To Lecture Notes

IT 372 -- Apr 15, 2019

Review Questions

1. How does an Android App know which startup activity file to use?
   Ans: The startup activity has these tags between <activity> and </activity>:
   

   <intent-filter>
       <action android:name="android.intent.action.MAIN" />
       <category android:name="android.intent.category.LAUNCHER" />
   </intent-filter>
   

2. How does an Android App know which layout to use for the startup app.
   Ans: The following method call is included in the onClick event handler:
   

   setContentView(R.layout.activity_main);
   

3. What does the gravity attribute do?
   Ans: It determines how the contents of the view are positioned within the view. Contrast this with the layout_gravity, which determines now the view is positioned within its parent.
4. What is wrong with this linear layout?
   

   <!-- Main layout file -->
   <LinearLayout
       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:padding="50dp"
       android:orientation="vertical"
       tools:content=".MainActivity">
   
   </LinearLayout>
   

   Ans: The validation header at the top is missing:
   

   <?xml version="1.0" encoding="utf-8"?>
   

   The validation header must be the first tag in the XML file. All other tags, even comments, must come after it.
5. Set up a linear layout having horizontal orientation that contains two nested linear layouts each having vertical orientation. Each nested layout should have three buttons. The button texts are Button1, Button2, ... , Button 6
   Ans: See the Buttons Example (to be posted soon).

Extended Color Names

• Look at where standard colors are located on the HSL color wheel.
• HSL means Hue, Saturation, Lightness.
• A table of HTML Extended Color Names
• Here is the color wheel that we discussed in class,with hex color codes.
• A color entry in the colors.xml file looks like this:
  

   <color name="color1">#e0e0ff</color>
  

Setting the Background Color

1. We know how to set the background color in the layout file:
   

   android:background="ffffd0"
   

   or

   android:background="@color/color1"
   

2. To set or change the background color in the Java code, first obtain a View object for the layout or widget for which you wish to set the background color:
   

   LinearLayout layout =
       (LinearLayout) findViewById(R.id.main_layout);
   

   or

   Button btnSubmit =
       (Button) findViewById(R.id.submit);
   

   Then call the View setBackground method:
   

   layout.setBackgroundColor(Color.GREEN);
   

   or

   layout.setBackgroundColor(Color.parseColor("#ffffe0"));
   

   or

   layout.setBackgroundColor(getResources( ).
       getColor(R.color.color1));
   

3. Practice Problem 1: Create an Android app with a button. When the button is pressed repeatedly, the background color of the linear layout cycles through three colors or your choice.

FontSizes Example

• Look at the at the FontSizes Example

Future Example

• On Wednesday, we will look at three examples:
      BackColor     TempConverter     SendMessge