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Parsing an XML file with DOM

From: Java Essential Training

Video: Parsing an XML file with DOM

There are many ways to read XML files in Java, there are many programming interfaces and packages available. They are known by their acronyms, there is JDOM and Stacks and Sacks, but the most venerable, the one that's been around the longest is known as DOM or Document Object Model. The Document Object Model approach to reading XML isn't the most fun or the most concise, but it's the most deeply embedded in the Java libraries. And so that's the version I'm going to show you here. I'm not going to show you everything about DOM that's something that would require a whole separate course, but I'll introduce you to some the most critical classes that are available.

Parsing an XML file with DOM

There are many ways to read XML files in Java, there are many programming interfaces and packages available. They are known by their acronyms, there is JDOM and Stacks and Sacks, but the most venerable, the one that's been around the longest is known as DOM or Document Object Model. The Document Object Model approach to reading XML isn't the most fun or the most concise, but it's the most deeply embedded in the Java libraries. And so that's the version I'm going to show you here. I'm not going to show you everything about DOM that's something that would require a whole separate course, but I'll introduce you to some the most critical classes that are available.

And as with the classes that I've shown you to read files from the local file system and from the network these are classes that are available in the core Java class library. As with the previous video, I'm going to be working with this XML feed at services.explorecalifornia.org/rss/tours.php. The code I'll show you will work with any RSS feed because the structure of an RSS file is standardized. So let's start with this empty glass. So the first step in reading an XML file is retrieving it and when you're working in the document object model world, your first goal is to create a document object.

It takes a few classes to get there. I'm working in an empty project named parsexml in its class read XML and its empty main method. The first step is to create an instance of a class called document builder factory. I'll type outside DocumentBuild and press Ctrl + Spacebar and I'll choose the DocumentBuilder factory class from Javax.xml.parsers. And I'll name the variable factory. Now this code is going to get a little bit wide, so I'll maximize my editor and then I'll press Enter or Return and go to the next line and the DocumentBuilderFactory class is a class that you instantiate by calling it Static Method New Instance.

So I'll call DocumentBuilderFactory.newInstance(). Now I have an instance of that class and I am ready to go onto the next step, creating an instance of a class called DocumentBuilder. I'll type in the name of the class and I'll choose the class from Javax.xml.parsers the same package as the factory class. I'll name this one builder and I'll get its reference from factory.newDocumentBuilder. Now I'm ready to create my XML document object. The data type for this will be document, I'll type in the name of the class and press Ctrl + Spacebar and I'll make sure that I'm choosing the document class from org.w3c.dom.

This document class implements the document object model programming interface as defined by the W3C. It's the same programming interface that you might use in JavaScript and in other programming languages, but it's specifically implemented for Java. I'll name this variable doc and I'll get its reference by calling the document builder classes parse method like this builder.parse and then I'll pass in the URL to the parse method. I'll go back to my browser, I'll copy the URL to the clipboard, I'll return to Eclipse, type in a double quote, paste in the URL close the string and close the statement.

When you call the parse method, it takes care of the downloading of the file and parsing the document and turning it into a hierarchical set of objects that you can traverse using document object model programming. Once you have the document in memory, the next step is to get the data, you could walk down the XML tree one level at a time, but there's a really great convenience method that's a part of the document class named get elements by tag name. If you know the tags that is the element names in your XML file, then you know how to get the data out.

So I'm going to create an object which is data typed as NodeList. In DOM a node list is kind of like an array, it's an ordered collection of objects. It has its own API which is a little bit different from an array as you'll see in a moment, I'll type in NodeList and press Ctrl + Spacebar and choose the Node List from org.W3C.dom. I'll name the variable list, and I'll populate it using the syntax doc.GetElementsByTagName and I'll pass in the name of an element I'm looking for, title.

In RSS, each news item is named an item and each item element has a sub element named title. For this exercise, I'm only interested in the titles of the news articles. So that's what I'm retrieving. And now let's find out how many items we got back. I'll output a string, "There are" and then here's a way in which the node list class is different from an array. With an array, you determine the size of the array with its length property.

With the node list you call a getLength method. It looks like this list.getLength I pressed Ctrl + Spacebar to auto complete the code there. Then I'll append the string items. I'll save my changes and see if there are any errors, I see one right here that I need to fix. It tells me that there's a syntax error here and I just missed my plus operator, here we go. I'll save that change, and see that there are errors from unhandled exceptions as I've done earlier exercises, I'll handle that by selecting the code and then surrounding the code with a Try Catch block.

There are three possible exceptions and Eclipse generates catch block for each of them. I'll get rid of the TODO comments to shorten the code and then I'm ready to save and run the application. And I'm told that there are 27 items. So now, the next step is to go get the text value of each title element. In the document object model, each part of the XML file is seen as something called a node, and each node is of a particular type. So for example, let's say that you had an XML node that looked like this starting with title and ending with title and then between those tags some text.

In the document object model, that's actually two separate notes, one parent and one a child. Title is an element node and the text within it is a text node, and if you know then you know how to get the text out from the title. I'm going to create a loop. I'll use a for loop to loop through the node list contents, I'll choose iterate over array and I'm going to keep on looping as long as my counter variable doesn't exceed the length of the node list.

