## The problem

Suppose that you need a easy way to persist some objects in the file system. Not just one, but a whole collection. The real problem arrives when you start using java.io api in order to create one output stream for each object, showing itself to be really painful - although simple.

Imagine that you have the following Java class, a basic Author class (stolen from some other tutorial):

    package com.thoughtworks.xstream;

    public class Author {
            private String name;
            public Author(String name) {
                    this.name = name;
            }
            public String getName() {
                    return name;
            }
    }

By using the XmlArrayList implementation of java.util.List you get an easy way to write all authors to disk

The XmlArrayList (and related collections) receives a StreamStrategy during its construction. This Strategy decides what to do with each of it's elements. The basic implementation - our need - is the FileStreamStrategy, capable of writing different files to a base directory.

    // prepares the file strategy to directory /tmp
    StreamStrategy strategy = new FileStreamStrategy(new File("/tmp"));

We can easily create an XmlArrayList from that strategy:

    // prepares the file strategy to directory /tmp
    StreamStrategy strategy = new FileStreamStrategy(new File("/tmp"));
    // creates the list:
    List list = new XmlArrayList(strategy);

## Adding elements

Now that we have an XmlArrayList object in our hands, we are able to add, remove and search for objects as usual. Let's add five authors and play around with our list:

    package org.codehaus.xstream.examples;

    public class AddAuthors {

    	public static void main(String[] args) {

    		// prepares the file strategy to directory /tmp
    		StreamStrategy strategy = new FileStreamStrategy(new File("/tmp"));
    		// creates the list:
    		List list = new XmlArrayList(strategy);

    		// adds four authors
    		list.add(new Author("joe walnes"));
    		list.add(new Author("joerg schaible"));
    		list.add(new Author("mauro talevi"));
    		list.add(new Author("guilherme silveira"));

    		// adding an extra author
    		Author mistake = new Author("mama");
    		list.add(mistake);

    	}
    }

If we check the /tmp directory, there are five files: 1.xml, 2.xml, 3.xml, 4.xml, 5.xml, each one containing the XML serialized form of our authors.

## Playing around

Let's remove mama from the list and iterate over all authors:

    package org.codehaus.xstream.examples;

    public class RemoveMama {

    	public static void main(String[] args) {

    		// prepares the file strategy to directory /tmp
    		StreamStrategy strategy = new FileStreamStrategy(new File("/tmp"));
    		// looks up the list:
    		List list = new XmlArrayList(strategy);

    		// remember the list is still there! the files 1-5.xml are still in /tmp!
    		// the list was persisted!

    		for(Iterator it = list.iterator(); it.hasNext(); ) {
    			Author author = (Author) it.next();
    			if(author.getName().equals("mama")) {
    				System.out.println("Removing mama...");
    				it.remove();
    			} else {
    				System.out.println("Keeping " + author.getName());
    			}
    		}

    	}
    }

The result?

    Keeping joe walnes
    Keeping joerg schaible
    Keeping mauro talevi
    Keeping guilherme silveira
    Removing mama...

## Going further

From this point on, you can implement different StreamStrategies in order to generate other behaviour using your XmlArrayList collection, or try the other implementations: XmlSet and XmlMap.