Question

I am trying to come up with an exception handling strategy for my Android application (although this may also apply to any Java app). For example, I am looking at the delete() ContentProvider function in the example Notepad application:

public int delete(Uri uri, String where, String[] whereArgs) {
    SQLiteDatabase db = mOpenHelper.getWritableDatabase();
    int count;
    switch (sUriMatcher.match(uri)) {
        case NOTES:
            count = db.delete(NOTES_TABLE_NAME, where, whereArgs);
            break;

        case NOTE_ID:
            String noteId = uri.getPathSegments().get(1);
            count = db.delete(NOTES_TABLE_NAME, NoteColumns._ID + "=" + noteId
                    + (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
            break;

        default:
            throw new IllegalArgumentException("Unknown URI " + uri);
    }

    getContext().getContentResolver().notifyChange(uri, null);
    return count;
}

What happens if the Uri is null? or getContext()? or getContentresolver()?

I have come to the conclusion that the ContentResolver is not the place to catch the exceptions, but it should re-throw them or throw new exceptions so the app can display a meaningful error message.

Would the following or something similar be a bad approach (overkill) – should I just let NullPointerExceptions, etc. bubble up to the top to be handled in a more generic way (as per the example)?.

public int delete(Uri uri, String where, String[] whereArgs) 
    throws SQLiteException, IllegalArgumentException, NotifyException {

    if (null != mOpenHelper) {
       SQLiteDatabase db = mOpenHelper.getWritableDatabase();
       if (null != db) {
           if (null != uri) {
               int count = 0;
               switch (sUriMatcher.match(uri)) {
                   case NOTES:
                       count = db.delete(NOTES_TABLE_NAME, where, whereArgs);
                       break;

                   case NOTE_ID:
                       String noteId = uri.getPathSegments().get(1);
                       count = db.delete(NOTES_TABLE_NAME, NoteColumns._ID + "=" + noteId
                        + (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
                       break;

                   default:
                       throw new IllegalArgumentException("Unknown URI " + uri);
               }

               if (null != getContext()) && (null != getContentResolver()) {
                   getContext().getContentResolver().notifyChange(uri, null);
               } else {
                   throw NotifyException("Failed to notify change");
               }
               return count;
            } else {
                throw new IllegalArgumentException("Must provide URI");
            }
        } else {
            throw new SQLiteException("Failed to get database");
        }
    } else {
        throw new SQLiteException("Invalid database helper");
    }
}

Disclaimer: this code may not compile! it is an example.

It certainly is harder to read! I don’t know what the right balance is and need some help!.

Update: I read through Android’s recommended practices (see http://source.android.com/source/code-style.html#java-language-rules) but it just confused me further!

Answer 1

The idea of handling exception is to show a clear reason why an error is happened and maybe to perform some actions depending on the error. So, please consider:

1. Define your own exception class derived from Java exception

2. Put your code in try/catch blocks. When you catch an exception you log it with a trace and then throw your own exception with clear explanation.

3. At some point you catch your own exception, extract information and
   show the information either on a screen or send back to a program user if you provide a Web Service

Code Example:

public class MyException extends Exception {

public static final String MYSeparator = "!@#!";
public MyException() {
    super();
}

public MyException(String message) {
    super(message);
}

public MyException(Throwable cause) {
    super(cause);
}

public MyException(String message, Throwable cause) {
    super(message, cause);
}

public MyException(String errorCode, String errorDescription,
        Throwable cause) {
    super(errorCode + MYSeparator + errorDescription, cause);
}

public MyException(String errorCode, String errorDescription) {
    super(errorCode + MYSeparator + errorDescription);
}

}