The Vital Guide to Android Interviewing - 2

// A basic data model for illustrative purposes
class Contact {
publicint id;
public String name;
public String phone;
}

publicclassContactAdapterextendsBaseAdapterimplementsListAdapter {
    Context mContext;
    List mContacts;

publicContactAdapter(Context context) {
        mContext = context;
    }

// Call this method to attach data to this adapter, and call when the data changes.
publicvoidnewData(List data) {
        mContacts = data;
// Inform AdapterView that data has change and to redraw if necessary.
        notifyDataSetChanged(); 
    }

// The crux of the adapter -- bind data to the view layout.
@Override
public View getView(int position, View convertView, ViewGroup parent) {
        Contact contact = mContacts.get(position);
        LinearLayout view = (LinearLayout) convertView;
if (view == null) {
// "Inflate" our contact.xml view into existence.
            LayoutInflater inflater = (LayoutInflater)
                mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            view = (LinearLayout) inflater.inflate(R.layout.contact, null);
        }
// Bind the data to their appropriate TextViews in the layout.
        ((TextView)view.findViewById(R.id.name)).setText(contact.name);
        ((TextView)view.findViewById(R.id.phone)).setText(contact.phone);
// ... etc. ...
return view;
    }
}

Performance and Responsiveness

Technique #1: Optimize views in layouts.

Technique #2: Use thetag in your layout files.

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

<ImageView
android:id="@+id/image"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />

RelativeLayout>

<mergexmlns:android="http://schemas.android.com/apk/res/android">

<ImageView
android:id="@+id/image"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />

merge>

Technique #3: Consider using a ViewHolder pattern, but do so judiciously.

• A concreteAdapterViewsubclass such asListView,GridView, etc.
• An underlying data set, typically an array ofPOJOsor aCursor(a set of database rows)
• AnAdapterclass responsible for creating and populating Views with specified data elements

• Incurring the cost of garbage collection
• Incurring the cost of layout inflation

• Code clutter
• Breaking theSeparation of Concerns Principle
• Breaking theSingle Responsibility Principle

staticclassViewHolder {
    TextView text;
    TextView timestamp;
    ImageView icon;
    ProgressBar progress;
int position;
}

ViewHolder holder = new ViewHolder();
holder.icon = (ImageView) convertView.findViewById(R.id.listitem_image);
holder.text = (TextView) convertView.findViewById(R.id.listitem_text);
holder.timestamp = (TextView) convertView.findViewById(R.id.listitem_timestamp);
holder.progress = (ProgressBar) convertView.findViewById(R.id.progress_spinner);
convertView.setTag(holder);

ViewHolder holder = (ViewHolder) convertView.getTag(); 

@Override
public View getView(int position, View convertView, ViewGroup parent) {
   Contact contact = mContacts.get(position);
   ContactView view = (ContactView) convertView;
if (view == null) {
// "Inflate" our contact.xml view into existence.
       LayoutInflater inflater = (LayoutInflater)
           mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
       view = (LinearLayout) inflater.inflate(R.layout.contact, null);
        }
// Bind contact data to appropriate child Views in the layout.
   view.setModel(contact);
return view;
}

publicclassContactViewextendsLinearLayout {
private Contact model;
private TextView name;
private TextView phone;

@Override
protectedvoidonFinishInflate() {
this.name = (TextView)view.findViewById(R.id.name);
this.phone = (TextView)view.findViewById(R.id.phone);
  }

publicvoidsetModel(Contact model) { 
this.model = model; 
    bindModel();
  }

privatevoidbindModel() {
this.name.setText(model.name);
this.phone.setText(model.phone);
  }
}

• Uploading/loading data to/from the cloud
• Saving/loading cached data to/from disk
• Sorting, filtering and searching data

class VotingActivity {
private Runnable processVotes = new Runnable() {
publicvoidrun() {
        }
    };

publicvoidonCreate (Bundle savedInstanceState) {
        HandlerThread handlerThread = new HandlerThread("Voting Tasks");
        handlerThread.start();
        Handler handler = new Handler(handlerThread.getLooper());
// Now let's execute a Runnable task ASAP on the new thread we just created:
        handler.post(new Runnable() {
publicvoidrun() {
// Accept a vote, say.
            }
        });
// Let's also schedule a task to execute 10 seconds in the future on the new thread:
        handler.postDelayed(processVotes, 10000);
    }
}

classDownloadThreadextendsHandlerThread {
publicstaticfinalint START_MSG = 0;
publicstaticfinalint STOP_MSG = 1;

