How to force derived class to call super method? (Like Android does)

Question

I was wondering, when creating new Activity classes and then overriding the onCreate() method, in eclipse I always get auto added: super.onCreate(). How does this happen? Is there a java keyword in the abstract or parent class that forces this?

I don't know if it is illegal not to call the super class, but I remember in some methods that I got a exception thrown for not doing this. Is this also built-in into java? Can you use some keyword to do that? Or how is it done?

Solution 1

Here's the source of Activity#onCreate() - it is almost all comments (original - see line ~800):

/**
 * Called when the activity is starting.  This is where most initialization
 * should go: calling {@link #setContentView(int)} to inflate the
 * activity's UI, using {@link #findViewById} to programmatically interact
 * with widgets in the UI, calling
 * {@link #managedQuery(android.net.Uri , String[], String, String[], String)} to retrieve
 * cursors for data being displayed, etc.
 *
 * <p>You can call {@link #finish} from within this function, in
 * which case onDestroy() will be immediately called without any of the rest
 * of the activity lifecycle ({@link #onStart}, {@link #onResume},
 * {@link #onPause}, etc) executing.
 *
 * <p><em>Derived classes must call through to the super class's
 * implementation of this method.  If they do not, an exception will be
 * thrown.</em></p>
 *
 * @param savedInstanceState If the activity is being re-initialized after
 *     previously being shut down then this Bundle contains the data it most
 *     recently supplied in {@link #onSaveInstanceState}.  <b><i>Note: Otherwise it is null.</i></b>
 *
 * @see #onStart
 * @see #onSaveInstanceState
 * @see #onRestoreInstanceState
 * @see #onPostCreate
 */
protected void onCreate(Bundle savedInstanceState) {
    mVisibleFromClient = !mWindow.getWindowStyle().getBoolean(
            com.android.internal.R.styleable.Window_windowNoDisplay, false);
    mCalled = true;
}

so, my guess would be that the ADT Eclipse plugin is what's auto-adding that call to super.onCreate() for you. It's a total guess, though.

Solution 2

This is added in the support annotation library:

dependencies {
    compile 'com.android.support:support-annotations:22.2.0'
}

http://tools.android.com/tech-docs/support-annotations

@CallSuper

Solution 3

If you want to force subclasses to execute the parent class' logic, a common pattern is something like the following:

public abstract class SuperClass implements SomeInterface
{
    // This is the implementation of the interface method
    // Note it's final so it can't be overridden
    public final Object onCreate()
    {
        // Hence any logic right here always gets run
        // INSERT LOGIC

        return doOnCreate();

        // If you wanted you could instead create a reference to the
        // object returned from the subclass, and then do some
        // post-processing logic here
    }

    protected abstract Object doOnCreate();
}

public class Concrete extends SuperClass
{
    @Override
    protected Object doOnCreate()
    {
        // Here's where the concrete class gets to actually do
        // its onCreate() logic, but it can't stop the parent
        // class' bit from running first

        return "Hi";
    }
}

This doesn't actually answer your question about what prompts Eclipse to automatically insert a superclass call into the implementation; but then I don't think that's the way to go anyway as this can always be deleted.

You can't actually enforce that a method must call the superclass' version with a Java keyword, or anything like that. I suspect that your exceptions simply came from some code in the parent class checking expected invariants, or something, that were invalidated by your approach. Note that this is subtly different from throwing an exception because you failed to call super.onCreate().

Solution 4

If you want to make absolutely sure that the superclass-method is called as well, you must trick a bit: Don't allow the superclass-method to be overwritten, but have it call an overridable protected method.

class Super
{
   public final void foo() {
      foo_stuff();
      impl_stuff();
   }

   protected void impl_stuff() {
      some_stuff_that_you_can_override();
   }
}

class Base extends Super
{
  protected void impl_stuff() { 
     my_own_idea_of_impl();
  }
}

That way, the user must call Super.foo() or Base.foo() and it will always be the base-class version as it was declared as final. The implementation-specific stuff is in impl_stuff(), which can be overriden.

Solution 5

To answer your actual question, the auto-creation of the call to super.onCreate() is a feature of the ADT plugin. In java, you cannot directly force a subclass to call the super implementation of a method, afaik (see the pattern described in other answers for work-around). However, keep in mind that in Android, you are not instantiating Activity objects (or Service objects) directly - you pass an Intent to the system and the system instantiates the object and calls onCreate() upon it (along with other lifecycle methods). So the system has a direct object reference to the Activity instance and is able to check (presumably) some Boolean that is set to true in the superclass implementation of onCreate(). Although I don't know exactly how it is implemented, it probably looks something like this:

class Activity
{
  onCreate()
  {
    superCalled = true;
    ...
  }
  ...
}

And in the "system" level class that receives the Intent and instantiates the Activity object from it:

...
SomeActivitySubclass someActivitySubclassObject = new SomeActivitySubclass();
someActivitySubclassObject.onCreate();
if (!someActivityObject.isSuperCalled())
{
  Exception e = new Exception(...) //create an exception with appropriate details
  throw e;
}

My guess is it's probably slightly more complex than that, but you get the idea. Eclipse automatically creates the call because the ADT plugin tells it to, as a convenience. Happy coding!

View more solutions

Comments

[Peterdk]

I guess the mCalled = true also is used for a possible exception. Maybe not in the onCreate() but when indeed such a exception is thrown it will use that simple pattern.

[SMBiggs]

Yep, you hit the nail on the head here. Thanks.

[Rizwan Sohaib]

Not tried yet but read the documentation. This is going to be awesome I guess. This should be the accepted answer.

[Rizwan Sohaib]

Sorry but I need to change my comment. It didn't forced to implement super method. I was able to run app without supper call.

[Le_Enot]

All of the other answers are totally crap, except this. Should be accepted.

[frostymarvelous]

@RizwanSohaib it won't force you to do anything. It will highlight and let you know you need to call it. To do anything more complex, you will need either an annotation processor or to implement the logic yourself. depending on the IDE, it should prevent you from building as well.

[Võ Quang Hòa]

Perfect answer. Thanks a lot

[fthdgn]

You can enforce @CallSuper on release builds by using lint check "MissingSuperCall" tools.android.com/tips/lint-checks

[smarteist]

Thanks , this is exactly what i want.

[sebasira]

This should be the answer!

[ankitkpd]

Looks like Template method Design pattern

[adi9090]

There is an issue with this approach if you have multi module project, and you need to use this in non Android module, it will not work.