Hello World

Create Android Application

The first step is to create a simple Android Application using Android Studio.
Follow the option File -> New Project and finally select Android New Application wizard from the wizard list.
Now you get a chance to name your application and specify a project location, I have named my application as Hellow World Example using the wizard window as follows:

new project

Next, follow the instructions provided and keep all other entries as default till the final step.
phone or tablet name activity

Once your project is created successfully, you will have following project screen:

main screen

The Main Activity File

The main activity code is a Java file MainActivity.java.

This is the actual application file which ultimately gets converted to a Dalvik executable and runs your application.

Following is the default code generated by the application wizard for Hello World! application:

package com.example.com.hellowworldexample;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity {

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

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Here, R.layout.activity_main refers to the activity_main.xml file located in the res/layout folder which we will show you further down.

The Manifest File

Whatever component you develop as a part of your application, you must declare all its components in a manifest.xml which resides at the root of the application project directory.

This file works as an interface between Android OS and your application, so if you do not declare your component in this file, then it will not be considered by the OS.

For example, a default manifest file will look like as following file:

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

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

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

</manifest>

Here <application>…</application> tags enclosed the components related to the application. Attribute android:icon will point to the application icon available drawable/drawable-hdpi or drawable/mipmap-hdpi depending on if you have android:icon=”@drawable/ic_launcher” or android:icon=”@mipmap/ic_launcher”. The application uses the image named ic_launcher.png located in the drawable or mipmap folders.

The <activity> tag is used to specify an activity and android:name attribute specifies the fully qualified class name of the Activity subclass and the android:label attributes specifies a string to use as the label for the activity. You can specify multiple activities using <activity> tags.

The action for the intent filter is named android.intent.action.MAIN to indicate that this activity serves as the entry point for the application. The category for the intent-filter is named android.intent.category.LAUNCHER to indicate that the application can be launched from the device’s launcher icon.

The @string refers to the strings.xml file explained below. Hence, @string/app_name refers to the app_name string defined in the strings.xml, which is “Hellow World Example”.

Similar way, other strings get populated in the application.

The Strings File

The strings.xml file is located in the res/values folder and it contains all the text that your application uses. For example, the names of buttons, labels, default text, and similar types of strings go into this file. This file is responsible for their textual content. For example, a default strings file will look like as following file:

<resources>
    <string name="app_name">Hellow World Example</string>

    <string name="hello_world">Hello world!</string>
    <string name="action_settings">Settings</string>
</resources>

The Layout File

The activity_main.xml is a layout file available in res/layout directory, that is referenced by your application when building its interface. You will modify this file very frequently to change the layout of your application. For your “Hello World!” application, this file will have following content related to default layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

    <TextView android:text="@string/hello_world" android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</RelativeLayout>

This is an example of simple RelativeLayout which we will study in a separate chapter. TheTextView is an Android control used to build the GUI and it have various attributes like android:layout_width, android:layout_height etc which are being used to set its width and height etc. The @string refers to the strings.xml file located in the res/values folder. Hence, @string/hello_world refers to the hello string defined in the strings.xml, which is “Hello World!”.

Running the Application

Let’s try to run our Hello World! application we just created. I assume you had created your AVD while doing environment setup. To run the app from Android studio, open one of your project’s activity files and click Run play icon from the toolbar. Android studio installs the app on your AVD and starts it and if everything is fine with your setup and application, it will display following Emulator window

Android Hello World