Let's start by adding the RxJava 2.0 dependencies with a line:

compile 'io.reactivex.rxjava2:rxjava:2.0.3'

Also, we will want to use Android-specific schedulers, so let's add the following line:

compile 'io.reactivex.rxjava2:rxandroid:2.0.1'

To interact with UI elements, we will use two great libraries: RxBinding (which can be found at https://github.com/JakeWharton/RxBinding ) and Butterknife (which can be found at https://github.com/JakeWharton/butterknife ).

ButterKnife will let us easily import UI elements into Java code (no more pesky findViewById()!) and RxBinding will let us make all UI interactions RxJava friendly.

To include ButterKnife, let's add these lines:

compile 'com.jakewharton:butterknife:8.4.0'
annotationProcessor 'com.jakewharton:butterknife-compiler:8.4.0'

To include RxBinding, put in the following:

 compile 'com.jakewharton.rxbinding2:rxbinding:2.0.0'
 compile 'com.jakewharton.rxbinding2:rxbinding-recyclerview-v7:2.0.0'

Finally, we will be using the RecyclerView and CardView elements from Android Support Repository, so these will be needed as well:

compile 'com.android.support:cardview-v7:25.1.0'
compile 'com.android.support:recyclerview-v7:25.1.0'

In the future, we might need to mix different versions of RxJava, so this line (under android block) will help when both the libraries will be included at the same time:

packagingOptions {
    exclude 'META-INF/rxjava.properties'
}

In the end, the top of the build.gradle file will look like this:

plugins {
    id "me.tatarka.retrolambda" version "3.4.0"
}

apply plugin: 'com.android.application'

Also, the android block will be as follows:

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.2"
    defaultConfig {
        applicationId "packt.reactivestocks"
        minSdkVersion 15
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner 
          "android.support.test.runner.AndroidJUnitRunner"
    }

    packagingOptions {
        exclude 'META-INF/rxjava.properties'
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-
              android.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        targetCompatibility 1.8
        sourceCompatibility 1.8
    }
}

The dependency block will be set to the following:

dependencies {
    compile 'com.android.support:appcompat-v7:25.1.0'

    compile group: 'io.reactivex.rxjava2', name: 'rxjava', version: 
      '2.0.3'
    compile group: 'io.reactivex.rxjava2', name: 'rxandroid', version: 
      '2.0.1'

    compile 'com.jakewharton:butterknife:8.4.0'
    annotationProcessor 'com.jakewharton:butterknife-compiler:8.4.0'

    compile 'com.jakewharton.rxbinding2:rxbinding:2.0.0'
    compile 'com.jakewharton.rxbinding2:rxbinding-recyclerview-
      v7:2.0.0'

    compile 'com.android.support:cardview-v7:25.1.0'
    compile 'com.android.support:recyclerview-v7:25.1.0'

    compile "com.github.akarnokd:rxjava2-interop:0.8.1"
}

Since the application will be displaying changes of financial stocks over time, it means that we will be working with data that's represented best as a list. For this, we will use RecyclerView. 

First of all, add it to your activity_main.xml file:

<android.support.v7.widget.RecyclerView
    android:id="@+id/stock_updates_recycler_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_below="@id/hello_world_salute" />

Then, inject it into your code with the following:

@BindView(R.id.stock_updates_recycler_view)
RecyclerView recyclerView;

The next step is to initialize it in your activity:

private LinearLayoutManager layoutManager;
private StockDataAdapter stockDataAdapter;

...
recyclerView.setHasFixedSize(true);

layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);

stockDataAdapter = new StockDataAdapter();
recyclerView.setAdapter(stockDataAdapter);

The code for the StockDataAdapter is as illustrated:

public class StockDataAdapter extends RecyclerView.Adapter<StockUpdateViewHolder> {
private final List<String> data = new ArrayList<>();

@Override
    public StockUpdateViewHolder onCreateViewHolder(ViewGroup parent, 
      int viewType) {
        View v = LayoutInflater.from(parent.getContext())
          .inflate(R.layout.stock_update_item, parent, false);
        StockUpdateViewHolder vh = new StockUpdateViewHolder(v);
        return vh;
    }

