Android: App Programming

Programming Android

2011
Zigurd Mednieks, Laird Dornin, G. Blake Meike, and Masumi Nakamura

Part I. Tools and Basics

Part II. About the Android Framework

Part III. A Skeleton Application for Android

Part IV. Advanced Topics

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Beginning Android Application development

2011
Wei-Meng Lee

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Teach Yourself Android Application Development in 24 Hours

2012

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Android Programming: The Big Nerd Ranch Guide

2013
by Bill Phillips and Brian Hardy

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Head First Android Development

2015
Dawn Griffiths
David Griffiths

The source code can be downloaded:

  git clone https://github.com/dogriffiths/HeadFirstAndroid2ndEdition.git

Chapter 1 Getting Started: Diving In

We are going to build Android apps using a mixture of Java and XML.
An Android app is composed of one or more screen.
Each screen is designed with a layout which is defined using XML.
Layouts can contain GUI components such as buttons and labels.

Layouts only defined the look of the app, a Java class called activity decides how the GUI component should respond to the UI event.
Other resources such as image files and application data can be bundled with when you build your app.

Setup development environment

Android device run the byte code which is the format generated from .class and .jar files.
Android SDK contains tools and libraries to convert the compiled Java code.
The Android build system compiles app resources and source code, and packages them into APKs that you can test, deploy, sign, and distribute. Android Studio uses Gradle, an advanced build toolkit, to automate and manage the build process, while allowing you to define flexible custom build configurations.

Android Studio is the official Integrated Development Environment (IDE) for Android app development, based on IntelliJ IDEA which is the popular IDE for Java development.
You need to make sure correct versions of JDK and JRE are installed before to use the Android Studio.
JDK and JRE (Java Standard Edition) can be downloaded:

                                                                  https://www.oracle.com/technetwork/java/javase/downloads/index.html

Once Java is running, download Android Studio:

https://developer.android.com/studio

Build an App

Start Android Studio:

1. Create a new project
"Start a new Android Studio project", choose the "Empty Activity" template from "Phone and Tablet".
2. Configure the project
Android Studio uses the company name and application name to form the qualified name of a package. Enter the application name, a company name, and accept the default project location.
3. Specify the API level
The API level refers to the PI version used by the app. The minimum required SDK is the lowest version you app will support. Your app will run on devices withe the level API or higher.

After the Wizard created the project, it created all needed files . The work space will show 3 tab pages:

• The layout file *.xml
• The activity file *.java
• AndroidManifest.xml

Android Studio use the Gradle to compile and build your apps.
Gradle projects have a standard layout


The layout can be designed by editing the XML file or the design editor. The design editor allows you to drag GUI components onto your layout.



Android Studio includes the emulator which enables you to set up one or more Android virtual devices(AVDs) and then run your app in the emulator. The emulator is a kind of virtual machine application, each emulator emulates a different type of device.

To set up an AVD, start the AVD manager on the Tool menu:

• create a virtual device
• choose a device
Phone, Tablet, Wear or TV.
• choose a system image

Choose "Run app" from the Run menu, lists of connected devices and AVD will be listed, select the AVD you want the emulator to load.
During the running, some initial tasks are done:

• Java source files get compiled to byte code
The byte code is in DEX format which runs on the Android RunTime. ART can convert the DEX bytecode into native code that can run on the CPU directly. ART is smaller than the JVM.
• emulator is launched with an AVD
It takes time to start the AVD, the console tells you what the Gradle build system is doing.
• APK is created
• the APK file is installed to the AVD
DEVICE SHELL COMMAND: pm install -r ...
• AVD starts the application then triggers the main activity associated with the app
DEVICE SHELL COMMAND: am start ...

The activity specifies that it uses the layout "activity_main.xml".

Change the Layout

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

Try to change the text:

• res/values/strings.xml
strings.xml is the default resource file used to hold name / value pairs of strings which can be referenced throughout your app. Define a new text in the resource,

<resources>
    <string name="app_name">My Application</string>
    <string name="app_text">Hi, Jerry.</string>
</resources>

Each name/value pair take the form:

    <string name="name">value</string>

Besides, any file containing the above format can be identified by Android to have the String Resource.
• res/layout/activity_main.xml
Use the resource string in layout,

        android:text="@string/app_text"

This tells Android to look up the value of a string resource with the name "app_text". The advantage using string.xml:
• i18n for different languages
You can have one string resource file for each language. The default English resource file in res/values/ folder, other string resource file will be in the folder named res/values-language.
• text change across the whole application

Chapter 2 Building Interactive Apps: Apps that Do Something

First, setup a new project:

• Start a new project
• Choose a "Blank Activity"
• Enter the application name "Beer Adviser"

After a new project is created, the design of an app will be structured as:

• layout
res/layout/activity_main.xml is created. It include all GUI components you need. You can change the layout using the Design editor:
• Move the TextView component
• Drag a Button component into the design editor.
Then, the changes in the design editor are reflected in the layout XML file.

