To Lecture Notes

IT 372 -- Apr 22, 2024

Review Questions

1. The following code copies the items from an arraylist declared by
   

   ArrayList<Integer> al = new ArrayList<Integer>( );
   

   into a string:
   

   String output = "";
   for(Integer n : al) {
       output += n + "\n";
   }
   

   Perform this operation using a StringBuilder object instead. If there are many items in the arraylist, using a stringbuilder object is much faster.
2. Suppose you have a hashmap collection:
   

   HashMap<String, Integer> map = 
       new HashMap<String, Integer>( );
   

   How do you add an Integer object to the collection using the key "K123"? How do you retrieve the Integer object? We will use a similar concept when we store information from an activity in a storedInstanceState object.
3. Add these strings to an ArrayList collection:

   c  java  kotlin  python  
   

   Then use the ArrayList method replaceAll that accepts a lambda function to convert each of the strings to uppercase.
4. What is an Android drop-down menu called? How do you set one up?
   Answer: It is called a Slider. See the BeerAdvisor and ChineseHoroscope Examples. The Chinese Horiscope Example is in the ArrayAdapters section of the Apr 13 Lecture Notes.
5. Create an app named TestWidgets. Switch the layout to LinearLayout, with vertical orientation. Add a ScrollView that includes the linear layout. Then place each of these widgets in the linear layout:
   

   Switch  ToggleButton  Button
   

   When the button is clicked, show the states of the switch and the toggle button.
6. List the XML attributes that we use to position items in a layout.
7. What is an Android Toast. How do you display one?
8. What is an inner class? Give an example of an inner class used to implement an Android event handler.
9. Create an Android app with four radio buttons with text Freshman, Sophomore, Junior, Senior. Place them in a radio button group. Add a button with text Show Year that shows the selected year using the codes 1, 2, 3, and 4.
10. How do you restrict the inputs to an EditText widget? For example, how do you set it up so that only numbers can be entered?

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);
     

The Chinese Horoscope Example

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

The Activity Lifecycle

• We know about the onCreate method. It is a required method in every 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.
• 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.

The SendMessage Example

• Look at the SendMessage Example.