    @Override
    public void onBindViewHolder(StockUpdateViewHolder holder, int 
      position) {
        holder.stockSymbol.setText(data.get(position));
    }

    @Override
    public int getItemCount() {
        return data.size();
    }

    public void add(String stockSymbol) {
        this.data.add(stockSymbol);
        notifyItemInserted(data.size() - 1);
    }
}

Finally, we need the layout XML for the RecyclerView items and a ViewHolder for them:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="3dp">

    <TextView
        android:id="@+id/stock_item_symbol"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_margin="20dp" />
</android.support.v7.widget.CardView>

To fill in the data for this view, we will have to utilize ViewHolders from the RecyclerView class:

public class StockUpdateViewHolder extends RecyclerView.ViewHolder {
@BindView(R.id.stock_item_symbol)
    TextView stockSymbol;

public StockUpdateViewHolder(View v) {
        super(v);
        ButterKnife.bind(this, v);
    }
}

The very last step is to populate it with the data. We will do that in a Reactive way:

Observable.just("APPL", "GOOGLE", "TWTR")
        .subscribe(new Consumer<String>() {
            @Override
            public void accept(String stockSymbol) {
                stockDataAdapter.add(stockSymbol);
            }
        });

This is rather simple code and I hope that most of the developers had no trouble following this. In case there were some not-so-clear bits, feel free to check out the code that's provided with this book. The examples there are fully working.

One will notice pretty quickly that it is quite unwieldy to work just with string data as the app will process stock updates. For the purpose of a proper way to track stock updates, we will create a Value object that will keep that information. It will contain the following fields for now:

The list might be a bit too naive for professional use, but it will serve as a starting point for upcoming work. The final class can look like this:

public class StockUpdate implements Serializable {
    private final String stockSymbol;
    private final BigDecimal price;
    private final Date date;

public StockUpdate(String stockSymbol, double price, Date date) {
        this.stockSymbol = stockSymbol;
        this.price = new BigDecimal(price);
        this.date = date;
    }

public String getStockSymbol() {
        return stockSymbol;
    }

public BigDecimal getPrice() {
        return price;
    }

public Date getDate() {
        return date;
    }
}

Obviously, we then need to update other parts of the code to make use of this refactoring:

@Override
public void onBindViewHolder(StockUpdateViewHolder holder, int position) {
    StockUpdate stockUpdate = data.get(position);
    holder.setStockSymbol(stockUpdate.getStockSymbol());
    holder.setPrice(stockUpdate.getPrice());
    holder.setDate(stockUpdate.getDate());
}

Also, StockUpdateViewHolder now has additional methods and view fields:

@BindView(R.id.stock_item_date)
TextView date;
@BindView(R.id.stock_item_price)
TextView price;

private static final NumberFormat PRICE_FORMAT = new DecimalFormat("#0.00");

public void setStockSymbol(String stockSymbol) {
    this.stockSymbol.setText(stockSymbol);
}

public void setPrice(BigDecimal price) {
    this.price.setText(PRICE_FORMAT.format(price.floatValue()));
}

public void setDate(Date date) {
    this.date.setText(DateFormat.format("yyyy-MM-dd hh:mm", date));
}

Also, our file stock_update_item.xml will change slightly to nicely display all this new data. The stock_item_symbol is now as shown:

<TextView
    android:id="@+id/stock_item_symbol"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:text="GOOGLE"
    android:textSize="18sp" />

At the same time, stock_item_price and stock_item_date are handled by the following:

<TextView
    android:id="@+id/stock_item_price"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_alignParentRight="true"
    android:layout_centerVertical="true"
    android:text="18.90"
    android:textColor="@android:color/holo_green_dark"
    android:textSize="22sp" />

<TextView
    android:id="@+id/stock_item_date"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_below="@id/stock_item_symbol"
    android:layout_marginTop="5dp"
    android:text="2012-12-01"
    android:textSize="12sp" />

