CS 185C/286 - Lecture 4

Cay S. Horstmann

Lecture 4 Quiz 1

Put your answer into Piazza.

In which Activity method should you save user edits?

1. onPause
2. onStop
3. onDestroy
4. all of them

Lecture 4 Quiz 2

Suppose you write an e-book reader. What happens when the user hits the back button, assuming that you didn't override the default behavior?

1. Nothing.
2. The previous book page is displayed.
3. The previous activity in your application is displayed
4. The previous user activity is displayed (which may be in a different application)

Today's Lecture/Lab

• Lists
• Parsing XML

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;
  }

XML Parsing

• XML = syntax for structured data
• Elements can have
  • attributes <img src='hamster.jpg'/>
  • text <p>Hello</p>
  • child elements <ul><li>...</li><li>...</li></ul>
• Android has three kinds of parsers:
  • SAX (event-driven)
  • DOM (builds tree)
  • Pull (pull tokens)
• Android recommends pull parser, but it's not as easily testable outside the phone.
• SAX is fine for simple documents and a part of the JDK

SAX Parsing

• Callbacks
  • void startElement(String uri, String localName, String qName, Attributes attributes)
  • void endElement(String uri, String localName, String qName)
  • void characters(char[] ch, int start, int length)
• characters can be called multiple times. Use my BasicHandler convenience class.
• Subclass BasicHandler. Add fields for whatever you want to collect, or make inner class in the object that you are building.
• Need to catch attributes in startElement
• Catch text in endElement

SAX Parsing Example

public class Question implements Serializable {
    private String text;
    private List<String> choices = new ArrayList<String>();
    private int correctChoice;

    public void readFromXml(InputStream in) throws IOException {
        class QuestionHandler extends BasicHandler {
            @Override
            public void startElement(String uri, String localName,
                    String qName, Attributes attributes) throws SAXException {
                super.startElement(uri, localName, qName, attributes);
                if ("true".equals(attributes.getValue("value")))
                    correctChoice = choices.size(); 
            }
            @Override
            public void endElement(String uri, String localName, String qName)
                    throws SAXException {
                if (qName.equals("text")) text = lastString();
                else if (qName.equals("choice")) choices.add(lastString());
            }
        }
        
        try {       
            SAXParserFactory factory = SAXParserFactory.newInstance();
            SAXParser parser = factory.newSAXParser(); 
            parser.parse(in, new QuestionHandler());
        } catch (Exception ex) {
            IOException ioEx = new IOException(ex.getMessage());
            ioEx.initCause(ex);
            throw ioEx;
        }       
    }
	  ...
}

Reading Before Next Class

• Android Recipes 3-8. XML Parsing

Lab

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

Displaying the Choices in a List

1. Continue with the code from lab 3.
2. Tip: Have a look in AndroidManifest.xml. You can use android:name=".MainActivity" instead of android:name="packageName.MainActivity". A name starting with a period is relative to the package attribute of the manifest element. That way, you don't have to change so many things when you rename your package.
3. Make a layout containing just a text view. (File → New → Other → Android → Android XML Layout file)
4. 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.
5. Comment out the button handler for now and replace the text field initialization in onPostExecute with the initialization of the list view. Make an ArrayAdapter out of the question choices. What is your code?
6. What happens when your application runs?
7. What happens when you select one of the questions?
8. Now add an OnItemClickListener. Move the appropriate code from your button listener. What is your code?
9. Now what happens when you run and select one of the questions?

Reading XML

• Now we want to read the question from this file. Change the URL in the code.
• Add the BasicHandler to your project.
• Add the readFromXml method from the slide to the Question class.
• Call the method from doInBackground. What is your code?
• What happens when you run your program?
• Now look inside readFromXml more closely. Why does the startElement method call super.startElement but the endElement doesn't?
• Why doesn't the startElement method look at qName?
• Why isn't the if condition in startElement

  attributes.getValue("value").equals("true")

  Try it out if you don't know.