SQLiteDatabase: Handling upgrades

When you're looking for tutorials online on how to create an SQLiteOpenHelper. You will probably find one where the onUpgrade() method drops your whole table and recreates it. In a lot of cases, you don't want the user to lose all their data whenever a database change has been made in a new version of the app.
It would be really bad practice when this would happen.

Now, how can we handle database upgrades then?
For this, let's have a look at the arguments that are passed in the onUpgrade() method. We have our SQLiteDatabase, the version number of the old database and the version number of the new database.

You could use a switch, checking the new version number and run some SQLite ALTER methods in the case block. That way, every time a new version is installed it will only run the ALTER methods of that new version. But when a user switches from version number 1 to version number 3, the changes for version number 2 will not have happened.

Next option is using a fall-through switch, where it jumps in on the right version number and runs all the changes that have happened since then. So suppose the version number is 3, then it will run all the changes for version 3 and version 2. The problem is, if the changes have already been done, it will try to run them again. This might cause a SQLiteDatabaseException for example when you're adding a new column, it might have a duplicate column name.

There's one way to solve this:
Use a fall-through switch and check if the old version number is the same as the case. When this happens, break out of the switch before running the changes.

Here's an example of this:

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
  switch (newVersion) {
  case 3:
   if(oldVersion == 3) {
    break;
   }
   db.execSQL("ALTER TABLE " + CONTACTS_TABLE + " ADD COLUMN " + CONTACTS_EMAIL + " TEXT");
  case 2:
   if(oldVersion == 2) {
    break;
   }
   db.execSQL("ALTER TABLE " + CONTACTS_TABLE + " ADD COLUMN " + CONTACTS_AGE + " INTEGER");
   
  }
 }

This method might be helpful to you and your users.

At all costs, try to avoid this if it's possible:

@Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + CONTACTS_TABLE);
        onCreate(db);
    }

ImageView ScaleTypes

When there are images that need to be displayed in an Android app, we need to be careful with the way they are scaled. A first basic step is setting the right scaletype in your layout file.

The different scaletypes are:

• center
• centerInside
• centerCrop
• fitXY
• fitStart
• fitCenter (which is the default)
• fitEnd
• matrix (can be used for transformations but is not covered here)

This post will just simple show the different scaletypes with a screenshot of how it looks in the app.

Here's the code of my layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:src="@drawable/logo"
        android:scaleType="centerCrop" />

</RelativeLayout>

Image larger than its given space

center

centerInside

centerCrop

fitXY

fitStart

fitCenter (default)

fitEnd

Image smaller than its given space

center

centerInside

centerCrop

fitXY

fitStart

fitCenter

fitEnd

Create Code Template for Log.d in Eclipse and Android Studio

Hi,

I've just found a way to save a lot of time while developing Android apps using Eclipse or Android Studio, by creating a shortcut for the typical Log.d statement.

Eclipse (or ADT)

In Eclipse go to Preferences > Java > Editor > Templates and click on "New...".
This will let you create a new Code Template.

Fill in the form as followed:

• Name = logd
• Context = Java statements
• Select 'Automatically insert'
• Description = Log.d shortcut
• Pattern = ${:import(android.util.Log)}Log.d("${enclosing_type}", "${enclosing_method}: ${cursor}");

Press OK to save the template and choose OK to close and apply the changes.

Now go inside your Java-file where you want to place a new Log.d, type "logd" and press CTRL + Space.

The code template will be printed in your editor and you can start writing what you want. For example it will print Log.d("MainActivity", "onCreate: "); and your cursor will be ready after "onCreate: "

It will also automatically import android.Util.Log.

Android Studio (or intelliJ)

In Android Studio it's pretty much the same. Go to Settings and select Live Templates in the list under IDE Settings. I've selected output in the list of Live Templates and pressed the plus sign on the right-hand side to add a new Live Template.

Fill in the form as followed:

• Abbreviation = logd
• Description = Prints a Log.d statement
• Template text = android.util.Log.d("$CLASS_NAME$", "$METHOD_NAME$ (line $LINE$): $MESSAGE$");$END$
• Shorten FQ names = checked

Next, go to Edit Variables and fill in the table as followed:

• MESSAGE
• Default value: "Write a log message"
• LINE
• Expression: lineNumber()
• Skip if defined: checked
• METHOD_NAME
• Expression: methodName()
• Skip if defined: checked
• CLASS_NAME
• Expression: groovyScript("if(_1.length() > 23) { return _1.substring(0, 23)} else { return _1}", className())
• Skip if defined: checked

The reason why I've used a groovy script is because Android Studio will give an error if the tag name is longer than 23 characters.

