使用像这样的继承是一个坏主意

Question

Hello I'm inexperienced in programming and learning slowly by making a project for android.

The problem is I'm using a lot of the same code in my activities. Is it a bad idea to share code using inheritance.

example:

Class Main extends Activity
       //shared code

           _
Class a     |
Class b     | - extends Main
Class c    _|

my usage example:

public class Global extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    setVolumeControlStream(AudioManager.STREAM_MUSIC);
}

public void toastMessage(String tMsg, int tLength) {
    Toast.makeText(getApplicationContext(), tMsg, tLength).show();
}

public void nextActivity(Class<?> classId, ImageButton btnId) {
    AlphaAnimation alpha;
    Vibrator shake = (Vibrator) getSystemService(Context.VIBRATOR_SERVICE);
    if (btnId!= null) {
        alpha = new AlphaAnimation(0.5F, 1.0F);
        alpha.setDuration(50);
        alpha.setFillAfter(false);
        btnId.startAnimation(alpha);
    }
    shake.vibrate(50);
    Intent goToNextActivity = new Intent(getApplicationContext(), classId);
    startActivity(goToNextActivity);
}

Class a,b,c could extend Global and use:

  nextActivity(a.class, btnA);
  toastMessage("example", Toast.LENGTH_SHORT);

Answer 1

In general is not a good idea use inheritance only for code reuse. Inheritance use the "is a" paradigm. Look if is better for you to use Composition. Follow the advice of Matthijs, post always your code.

Answer 2

Inheritance is used to say that a class belongs to some "category" for example:

a class named "goblin" extends the class "enemy". Inside "enemy" there is simple code that applies to all enemys (walking, ai, attacking etc), inside "goblin" class there is code like the graphics, maybe some dialog, animation or stuff that are not shared to all enemies.

There are cases where this is very useful like controlling all enemies, despite some of them are actually goblins or orcs or whatever. About code re-use you can use methods to do it, keep in mind that your code must be easy to read (even if it's just for you) so don't put all steps inside one class, for example (it's going to be a video game example again :P )

while(game == true)
{
    getInput();

    moveHero();

    moveEnemies();

    applyGravity();

    drawFrame();
}

Even if there are cases like "moveHero" and "moveEnemies" are going to be applied one after another almost always, it's not a good idea to group them together, so you can easily tell what's going on in your program.