   <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        tools:layout_editor_absoluteX="174dp"
        tools:layout_editor_absoluteY="98dp" />

The android:id property enables you to control what components do via activity code. Besides, you can change the layout XML file directly. For example, Spinners provide a quick way to select one value from a set. Touching the spinner displays a dropdown menu with all other available values. You usually add a spinner in your XML layout with a element.

<Spinner
    android:id="@+id/planets_spinner"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content" />

A To define a view's position in ConstraintLayout, you must add at least one horizontal and one vertical constraint for the view. ConstraintLayout is a ViewGroup which allows you to position and size widgets in a flexible way. To define a view's position in ConstraintLayout, you must add at least one horizontal and one vertical constraint for the view. Each constraint represents a connection or alignment to another view and defines the view's position along either the vertical or horizontal axis. When you drop a view into the Layout Editor, that view has no constraints, the Layout Editor indicates "missing constraints in Constraintlayout" as an error in the toolbar. You can move each view into the positions you desire, and then click Infer Constraints to automatically create constraints. To add a constraint,
1. Click the view
2. Click the edge of the bounding box in the attribute window to add constrains.
The changes in the layout XML file:

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
...
    tools:context=".MainActivity">

    <TextView
...
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.047"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.051" />

    <Spinner
...
        app:layout_constraintBottom_toTopOf="@+id/button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textview" />

    <Button
...
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/planets_spinner" />

</android.support.constraint.ConstraintLayout>

You can use constraints to achieve different types of layout behavior:
• Parent position
A horizontal constraint to the parent
• Order position
B is constrained to always be to the right of A, and C is constrained below A.
• Alignment
A horizontal alignment constraint: An offset horizontal alignment constraint:
• Baseline alignment
Select the text view you want to constrain and then click Edit Baseline:
• Constrain to a guideline

• Constrain to a barrier
A barrier is an invisible line that you can create to constrain views.
• string.xml
Add string resources:

<resources>
    <string name="app_name">Beer Adviser</string>
    <string name="find_beer">Find Beer !</string>
    <string name="brands"></string>

</resources>

Then, change the layout XML file:

...
        android:text="@string/brands"
...
        android:text="@string/find_beer"
...

• activity
java/com/example/beeradviser/MainActivity.java is created.
• java code

At this point, the layout includes a spinner, but it does not include anything in it.
We can specify an array of String values in the resource XML file so that the spinner can reference it.

    <string-array name="beer_colors">
        <item>light</item>
        <item>amber</item>
        <item>brown</item>
        <item>dark</item>
    </string-array>

android:entries is the human-readable array to present as a list. Change the layout XML:

        android:entries="@array/beer_colors"

The next step is to make the app to react to the value selected in the spinner.
We want the button to call a method when it is clicked, 2 files needed changes:

• the layout file activity_main.xml
Specify the method to be called when the button is clicked. Add the android:onClick attribute to the button element:

<Button
...
        android:onClick<="onClickFindBeer"
...
/>

• the activity file MainActivity.java
Implement the method to be called when the button is clicked. The onCreate() method is called when the activity object gets created. This is used to do some setup works. setContentView(R.layout.activity_main) tells Android this activity is associated to the layout "activity_main". If you want a method to respond to a button click, it needs the following signature:

  public void method_name(View view)

We can get a handle of the GUI component using a method findViewById(). ID is a number used to identify the view, the ID passed in this function must be in the form R.id.xxx. R.java is a special java file generated by Android, Android use R.java to keep track of resources that enables you to get the reference of GUI components. Add the onClick method in the activity MainActivity.java:
• This method will retrieve the selected value from the spinner then put it in the textview

...
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Spinner;
import android.widget.TextView;
...

public class MainActivity extends AppCompatActivity {
...
  public void onClickFindBeer(View view){
        //Get a reference to the TextView
        TextView brands = (TextView) findViewById(R.id.textview);

        //Get a reference to the Spinner
        Spinner color = (Spinner) findViewById(R.id.planets_spinner);

        //Get the selected item in the Spinner
        String beerType = String.valueOf(color.getSelectedItem());

        brands.setText(beerType);

  }
}

The activity will take the selected value from the spinner, display the result in a text view.