Last but not least, select an applicable context. I've just selected the whole Java field.

Here's a screenshot of what it should look like

Apply changes and you're done.

Just type logd and hit tab (unless you've changed it to something else). Your Log.d message will be displayed, start typing the preferred message and hit Enter. Here's an example of output:

Log.d("MainActivity", "onCreateOptionsMenu (line 43): Write a log message");

Or

Log.d("MyWayTooLongClassNameAc", "onCreateOptionsMenu (line 43): Write a log message");

Once again, the Log class will automatically be added to the imports.

Good luck with this and don't forget to share ;-)

Android Bug: StringSet in SharedPreferences

I've been using SharedPreferences a lot last week and  I think I've noticed a bug in the Android platform.

I had to update a Set<String> in my SharedPreferences and there seems to be an error when I tried to remove a String out of the Set<String>

Set<String> set = prefs.getStringSet("set", new HashSet<String>());
set.add("one");
set.add("two");
set.add("three");
editor.putStringSet("set", set);
editor.commit();
showSet(prefs.getStringSet("set", new HashSet<String>()));

This shows the three elements correctly.

After restarting the application, the Set is correctly stored in SharedPreferences.
But then I ran this code:

Set<String> set = prefs.getStringSet("set", new HashSet<String>());
set.remove("three");
editor.putStringSet("set", set);
editor.commit();
showSet(prefs.getStringSet("set", new HashSet<String>()));

At first this seems to work fine as well. It shows the elements "one" and "two".
The strange thing happens when I now stop the application from running, and restart it.
Now when I want to get the Set<String> out of the SharedPreferences, it has all three elements again.
"one", "two" and "three". Although I removed it...

Note: if you have this error, you can make your code work by first removing the Set<String>, commiting it, and then putting it in the SharedPreferences again

You can download a sample project on this here

The dark side of AsyncTask

Hi everyone,

In this post I'm going to talk about some of the (less known) problems of AsyncTask. I've written my first post on how to use AsyncTask about half a year ago. Now I'm going to tell you what problems they might cause, how you can fix this and what other options you have.

AsyncTask (°2009 - ...)

AsyncTask was added to Android in API level 3 (Android 1.5 Cupcake) and was said to help developers manage their threads. It's actually also available for 1.0 and 1.1, under the name of UserTask. It has the same API as AsyncTask. You should just copy the source code in your application to use it. But according to the Android Dashboards, there are less 0.1% of all Android devices that run on these API levels, so I mostly don't bother to support them.

Right now, AsyncTask is probably the most commonly used technique on Android for background execution. It's really easy to work with and that's what developers love about it. But this class has a couple of downsides and we are not always aware of them.

Lifecycle

There is quite a misunderstanding about our AsyncTask. Developers might think that when the Activity that created the AsyncTask has been destroyed, the AsyncTask will be destroyed as well. This is not the case. The AsyncTask keeps on running, doing his doInBackground() method until it is done. Once this is done it will either go to the onCancelled(Result result) method if cancel(boolean) is invoked or the onPostExecute(Result result) method if the task has not been cancelled.

Suppose our AsyncTask was not cancelled before our Activity was destroyed. This could make our AsyncTask crash, because the view it wants to do something with, does not exist anymore. So we always have to make sure we cancel our task before our Activity is destroyed.  The cancel(boolean) method needs one parameter: a boolean called mayInterruptIfRunning. This should be true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete. If there is a loop in our doInBackground() method, we might check the boolean isCancelled() to stop further execution.

So we have to make sure the AsyncTask is always cancelled properly. 

Does cancel() really work?

Short answer: Sometimes.

If you call cancel(false), it will just keep running until its work is done, but it will prevent onPostExecute() to be called. So basically, we let our app do pointless work. So let's just always call cancel(true) and the problem is fixed, right? Wrong. If mayInterruptIfRunning is true, it will try to finish our task early, but if our method is uninterruptable such as BitmapFactory.decodeStream(), it will still keep doing the work. You could close the stream prematurely and catch the Exception it throws, but this makes cancel() a pointless method.

Memory leaks

Because an AsyncTask has methods that run on the worker thread (doInBackground()) as well as methods that run on the UI (e.g. onPostExecute()), it has took keep a reference to it's Activity as long as it's running. But if the Activity has already been destroyed, it will still keep this reference in memory. This is completely useless because the task has been cancelled anyway.

