To Lecture Notes

IT 372 -- Apr 16, 2025

Review Exercises

1. Translate these formulas into Java:
   
   1. a = √(s + 3 t³ⁿ)
      Answer:
      

      double a = Math.sqrt(s + 3 * Math.pow(t, 3 * n));
      

   2. b

      =

      2 s t1+n 1 - 5(u + 2)2n

      
      Answer:
      

      double b = (2 * s * Math.pow(t * (1 + n)) / 
                 (1 - 5 * Math(u + 2, 2 * n));
      

2. How does the Java Elvis operator work? The formal name for this operator is the tertiary conditional operator. Answer: The ? operator looks like Elvis Presley's waivy hair when rotated 90 degrees counterclockwise. The goal of this operator is to express an if..else statement in one line. Here is an example if..else statement:
   

   int n = 476;
   String s = "";
   if (n >= 500) {
       s = "large"; 
   }
   else {
       s = "small";
   }
   

   Here is this if..else statement rewritten using the Elvis operator:
   

   int n = 476;
   String s = (n >= 500) ? "large" : "small";
   

3. Create an app with a radio button group containg four radio buttons with text Freshman, Sophomore, Junior, Senior. Answer:
         activity_main.xml  MainActivity.java
4. Set up an app with an EditText widget that only accepts decimal numbers. Answer:
         activity_main.xml 
5. 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

BeerAdvisor Example

• Look at the BeerAdvisor Example.

Array Adapters

• Practice Problems
  
  1. Add items programatically to a Spinner widget? Use an ArrayAdapter object.
     Answer: Add a Spinner widget to the layout, with id set to spinner1. Then
     
     1. place the array of Integer objects to display in the spinner in an array list,
     2. set up an ArrayAdapter object that connects the spinner to the array list.
     Here is the Java code:
     

     // Set up array list.
     ArrayList<Integer> arrList = new ArrayList<Integer>( );
     for(int n = 1001; n <= 1005; n++) {
         arrList.add(n);
     }
     
     // Set up array adapter.
     ArrayAdapter<Integer> adapter = new ArrayAdapter<Integer>(
         this, android.R.layout.simple_list_item_1, arrList);
     spinner1.setAdapter(adapter);
     

• ChineseHoroscope:
  
  1. Input a year of birth using a Spinner widget. Use an array adapter to load the years into the spinner.
  2. Determine the Chinese horoscope animal corresponding to the year. The animal number can be obtained from year % 12.
  3. Display the animal in a TextView widget.
  
  Answer: The layout and activity files: activity_main.xml   MainActivity.java

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

• This section will be discussed at a later date.
• 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);
      System.out.println("In onCreate method");
  }
  

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

  @Override
  public void onStart() {
      super.onStart( );
      System.out.println("In onStart method");
  }
  

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

Android Platform

• Some Details about the Android Platform.
• To view files on the virtual device, use the Device Explorer.