So I'll change args.length to lists.getLength. Within the for loop, I'll get the item in the list. I am going to data type the item that I'm retrieving as an Element. Once again, this is a class of the DOM interface. I'll name the object item and I'll set its value using casting syntax. I'm going to be retrieving an item from the list, the node list will return it as a note, the superclass but I know it's an element the subclass.

I'll retrieve it, by calling the item method and passing in the "I" counter variable. Finally, I'll output the value within the element. To do this, I'm going to walk down the XML tree from the Element Node down to its Child Node, the Text Node. I know it's the first child of the element, because text nodes always are. So I'll use system.out.println and I'll output item.getFirstChild and then from there I'll call a method called getNodeValue.

When you're working with a Text Node, the GetNodeValue method returns the string value. I'll save and run the application and there is the result. A listing of all the titles, from all the items within the RSS feed. Now if that seems like a lot of code, just to get titles you're right, it's more code than is really needed and if you switched over to one of the third-party libraries for working with XML and Java such as JDOM you'd find your code was significantly smaller and easier to work with but it's important to know what's available in the Java class library.

When you're working with XML in the class library that's a part of the SDK the document object model is always available. It works in all Java development environments, Console applications, Web applications, mobile applications for Android and Blackberry, because as always it's just Java. There is obviously a lot to learn just about working with the document object model, but you can choose between that approach and the other libraries that are available in the Java community.

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This video is part of

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Java Essential Training

71 video lessons · 72299 viewers

David Gassner
Author

 
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  1. 10m 8s
    1. Welcome
      1m 3s
    2. Is this course for you?
      5m 35s
    3. Using the exercise files
      3m 30s
  2. 31m 24s
    1. The history of Java
      5m 19s
    2. Java compilation and syntax
      8m 54s
    3. Understanding the principles of Java
      8m 28s
    4. Choosing a development environment
      8m 43s
  3. 19m 5s
    1. Installing Java on Windows
      6m 42s
    2. Installing Eclipse on Windows
      3m 19s
    3. Exploring Java on Mac OS X Leopard and Snow Leopard
      2m 27s
    4. Installing Java on Mac OS X Lion
      3m 27s
    5. Installing Eclipse on Mac OS X
      3m 10s
  4. 46m 10s
    1. Creating a Hello World application
      11m 7s
    2. Exploring the Eclipse IDE
      8m 55s
    3. Compiling and running from the command line
      8m 2s
    4. Passing arguments to the application
      8m 17s
    5. Using the Java API documentation
      4m 5s
    6. Memory management and garbage collection
      5m 44s
  5. 58m 57s
    1. Everything is an object
      5m 59s
    2. Declaring and initializing variables
      9m 15s
    3. Working with numbers
      8m 32s
    4. Converting numeric values
      6m 40s
    5. Understanding operators
      7m 58s
    6. Working with character values
      5m 14s
    7. Working with boolean values
      5m 13s
    8. Outputting primitive values as strings
      5m 33s
    9. Creating a simple calculator application
      4m 33s
  6. 53m 40s
    1. Writing conditional code
      5m 35s
    2. Using the switch statement
      8m 50s
    3. Repeating code blocks with loops
      7m 35s
    4. Creating reusable code with methods
      6m 31s
    5. Declaring methods with arguments
      5m 41s
    6. Overloading method names with different signatures
      5m 53s
    7. Passing arguments by reference or by value
      5m 35s
    8. Creating a more complex calculator application
      8m 0s
  7. 20m 30s
    1. Using the String class
      5m 44s
    2. Building strings with StringBuilder
      3m 34s
    3. Parsing string values
      3m 19s
    4. Working with date values
      7m 53s
  8. 20m 44s
    1. Understanding compile-time vs. runtime errors
      4m 5s
    2. Handling exceptions with try/catch
      4m 55s
    3. Throwing exceptions in methods
      2m 50s
    4. Using the debugger
      8m 54s
  9. 32m 22s
    1. Using simple arrays
      4m 47s
    2. Using two-dimensional arrays
      6m 17s
    3. Managing resizable arrays with ArrayList
      7m 14s
    4. Managing unordered data with HashMap
      6m 5s
    5. Looping through collections with iterators
      7m 59s
  10. 52m 2s
    1. Understanding encapsulation
      5m 59s
    2. Creating and instantiating custom classes
      8m 8s
    3. Organizing classes with packages
      6m 47s
    4. Creating and using instance methods
      6m 52s
    5. Storing data in instance variables
      6m 56s
    6. Using constructor methods
      5m 40s
    7. Managing instance data with getter and setter methods
      8m 26s
    8. Using class variables and Enum classes
      3m 14s
  11. 41m 15s
    1. Understanding inheritance and polymorphism
      9m 12s
    2. Extending custom classes
      9m 1s
    3. Overriding superclass methods
      3m 8s
    4. Casting subclass objects
      5m 3s
    5. Understanding interfaces and implementing classes
      4m 2s
    6. Creating your own interfaces
      4m 14s
    7. Using abstract classes and methods
      6m 35s
  12. 32m 17s
    1. Managing files with the core class library
      7m 46s
    2. Managing files with Apache Commons FileUtils
      7m 32s
    3. Reading a text file from a networked resource
      7m 52s
    4. Parsing an XML file with DOM
      9m 7s
  13. 17m 39s
    1. Creating your own JAR files
      4m 54s
    2. Understanding the classpath
      5m 2s
    3. Documenting code with Javadoc
      7m 43s
  14. 47s
    1. Goodbye
      47s

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