to test A class.

I did not know how to mock this situation. Previously, I did use FieldSetter in Mockito to solve simple cases.

Answer 1

The first option would be to declare D as a constructor argument. This is also called dependency injection and can be useful for testing. Additionally, you can still have a constructor without any arguments:

public class A {
    public A (D d) throws FileNotFoundException {
        this.d = d;
    }
    public A () throws FileNotFoundException {
        this(new D());
    }

    private final D d;
} 

@Test
void test() {
    D mock = mock(D.class);
    A tested = new A(mock);
    // ...
}

Alternatively, you can test this code even more elegantly, without any mocks. To do so, we need to apply the same technique to D and use dependency injection for passing a Reader:

public class D { 
    public D () throws FileNotFoundException {
        this(new InputStreamReader(new FileInputStream("filePath")));
    }
    public D (Reader reader) throws FileNotFoundException {
        // other code...
    }
}

Since Reader is an interface, we can use different implementations for it, inside the test. For instance, we can use a StringReader:

@Test
void test() {
    D d = new D(new StringReader("my test file content"));
    A tested = new A(d);
    // ...
}