Replace implementation of instantiated class without touching the code (java)

Question

I have legacy code I don\'t want to touch.

public class LegacyCode{
    public LegacyCode() {
        Service s = new ClassA();
        s.getMessage();
    }         


        

Answer 1

Using PowerMock you can create a mock (or stub) for a constructor code. The answer is taken from this link. I'll try to convert it to match exactly to your use case:

@RunWith(PowerMockRunner.class)
@PrepareForTest(ClassA.class)
public class LegacyTester {
    @Test
    public void testService() {
         // Inject your stub
         PowerMock.createMock(ClassA.class);
         Service stub = new MyServiceStub();
         PowerMock.expectNew(ClassA.class).andReturn(stub);
         PowerMock.replay(stub, ClassA.class);

         // Implement test logic here

         LegacyCode legacyCode = new LegacyCode();

         // Implement Test steps

        // Call verify if you want to make sure the ClassA constructor was called
        PowerMock.verify(stub, ClassA.class)
    }
}

This way you inject your stub when the call to ClassA constructor happens, without changing the legacy code. Hope that's what you needed.