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In the previous lession, we learn how to setting up Android Environment

Now we start to create the first Application: Change background

Step 1: create new project
android file new project

Configure new project
android configure new project

Target android devices
android target android devices

Add an activity to mobile
android add an activity to mobile

Customize the activity
android customize the activity

Click Finish.
Step 2: Create new Android Virtual Device
You can user AVD manager in Toolbar
avd manage toolbar
or Tools -> Android -> AVD manager
tools android avd manager

Click the button Create Virtual Device to create your Virtual device 😀 …

You can go to this tutorial to view detail in Step 4: How to fix error “HAXM is not working and emulator runs in emulation mode” for Android Emulator

Click Finish

Step 3: begin your Application
Now we get the screen of Android Studio:
android studio application start first

3.1: First, work with content_main.xml -> use tab text

Create the textview

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:textColor="#ff0000"
        android:text="Change backgroud tutorial (tutorialspots.com)" />

android textview

Create the green button
Use the tab Design and drag the button and drop to center of Layout
android create new button

Use the tab text, and change the code of the button:

<Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Green Button"
        android:id="@+id/greenbutton"
        android:layout_below="@+id/textView"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="44dp" />

Create the red and blue buttons

<Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Red Button"
        android:id="@+id/redbutton"
        android:layout_below="@+id/textView"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="104dp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Blue Button"
        android:id="@+id/bluebutton"
        android:layout_below="@+id/textView"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="164dp" />

android create 3 buttons

Use ID for RelativeLayout

<RelativeLayout 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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:id="@+id/background"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.tutorialspots.changebackgroundapplication.MainActivity"
    tools:showIn="@layout/activity_main">

3.2 Work with MainActivity.java
Define some variables:

After the line

public class MainActivity extends AppCompatActivity {

Add some new lines:

    public RelativeLayout background;
    public Button greenbutton;
    public Button redbutton;
    public Button bluebutton;

Note: Android studio will auto add:

import android.widget.Button;
import android.widget.RelativeLayout;

In method onCreate insert some new lines

background = (RelativeLayout) findViewById(R.id.background);
greenbutton = (Button) findViewById(R.id.greenbutton);
        redbutton = (Button) findViewById(R.id.redbutton);
        bluebutton = (Button) findViewById(R.id.bluebutton);

greenbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                background.setBackgroundColor(Color.GREEN);
            }
        });

        redbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                background.setBackgroundColor(Color.RED);
            }
        });

        bluebutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                background.setBackgroundColor(Color.BLUE);
            }
        });

Step 4: start a AVD
start avd

Wait for some minutes until you see the AVD and Android startup

Step 5: Run the application

android run application

Wait a few minutes and we’ll get the result:
change background application

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