Also, all the preceding elements will be wrapped inside the RelativeLayout that will be in the CardView:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="3dp">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="20dp">
         [...]
    </RelativeLayout>
</android.support.v7.widget.CardView>

Finally, our Observable--the piece that creates the data--will look like this:

Observable.just(
        new StockUpdate("GOOGLE", 12.43, new Date()),
        new StockUpdate("APPL", 645.1, new Date()),
        new StockUpdate("TWTR", 1.43, new Date())
)

The option we will be using is Retrolambda (https://github.com/orfjackal/retrolambda). We will enable it by using a plugin for Android (https://github.com/evant/gradle-retrolambda).

It can be done by simply adding the following at the very top of the application's app/build.gradle file:

plugins {
    id "me.tatarka.retrolambda" version "3.4.0"
}

apply plugin: 'com.android.application'

Also, let's not forget to tell the IDE that we will be using Java 8 by adding the following block:

android {
    ...
    compileOptions {
        targetCompatibility 1.8
        sourceCompatibility 1.8
    }
}

Finally, if proguard is being used, adding a line---dontwarn java.lang.invoke.*--to your proguard file might come in handy.

You might think that setting all Retrolambda can be just too much trouble for very little gain. Nothing could be further from the truth and it will be obvious after we take a look at a few examples of code.

Compare the following two samples of code. The first sample is as follows:

Observable.just("1")
        .subscribe(new Consumer<String>() {
            @Override
            public void accept(String e) {
                Log.d("APP", "Hello " + e);
            }
        });

Here's the next sample:

Observable.just("1")
        .subscribe(e -> Log.d("APP", "Hello " + e));

The latter is much more concise, but might fail to show the scale of the problem of the former approach. This example might be much better at doing so:

Observable.just("1")
        .map(new Function<String, String>() {
            @Override
            public String apply(String s) {
                return s + "mapped";
            }
        })
        .flatMap(new Function<String, Observable<String>>() {
            @Override
            public Observable<String> apply(String s) {
                return Observable.just("flat-" + s);
            }
        })
        .doOnNext(new Consumer<String>() {
            @Override
            public void accept(String s) {
                Log.d("APP", "on next " + s);
            }
        })
        .subscribe(new Consumer<String>() {
            @Override
            public void accept(String e) {
                Log.d("APP", "Hello " + e);
            }
        }, new Consumer<Throwable>() {
            @Override
            public void accept(Throwable throwable) {
                Log.d("APP", "Error!");
            }
        });

Now, take a look at this:

Observable.just("1")
        .map(s -> s + "mapped")
        .flatMap(s -> Observable.just("flat-" + s))
        .doOnNext(s -> Log.d("APP", "on next " + s))
        .subscribe(e -> Log.d("APP", "Hello " + e),
                throwable -> Log.d("APP", "Error!"));

That's 24 lines saved! 

The observable flows can get pretty complicated and long, so these lines add up pretty quickly.

If you are stuck with an old version of Java, there are other means to make these flows clear; we will cover more about this in the later chapters.

Since we have just enabled Retrolambda, let's replace the existing code in MainActivity to make use of it:

Observable.just("Please use this app responsibly!")
        .subscribe(s -> helloText.setText(s));

Observable.just(
        new StockUpdate("GOOGLE", 12.43, new Date()),
        new StockUpdate("APPL", 645.1, new Date()),
        new StockUpdate("TWTR", 1.43, new Date())
)
        .subscribe(stockDataAdapter::add);

Note that we could have used the helloText::setText method reference in the first .subscribe() call as well, but it was left as it is just to serve as an example.

In the case where multiple lines are required, the subscribe part can look like this:

.subscribe(stockUpdate -> {
    Log.d("APP", "New update " + stockUpdate.getStockSymbol());
    stockDataAdapter.add(stockUpdate);
});

However, in this particular case, I would recommend putting logging calls in .doOnNext() blocks.

Finally, the resulting app should look like this screenshot:

If it looks different, feel free to check out the code that's provided with the book.