To Lecture Notes

IT 372 -- Apr 13, 2026

Matching Problem

• A button is defined in the layout file like this:
  

  Button button1 = findViewById(R.id.btn_1);
  

  Each of the code fragments a, b, c, and d adds an event listener to a button.  Match these code fragments  to the methods:
       Method 1   Method 2   Method 3   Method 4

  // a.
  button1.setOnClickListener(new View.OnClickListener( ) {
      public void onClick(View view) {
          // body of click event listener
      }
  }
  
  // b. 
  // In the layout file activity_main.xml:
  android:onClick="onClick"
  
  // In the main activity file MainActivity.java:
  public void onClick(View view) {
      // body of click event listener
  }
  
  // c.
  button1.setOnClickListener(new MyListener( ));
  
  // The inner MyListener class is defined in the MainActivity 
  // class, but outside of the onCreate method like this:
  private class MyListener implements View.OnClickListener {
      public void onClick(View view) {
          // body of click event listener
      }
  }
  
  // d.
  button1.setOnClickListener( view -> {
      // body of click event listener
  });
  

Installing an App on an Android Phone

1. How do you start an Android app from the virtual device?
   Ans: On the virtual device, if an app is running, click the back arrow to stop the app. Then click in the icon for the app that you want to run.
2. How do you uninstall an app from the virtual machine?
   Answer: Click the Settings Icon >> See all ? apps, where ? is the actual number of apps. >> Click on the app icon. >> Click Uninstall App.
3. Look at the complete Magic8Ball Example, complete with all 20 possible predictions.
   If you have an Android phone, you may wish to install this app or other apps on your phone. To do this, turn on Developer options using these directions:
        https://developer.android.com/studio/debug/dev-options
   (You really must tap the Build Number option 7 times to enable Developer options). Then connect your phone to your PC with a USB cable and run the app in the Emulator. This will simultaneously install and run the app on your phone.

Adding a ScrollView

• If the height of a linear layout is too large to all be viewable on a device, add a ScrollView. Notice that the ScrollView node is now the root node and the LinearLayout is contained in the ScrollView node.
• You can add a "ghost view" to pad the bottom of the layout if the bottom of the layout is cut off:
  

  <View
      layout_width="match_parent"
      layout_height="25dp" />
  

Practice Problems

1. Display the numbers from 1 to 1000 in a textview control, one number per line. Add a ScrollView to allow the user to swipe up and down to view all the numbers.
   Answer: Here are the layout and activity files:
        activity_main.xml   MainActivity.java
2. Start with some Lorem Ipsum text in a textview widget:
   

   Lorem ipsum dolor sit amet, 
   consectetur adipiscing elit. 
   In nibh lectus, accumsan 
   at ligula sagittis, bibendum 
   suscipit enim.
   

   When the textview widget is clicked, concatenate 100 copies of this paragraph together and display the new repeated text in the textview. Add scrollbars so the user can see the entire textview.
   Answer: Here are the layout and activity files:
        activity_main.xml   MainActivity.java   strings.xml
3. Create a RollDice Example that randomly displays these dice output images displayed in two ImageView widgets:
        face1.jpg  face2.jpg  face3.jpg   face4.jpg  face5.jpg  face6.jpg
   
   Use this RollDice class:
   

   public class RollDice {
       public static ArrayList<Integer> roll(int numDiceFaces) {
           ArrayList<Integer> rolls = new ArrayList<Integer>( );
           Random r = new Random( );
           int die1 = r.nextInt(numDieFaces) + 1;
           int die2 = r.nextInt(numDieFaces) + 1;
           rolls.add(die1);
           rolls.add(die2);
           return rolls;
       }
   }
   

   Use android:orientation="horizontal" for the linear layout.
   Answer: Here are the layout, RollDice class, and activity files:
        activity_main.xml   RollDice.java   MainActivity.java

The Design Editor

• The Design Editor is an alternative to directly editing the XML code that defines an Android layout.
• Use the Design editor to convert the default ConstraintLayout to a LinearLayout:
  Using the Design Editor, in the Component Tree,
  Right click on ConstraintLayout >>
  Select Convert view... >>
  Select LinearLayout to Convert View to: LinearLayout >>
  Click Apply.
• Use the Design Editor to drag five widgets onto a LinearLayout:
       TextView  Button  CheckBox  RadioButton  Switch
• Use the Design Editor to set these attributes of the widgets:
       id  text  layout_marginLeft   layout_marginTop
  Add an click event listener that enters the state of the check box, radio button, and switch in the textview when the button is clicked.
• The layout file and screen shot of the UseDesignEditor app that we worked on in class:
  activity_main.xml   Screen Shot
• Use the Design Editor to create an app that converts a Celsius temperature into Fahrenheit. Enter the Celsius temperature into an EditText widget and show the Fahrenheit temperature in a TextView widget when a button is clicked.

