Android intent extra message versus static variable

By using extras to start SecondActivity, you make it more reusable. Many of the stock activities work this way, that's why you can reuse for example the camera activity to take and save photos, because it does not make assumptions about who is calling it.

In your case SecondActivity depends on FirstActivity having been loaded in the JVM. I would not count on that, and it's certainly not a recommended practice to have such dependency between activities. Don't do this. Use extras to pass values between activities, as recommended by the SDK.