java mocking custom objects

public class MainClass  {

    public void makeCall() {
        CustomObject obj = new CustomObject();
        obj.testMethod();
    }
}

I want to unit test the makeCall(). So i have to mock the CustomObject in the above sample. I am using Mockito as the mock framework.But when i mock the object like mock(CustomObject.class). makeCall() method always call the actual object not the mocked one.Is there anyway to mock the local objects. Please help