Using Web Services

Android provides a full set of Java-standard networking APIs, such as the java.net.HttpURLConnection package, that you can use in your programs. The tricky part is to make the calls asynchronously so that your program's user interface will be responsive at all times.

Consider what would happen if you just make a blocking network call in your main (GUI) thread. Until that call returns (and it might never return), your application cannot respond to any user interface events such as keystrokes or button presses. It will appear hung to the user. Obviously, that's something you'll have to avoid.

The java.util.concurrent package is perfect for this kind of work. First created by Doug Lea as a stand-alone library and later incorporated into Java 5, this package supports concurrent programming at a higher level than the regular Java Thread class. The ExecutorService class manages one or more threads for you, and all you have to do is submit tasks (instances of Runnable or Callable) to the executor to have them run. An instance of the Future class is returned, which is a reference to some as-yet-unknown future value that will be returned by your task (if any). You can limit the number of threads that are created, and you can interrupt running tasks if necessary.

To illustrate these concepts, let's create a fun little program that calls the Google Translation API.6 Have you ever laughed at strange translations to and from foreign languages, especially computer-generated translations? This program will let the user enter a phrase in one language, ask Google to translate to a second language, and then ask Google to translate it back into the first language. Ideally, you'd end up with the same words you started with, but this is not always the case, as you can see in Figure 7.5, on the next page.

6. http://code.google.com/apis/ajaxlanguage

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