Losing your results

Another problem is that we lose our results of the AsyncTask if our Activity has been recreated. For example when an orientation change occurs. The Activity will be destroyed and recreated, but our AsyncTask will now have an invalid reference to its Activity, so onPostExecute() will have no effect. There is a solution for this. You can hold onto a reference to AsyncTask that lasts between configuration changes (for example using a global holder in the Application object). Activity.onRetainNonConfigurationInstance() and Fragment.setRetainedInstance(true) may also help you in this case.

Serial or parallel?

There is a lot of confusion about AsyncTasks running serial or parallel. This is normal because it has been changed a couple of times. You could probably be wondering what I mean with 'running serial or parallel'. Suppose you have these two lines of code somewhere in a method:

new AsyncTask1().execute();
new AsyncTask2().execute();

Will these two tasks run at the same time or will AsyncTask2 start when AsyncTask1 is finished?
Short answer: It depends on the API level.

Before API 1.6 (Donut):

In the first version of AsyncTask, the tasks were executed serially. This means a task won't start before a previous task is finished. This caused quite some performance problems. One task had to wait on another one to finish. This could not be a good approach.

API 1.6 to API 2.3 (Gingerbread):

The Android developers team decided to change this so that AsyncTasks could run parallel on a separate worker thread. There was one problem. Many developers relied on the sequential behaviour and suddenly they were having a lot of concurrency issues.

API 3.0 (Honeycomb) until now

"Hmmm, developers don't seem to get it? Let's just switch it back." The AsyncTasks where executed serially again. Of course the Android team provided the possibility to let them run parallel. This is done by the method executeOnExecutor(Executor). Check out the API documentation for more information on this.

If we want to make sure we have control over the execution, whether it will run serially or parallel, we can check at runtime with this code to make sure it runs parallel:

 
 public static void execute(AsyncTask as) {
  if (Build.VERSION.SDK_INT <= Build.VERSION_CODES.HONEYCOMB_MR1) {
   as.execute();
  } else {
   as.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
  }
 }

(This code does not work for API lvl 1 to 3)

Do we need AsyncTasks?

Not really. It is an easy way to implement background features in our app without having to write a lot of code, but as you can see, if we want it to work properly we have to keep a lot of things in mind. Another way of executing stuff in background is by using Loaders. They were introduced in Android 3.0 (Honeycomb) and are also available in the support library. Check out the developer guide for more information. 

Parcelable versus Serializable

In this post I'm going to talk about the differences between Parcelables and Serializables.

Android Activities are great, aren't they? They allow you to structure your app in a nice and easy way. Switching between two Activities is also really easy. Just write this.startActvitiy(intent) somewhere in your first Activity and you're done.
There's only one thing that's not so easy to do, and that's passing data between these two Activities. If we want to pass data we have to put it in an Intent or a Bundle. This is quite easy for primitive classes such as String or int, but it's a bit harder for other (custom) objects.

If you have some experience in Java, you probably know something about 'serialization'. Since we're using Java in our Android app, we could let our object implement Serializable and we're done, right? Well, not quite. Android provides us with another interface called Parcelable to do this and there's a reason why.

Serializable

First of all let's have a look at our TestObject that implements Serializable.

public class TestObject implements Serializable{



 private static final long serialVersionUID = 643301981519798569L;

 private String mString;

 private int mInt;

 private List<String> mList;

 

 public TestObject() {

  this.mString = "This is a String";

  this.mInt = 777;

  mList = new ArrayList<String>();

  for(int i=0; i<100; i++){

   mList.add(String.valueOf(i));

  }

 }

    //Getters and Setters...

}

The only thing we had to do to make sure TestObject could be passed with an Intent or Bundle, was implementing it with java.io.Serializable and add a generated serialVersionUID. Now let's have a look at the same class but as a Parcelable.

Parcelable

This is our TestObject, implementing android.os.Parcelable.

public class TestObject implements Parcelable{



 private String mString;

 private int mInt;

 private List<String> mList;



 public TestObject() {

  this.mString = "This is a String";

  this.mInt = 777;

  mList = new ArrayList<String>();

  for(int i=0; i<100; i++){

   mList.add(String.valueOf(i));

  }

 }

 

 public TestObject(Parcel in) {

  mString = in.readString();

  mInt = in.readInt();

  mList = new ArrayList<String>();

  in.readStringList(mList);

 }



 @Override

 public int describeContents() {

  return 0;

 }



 @Override

 public void writeToParcel(Parcel dest, int flags) {

  dest.writeString(mString);

  dest.writeInt(mInt);

  dest.writeStringList(mList);

  

 }

 

 public static final Parcelable.Creator<TestObject> CREATOR = new Parcelable.Creator<TestObject>() {

        public TestObject createFromParcel(Parcel in) {

            return new TestObject(in); 

        }



        public TestObject[] newArray(int size) {

            return new TestObject[size];

        }

    };

    

    //Getters and Setters...

