CS 175/286 - Lecture 6

Cay S. Horstmann

Today's Lecture

• XML Parsing

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;
        }       
    }
	  ...
}

Lecture 6 Quiz 1

Put your answer into Piazza.

What is NOT true about RSS parser code example in Android Recipes Section 3.8?

1. If the author had used the BasicHandler convenience class, the startElement method would have been shorter.
2. The channel element must be handled in the startElement method.
3. The title element must be handled in the endElement method.
4. The inItem flag is necessary.

Lecture 6 Quiz 2

What is NOT true about the XML pull parser implementation of the RSS parser code?

1. It is longer than the SAX implementation.
2. Its structure is more aligned with the structure of the data to be parsed.
3. Unlike the SAX parser, the pull parser checks that the document is valid.
4. The statement

   if (parser.getEventType() != XmlPullParser.START_TAG) {
       continue;
   }

   is necessary.

Reading Before Next Class

• Jump into JUnit 4

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

Reading XML

• Continue with your solution for Lab 5.
• Add the BasicHandler to your project.
• We want to read the question from this file. Use the URL in your code.
• Add the readFromXml method from the slide to the Question class.
• Call the method from doInBackground. What is your code? (You did this in lab 3 and your homework.)
• 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.