This document summarizes the structure and key components of an Android application. It explains that the main activity is defined in MainActivity.java, which links to the layout defined in activity_main.xml through R.java. R.java contains resource identifiers that map to strings, colors, and other resources defined in files like strings.xml. The activity_main.xml layout displays the text "Hello World!" by referring to the string resource @string/hello defined in strings.xml. It demonstrates how resources, Java code, and XML layouts work together in an Android app.

1. 05.More on Application
Structure
Oum Saokosal
Master of Engineering in Information Systems, South Korea
855-12-252-752
oum_saokosal@yahoo.com

2. Structure of Android App
Controller: MainActivity.java
Liaison: R.java
View: activity_main.xml

3. R.java

4. R.java (1)
Do not change. Actually you can't change!

5. R.java (2)
• R stands for Resource.
• R.java is auto-generated class. Do not modify.
• R.java is a class containing the definitions for all
resources.
• Every time you add a resource, e.g. string, color,
picture, movie, mp3, etc, it will be noted into R.java
automatically.
• If errors with R.java, Project -> Clean!

6. R.java (3)
• All members are static final.
– static means: you can access it directly.
– final means: Constant; the value is unchangeable.
– Look at an example:
R.java
…….
public static final class string {
public static final int app_name=0x7f040001;
public static final int hello_world=0x7f040001;
}
• If you want hello, you can call:
R.string.hello_world

7. R.java (4)
• Wait…. please look at it again:
R.java
…
public static final class string {
public static final int app_name=0x7f040001;
public static final int hello_world=0x7f040001;
}
• You can see that app_name and hello are int. So
here, in R.java is where it holds the index of all
resources.

8. R.java (5)
• Remember R.string.hello_world!
So what is the value of hello_world?
• To know the value of hello_world, you need to
open strings.xml under res/values.

9. R.java (6)
• Bottom line, when you invoke
R.string.hello_world in an Activity,
you get the "hello World!" from
strings.xml

10. strings.xml

11. strings.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
<string name="hello“>This is my App.</string>
<string name="app_name">HelloWorld</string>
</resources>
• So when you invoke R.string.hello, you will get:
This is my App.
 R.string.hello = “This is my App”

12. Use Resources tool in strings.xml (1)
• in strings.xml, you can either hard-code or use
Resources tool to add code.

13. Use Resources tool in strings.xml (2)
• To add a new resource, click add -> Choose a
resource, e.g. Color -> OK

14. Use Resources tool in strings.xml (3)
• Give a name and value -> Save (Ctrl+S)

15. Use Resources tool in strings.xml (4)
• Let’s double check the code:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<string name="hello“>This is my App.</string>
<string name="app_name">HelloWorld</string>
<color name="mycolor">#F0F</color>
<color name="blue">#00F</color>
</resources>

16. activity_main.xml

17. main.xml (1) – A Call from HelloWorld.java
• But before you go to main.xml, let’s take a
quick look at HelloWorld.java :
package com.kosalab;
import android.app.Activity;
import android.os.Bundle;
public class HelloWorld extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}

18. main.xml (2) – R.layout.main
• setContentView(R.layout.activty_main);
• Can you figure out what is R.layout.activty_main?
In R.java:

19. main.xml (3) – setContentView()
• setContentView(int layoutResId):
By using setContentView(int layoutResId) method, a
layout will be displayed on the screen.
• setContentView(R.layout.activity_main)
means to add activity_main.xml layout onto the
screen.

20. main.xml (4) – setContentView()
So setContentView(R.layout.main) will include the code
below onto the screen:

21. main.xml (5)
• After all, it is to show this to you:

22. main.xml (6) - @string/hello
• So where did you get “Hello World!” from?
- It is from activity_main.xml:
...
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/hello_world"
……
/>
</LinearLayout>

23. main.xml (7) - @string/hello
• android:text="@string/hello"
@ : at, refer to, point to
@string : at string resource
• So android:text="@string/hello" refers to
hello from strings.xml:
…
<string name="hello">This is my App</string>
…

24. main.xml (8) – Skeleton
It is time to look at main.xml itself:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/hello"
/>
</LinearLayout>

25. main.xml (9) – Skeleton
• main.xml contains:
– layout
– view
• And a view is in a layout. A layout can wrap
many views.

26. <?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<TextView
android:id="@+id/textView01"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/hello"
/>
<ImageView
android:id="@+id/ImageView01"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/ferrari"
/>
</LinearLayout>
Layout
View
View
End of Layout

27. main.xml (10)

28. main.xml (11) – Understand Layout tool
Layout
View Component
Outline of Layout
and View Component
Property of the Layout or
View Component

29. Go on to the next slide