CS 175/286 - Lecture 5

Cay S. Horstmann

Lecture 5 Quiz 1

Consider an application that gives a quiz to the user. Are the quiz questions a part of

• The model
• The view
• The controller
• The view and the controller

Lecture 5 Quiz 2

• Button b = new Button(this); b.setTextColor(Color.RED);
• Button b = findViewById("@+id/button1");
• Button b = findViewById(R.id.button1);
• None of the above

Today's Lecture

• Transfering Data Between Activities
• Lists

Transferring Data to the Target Activity

• First parameter is a Context. Activity is a subclass of Context.
• Can add extra stuff to intent which can be retrieved by new activity

  intent.putExtra(keyString, obj);

• Retrieve extra stuff in onCreate of new intent:

  getIntent().getXXXExtra(keyString)

• See the API doc for the various getXXXExtra methods.
• Within one app, best to transfer domain objects, not arrays of strings
• Objects must be serializable.

Lists

• Long, often scrolling, sequence of items
• Put ListView into the activity XML
• Make a separate view for the item
• Use Adapter to populate list
• Add OnItemClickListener to get notified of item selection
• Override void onItemClick(AdapterView<?> parent, View view, int position, long id)

List Adapters

• Simplest case: List of strings
• new ArrayAdapter<String>(activity, itemLayoutId, strings)
• Item layout ID should be a single TextView
• Otherwise, override getView(int, View, ViewGroup) in subclass
• For example, if item view has two text views:

  public View getView(int position, View v, ViewGroup parent) {
      if (v == null) {
          LayoutInflater vi = LayoutInflater.from(getContext());
          v = vi.inflate(R.layout.itemLayoutId, parent);
      }
      Item item = items.get(position);
      ((TextView) v.findViewById(R.id.description)).setText(item.getDescription());
      ((TextView) v.findViewById(R.id.price)).setText(item.getPrice());
      return v;
  }

Reading Before Next Class

• Android Recipes 3-8. XML Parsing
• Do this by next Tuesday. On Thursday, we have a presentation by Microsoft.

Lab

• Bring your laptop to each class
• You will work with a buddy
• One of you writes code, the other types up answers
• Switch roles each lab
• Submit lab work to github

Transferring Data to the Target Activity

1. Continue with the code from the lab of lecture 4.
2. Right now, all the buttons do the same thing. We want each button to bring relevant information to the target activity so that it can do it's job. We'll overdesign the application a bit so that you can use the design in the future.
3. We want to transfer domain objects. Here is a Question class. Add it to your project. Turn off the warning in Window → Preferences → Java → Compiler → Errors/Warnings → Potential programming problems → Serializable class without serialVersionUID.
4. Add an instance variable private Question question to MainActivity. Change the code to construct a new Question(title, choice1, choice2, choice3, choice4), and set the question instance variable. (The correct choice should start with a *.) Read the text and button labels from the Question object.
5. Try your program. It should still work.
6. Now use putExtra to add the question and the index (0, 1, 2, 3) of the current button to the intent. The latter is a bit tricky. Use this outline:

   for (int i = 0; i < buttonIds.length; i++) {
       final int choiceIndex = i;
       ((Button) findViewById(buttonIds[i])).setOnClickListener(
           new View.OnClickListener() {
       	    @Override public void onClick(View arg0) {
               Intent intent = new Intent(MainActivity.this, ResponseActivity.class);
               intent.putExtra("choiceIndex", choiceIndex);
               ...
       }});
   }

7. To see why you need the final, try

   intent.putExtra("choiceIndex", choiceIndex)

   instead. What happens?
8. Now capture the information on the target activity. Call getIntExtra and getSerializableExtra. Then call

   ((TextView) findViewById(R.id.responseText)).setText(
       question.isCorrect(choiceIndex) ? "Good job!" : "Try again!");

9. Try it out. What happens?

Displaying the Choices in a List

1. Make a layout containing just a text view. (File → New → Other → Android → Android XML Layout file)
2. Remove the buttons and all but the top text view from the main layout, and instead put in a list view below the question text.
3. Comment out the button handler for now and replace the text field initialization with the initialization of the list view. Make an ArrayAdapter out of the question choices. What is your code?
4. What happens when your application runs?
5. What happens when you select one of the questions?
6. Now add an OnItemClickListener. Move the appropriate code from your button listener. What is your code?
7. Now what happens when you run and select one of the questions?