    DownloadThread(String name) {
super(name);
    }

    @Override
publicvoid handleMessage(Message msg) {
switch (msg.what) {
case START_MSG:
// Start downloading...
break;
case STOP_MSG:
// Stop downloading...
break;
default:
break;
        }
    }
}

classMainActivityextendsActivity {
publicvoid onCreate (Bundle savedInstanceState) {
// Set up our DownloadThread for receiving messages.
        DownloadThread downloadThread = new DownloadThread("Image Downloader");
        downloadThread.start();
        Handler downloadHandler = new Handler(downloadThread.getLooper());
    }

publicvoid startDownloading() {
        downloadHandler.sendEmptyMessage(DownloadThread.START_MSG);
    }

publicvoid stopDownloading() {
        downloadHandler.sendEmptyMessage(DownloadThread.STOP_MSG);
    }
}

Beyond Android Proper

• ActionBarSherlock ActionBarSherlock is an extension of Androidssupport librarydesigned to facilitate the use of the action bar design pattern across all versions of Android with a single API. It allows you to easily develop an application with an action bar for every version of Android from 2.x and up.
• DiskLruCache As mentioned above, DiskLruCache is a powerful, yet simple, least-recently-used (LRU) caching implementation for storing dataon disk, with bounded pool size.
• EspressoAnnounced by Google in October 2013and still in developer preview, Espresso is a new UI testing framework whose goal is to make it easy for developers to write reliable UI tests. Most importantly, Espresso removes the need to think about the complexity of multi-threaded testing. Espresso is already being used by a number of Google applications (Drive, Maps, Google+, and more).
• Guava Googles Guava project contains several core libraries actively used by Google internally: collections, caching, primitives support, concurrency libraries, common annotations, string processing, I/O, and somewhat of a mishmash of very useful classes. SeeGuavas wikifor more information. Its purpose is to make Java more pleasant, improvedefensive programming, add optimizations and improve efficiency. It fills out and improves upon common classes like Collections, Math, Reflection, strings, Ranges, Hashing, I/O and more.
• NineOldAndroids NineOldAndroids makes the dramatically new and improved Android Honeycomb (3.0) animation API available to older versions back to 1.0 and forward compatible to the latest release of Android. Animation prior to Honeycomb was pretty darn limited. The new API offers much more power and flexibility to animate objects and actually reposition views with rotation, translation, alpha and scale primitives, for example. The library provides a wrapper that works across all Android versions.
• Protocol Buffers Googles Protocol Buffers is a language-neutral, platform-neutral, extensible mechanism for serializing structured data, that provides a viable alternative to JSON or XML. Its an efficient way to transport object data, for instance in its original binary format without needing to (un)marshal. Data transported is smaller, faster, simplerin Googles words.
• RxJava As mentioned above, RxJava is a Reactive Extension that lets you create asynchronous, event-based code using observable sequences, which can be invaluable in many circumstances.
• Volley Volley is a relatively recent Google libraryintroduced at Google I/O May 2013that makes networking for Android apps faster and easier. Oddly, Google hasnt yet published its own tutorial or documentation other than the source, making some developers skeptical or wary of it, but several have written tutorials such asthisandthis. Highlights include a view that automatically loads images, HTTP request queuing with prioritization, and automatic Android-version-specific selection of the best HTTP library.

Debugging and Tuning

public void onCreate() {
    // Only enable when developing your app!
    if (DEVELOPER_MODE) {
        // Tell Android what thread issues you want to detect and what to do when found.
        StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()
.detectDiskReads()
.detectDiskWrites()
.detectNetwork()    // or use .detectAll() for all detectable problems
.penaltyLog()
.build());
        // Tell Android what VM issues you want to detect and what to do when found.
        StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder()
.detectLeakedSqlLiteObjects()
.detectLeakedClosableObjects()
.penaltyLog()       // Log the problem
.penaltyDeath()     // Then kill the app
.build());
    }
    super.onCreate();
}

• Detect unused files and resources to reduce app bloat
• Tell you where you should be recyclingTypedArraysafter use
• Identify where you might be overdrawing backgrounds of overlaying views
• plus many other issues

Wrap Up