Why do we always type cast in Android/Java?

Question

I am writing an Android app and just was curious about why we must always type-cast in Android. I understand that we need to be sure of the type so that our code runs properly, but is there, perhaps, another reason?

Example:

public class Navigation extends Activity {
    private DrawerLayout mDrawerLayout;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        // other irrelevant code
        mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);

In the context of this question, I am asking why we must type-cast the returned value of the findViewById. (I'm also curious how this type casting is done if one could explain that.)

Answer 1

findViewById loads resources from a file. It returns some kind of View, but the Java compiler has no way of knowing that it will be a DrawerLayout (because it is defined in a file outside of Java's compilation).

If you need it to be a DrawerLayout in your code, you have to cast it. That makes sure (at runtime) that it really is such an object (if it is not, you will get a ClassCastException and your program will abort at that point).

The reason you want to cast it as DrawerLayout is that you may want to call methods on the instance that are defined in that class. Sometimes, you may be fine with just the signature of the more general superclass. Then you don't have to cast it.

To be sure, the instance returned by findViewById is a DrawerLayout no matter if you cast it or not. But if you don't cast it, then Java won't let you treat it as a DrawerLayout. That is the point of a type-safe language: Unless the compiler can be sure that an object is of a certain class, it won't let you call methods (or access fields) for that class.

Answer 2

DrawerLayout is a subclass of View. A DrawerLayout object automatically has all the methods that a View has, but defines some methods of its own that are specific to DrawerLayouts. If you don't cast it, you can't use any of those DrawerLayout methods; you can only use the methods defined for all Views.

The type-casting doesn't do anything, really. findViewById is declared to return a View, since it can return all different kinds of Views. But in reality, it will often return an object of some other class, that is a subclass of View. The type-cast just checks to make sure that the returned object is really an instance of DrawerLayout (or one of its subclasses); if not, an exception is thrown, but if it is, you can now look at the object as a DrawerLayout and use the DrawerLayout methods.