}

The first thing we notice here is that our class just became a lot bigger. As you can see we have to override two methods, write a Parcelable.Creator and write a new Constructor. Why should you go through all this trouble if you can just simple use Serializable instead? The answer is performance.

Parcelable splits your object up into primitives, sends those primitives and rebuilds the object afterwards. By doing this, it should run a lot faster. Serializable creates a lot of temporary objects and this causes quite some garbage collection, which should always be avoided, especially on mobile devices.

Putting it to the test

Of course, I ran a small test on this. I created an instance of TestObject, added it to an Intent, started another Activity and took our TestObject out of the Intent. I used TraceView for to check how long this process took for Parcelable and Serializable.

Here's our result for using the Serializable interface:

So we can see this whole process took 248ms. 

Now let's have a look at the results of using the Parcelable interface:

As you can see, for only this one object, the process was completed in less than half of the time for our first way. And let's not forget, this is the whole process of switching to another Activity, not just writing and reading the object!

Conclusion

Parcelable works a lot faster than Serializable and is the best way to go. But if we don't need to be faster, and it's possible to just use Serializable (e.g. android.graphics.Bitmap causes problems), just use Serializable and you don't have to bother about anything anymore.

Important notes:

• Read from the Parcel in the same order that you wrote to it
• Don't use raw Creators (specify by adding <TestObject> in this example)

Image Caching with LruCache

Caching images

When we are building an application, whether it's a web app or a mobile app or whatsoever, we want to make sure it runs as fast as possible. One of the most important things we have to keep in mind is caching. If we are using certain resources in our application, we should know that it isn't always that easy to retrieve them. So we should always avoid to retrieve the same resources multiple times. This can be done by caching them. In this tutorial I'm going to give an example of how to cache images in your Android app.

Images can easily get quite large, so retrieving them can take up quite some time. Since mobile devices have limited resources (such as battery life, data transfers, etc), it's twice as important to make sure we cache images the right way.

SoftReferences?

Java already provides an easy way to do this. It's called SoftReferences. You can read all about it here. The only problem is that there is an issue in Android that causes the garbage collector not to respect these SoftReferences. You can read all about this issue here. The Android System provides another way of caching your objects, called LruCache.

LruCache

LRU stands for Least Recently Used. This already tells us a lot about how this cache would work. LruCaches have a fixed cache size and work like a queue, you might say.  If a new object is added to the cache, it's placed in the beginning of the queue. If the app asks to get an object from the cache, it's also placed in the beginning of this queue. This way the least recently used objects are left at the beginning of the queue.

If the cache doesn't have enough memory left, it deletes the least recently used objects until it has enough free space to store the new object(s).

API levels

LruCache was only added to API level 12 but don't worry, they have included this in the support library v4.

Code it!

Okay, it's nice to know how it works but now we still have to implement it in our code. For this tutorial I've made a small app that puts all the thumbnails of the images on the device in a large list. If the user scolls down, the images are loaded. This is actually really simple to implement. First let's have a look on how we could code the LruCache.

public class ThumbnailCache extends LruCache<Long, Bitmap>{



  public ThumbnailCache(int maxSizeInBytes) {

   super(maxSizeInBytes);

  }



  @Override

  protected int sizeOf(Long key, Bitmap value) {

   return value.getByteCount();

   

   //before API 11

   //return value.getRowBytes() * value.getHeight();

  }

  

  

 }

We have to add a value in the constructor of this LruCache. This is the maximum size of the cache in bytes. It can be hard to know how much this should be. You can get some help of the ActivityManager to do this.

ActivityManager am = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
int memoryClassBytes = am.getMemoryClass() * 1024 * 1024;

If we run this code, we'll get the memory limit of our application. This depends on what device you are using (on a Nexus S and Galaxy SII it's 48MB). Have a look here for more information.
It's obvious that we don't want our image cache to take up all of the memory of my application. So we only take a part of this. For this example let's say we use a sixth.

mCache = new ThumbnailCache(memoryClassBytes / 6);

Now place the image in our LruCache when it is retrieved using this line of code.

mCache.put(photoId, bitmap);

And check if it is in cache before retrieving it like this:

Bitmap cachedResult = mCache.get(photoId);

             if (cachedResult != null) {

                 imageView.setImageBitmap(cachedResult);

             }else{
              //retrieves the thumbnail and puts it in an imageview
              new ThumbnailTask(imageView).execute(photoId);

That's all you need to do. Give it a try!