Project 2

• Look at Project 2.
• On Wednesday, we will look at examples of how to use checkboxes, radio buttons, and spinners.

The Android Toast

• A toast is a short message that is displayed to the app user.
• A toast appears for a short amount of time (2 to 3.5 seconds), then it disappears.
• The term comes from a toast presented at a wedding or other meeting; it is a short speech.
• Create and display a toast like this:
  

  Toast t = Toast.makeText(getApplicationContext( ),
      "Testing the toast.", Toast.LENGTH_LONG);
  t.show( );
  

• If you don't want to save the Toast object, create and show it like this:
  

  Toast.makeText(getApplicationContext( ),
      "Testing the toast.", Toast.LENGTH_LONG).show( );
  

• Practice Problem: Create an app with a linear layout that displays a Toast when a LongClick event is detected. An Android Toast is a small message displayed on the screen, similar to a tool tip or popup.
  Answer: The layout, color resource, and activity files:
  activity_main.xml   colors.xml   MainActivity.java

The Activity Lifecycle

• We know about the onCreate method in an Android activity. What are some other Android lifecycle methods?
• Look at this document from the Android Developer website:
      Understand the Activity Lifecycle
• The six important events in the activity lifecycle:
  onCreate onStart onResume onPause onStop onDestroy
• An event handler is said to fire when it executes in response to its event occurring.

• Event	Event Handler	Description	Required
  ON_CREATE	onCreate	Fires immediately after the activity is first created.	Yes
  ON_START	onStart	Fires immediately after the activity becomes visible.  It prepares the activity to move into the foreground and become interactive.	No
  ON_RESUME	onResume	Fires when the activity moves into the foreground after going into the background.	No
  ON_RESTART	onRestart	Fires when a covered activity moves into the foreground after being stopped, but not destroyed.	No
  ON_PAUSE	onPause	Fires immediately before the activity moves into the background. It may still be visible.	No
  ON_STOP	onStop	Fires when the activity has finished running and is no longer visible to the user, but may still be in memory.	No
  ON_DESTROY	onDestroy	Fires immediately before an app has terminated and is about to be unloaded from memory.	No

• Transition Diagram of the Activity Lifecycle.
• Event handlers are callback methods. They are not called in the activity code, but in the Android framework when the event occurs that corresponds to the event handler.
• Callbacks epitomize the Hollywood Principle: "Don't call us, we'll call you."
• Here is an automobile analogy to these lifecycle events.  In this analogy, the "app" consists of driving a car from home to the office.
  

  @Override
  public void onCreate(...) {
      super.onCreate(...);
      openGarageDoor( );
      unlockCar( );
      getInCar( );
      closeCarDoor( );
      putOnSeatBelt( );
      putKeyInIgnition( );
  }
  
  @Override
  public void onStart( ) {
      super.onStart( );
      startEngine( );
      changeRadioStation( );
      switchOnLightsIfNeeded( );
      switchOnWipersIfNeeded( );
  }
  
  @Override
  public void onResume( ) {
      super.onResume( );
      applyFootbrake( );
      releaseHandbrake( );
      putCarInGear( );
      drive( );
  }
  
  @Override
  public void onPause( ) {
      super.onPause( );
      putCarInNeutral( );
      applyHandbrake( );
  }
  
  @Override
  public void onStop( ) {
      super.onStop( );
      switchOffLights( );
      switchOffRadio( );
      switchOffHeater( );
      turnOffEngine( );
      removeSeatBelt( );
      getOutOfCar( );
      lockCar( );
  }
  
  @Override
  public void onDestroy( ) {
      super.onDestroy( );
      goToOfficeBuilding( );
  }
  
  @Override
  protected void onRestart() {
      unlockCar( );
      getInCar( );
      closeCarDoor( );
      putOnSeatBelt( );
      putKeyInIgnition( );
  }
  

  Reference:
      danielsaidi.com/blog/2013/08/05/android-activity-lifecycle-car-analogy
• Create an app named Lifecycle methods. Add a print statement to the onCreate method:
  

   @Override
  public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      Log.i("OO1", "In onCreate method");
  }
  

• Add an onStart method with a print statement like this:
  

  @Override
  public void onStart() {
      super.onStart( );
      Log.i("OO1", "In onStart method");
  }
  

• Similarly add onResume and onPause methods.
• ClickCounter Example.
• StopWatch Example.