We can add a customer Java class to do more woks before returning the result in the onClick method.
To add the Java class, highlight the package com.example.beeradviser in app/java, and click Fiel -> New -> Class.

A Class template file "BeerExpert.java" will be created for you to work on it.

package com.example.beeradviser;

import java.util.ArrayList;
import java.util.List;

public class BeerExpert {
    List<String> getBrands(String color) {
        List<String> brands = new ArrayList<>();
        if (color.equals("amber")) {
            brands.add("Jack Amber");
            brands.add("Red Moose");
        } else {
            brands.add("Jail Pale Ale");
            brands.add("Gout Stout");
        }
        return brands;
    }
}

Then, change the MainActivity.jav to use the customer Java class:

import java.util.List;


public class MainActivity extends AppCompatActivity {
    private BeerExpert expert = new BeerExpert();
...
        String beerType = String.valueOf(color.getSelectedItem());

        //Get recommendations from the BeerExpert class
        List brandsList = expert.getBrands(beerType);
        StringBuilder brandsFormatted = new StringBuilder();
        for (String brand : brandsList) {
            brandsFormatted.append(brand).append('\n');
        }

        //Display the beers
        brands.setText(brandsFormatted);

...

Chapter 3. Multiple Activities and Intents: State Your Intent

This chapter is to build apps with multiple activities and get apps to talk with each other by using intents.

The app will contain 2 activities and 2 layouts:

• When the app gets launched, it starts activity "CreateMessageActivity"
The layout is "activity_create_message.xml".
• When the button in CreateMessageActivity is clicked, another activity "ReceiveMessageActivity" is launched
The layout is "activity_receive_message.xml".

Create a basic app with a single activity and layout

First, create a new Android Studio project named "Messenger" with a blank activity .
Then change the generated layout "activity_main.xml" and activity "MainActivity.java":

• Use the Design Editor to remove the element <TextView>
• Use the Design Editor to add <Button> and <EditText> elements
Move "Buttons -> Button" and "Text -> Plain Text" to the layout. Then, do the following changes:

    <Button
        android:id="@+id/send"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/Send"
        android:onClick="onClickSendMessage"
        tools:layout_editor_absoluteX="161dp"
        tools:layout_editor_absoluteY="357dp" />

    <EditText
        android:id="@+id/message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:text="@string/Message"
        tools:layout_editor_absoluteX="64dp"
        tools:layout_editor_absoluteY="199dp" />

"android:ems" describes how wide the EditText is.
• Update the resource file "strings.xml" to add the following changes

    <string name="Send">Send Message</string>
    <string name="Message">Input message...</string>

• Update the activity "MainActivity.java" to implement the onClick method

import android.view.View;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    //Call onSendMessage() when the button is clicked
    public void onClickSendMessage(View view) {
 
    }
}

Add a second activity and layout

Click "File -> New -> Activity -> Empty Activity", then, give the name of the activity "ReceiveMessage".
Android Studio will create a new activity file and layout file for you:

• ReceiveMessage.java
• activity_receive_message.xml

Besides, Android Studio changes the AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.messenger">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".ReceiveMessage"></activity>
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

All activities need to be declared in app's AndroidManifest.xml:

• android:name
Specify the name of the activity class. The class name is prefixed with "." because it will be combined with the package name to form a fully qualified class name.

Get the 1st activity to call the 2nd activity

Whenever you want an activity to start another activity, you use an Intent.
You start another activity by :

• creating an intent for a target activity

Intent intent = new Intent(src, dst);

• send the intent to Android

startActivity(intent);

If there is no activity to receive this intent, an ActivityNotFoundException generated.

Get the 1st activity to pass data to the 2nd activity

You can deliver more information to the target activity via the Intent.
To send data to another activity, all you need to do is specify the data and its type.
The most straightforward and common use of the ACTION_SEND action is sending text content from one activity to another. For example,

Intent sendIntent = new Intent();
sendIntent.setAction(Intent.ACTION_SEND);
sendIntent.putExtra(Intent.EXTRA_TEXT, "This is my text to send.");
sendIntent.setType("text/plain");
startActivity(sendIntent);

If there's an installed application with a filter that matches ACTION_SEND and MIME type text/plain, the Android system will run it.

Get the 2nd activity to display received data

Using Design Editor to move a TextView in the layout "activity_receive_message.xml". Then, modify it:

    <TextView
        android:id="@+id/received"
...
    />

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Android Development Patterns

2016
Phil Dutson