Stop the key-value pair insanity

The Properties class is an old favorite, around since the beginning of Java programming time with very few changes. The Tiger release of J2SE enhances this class, which allows you not only to use it to specify key-value pairs on a single line separated by an equal sign, but also to use XML files to load and save those key-value pairs. In this installment of Taming Tiger, John Zukowski demonstrates how to use this updated work horse. Share your thoughts on this article with the author and other readers in the accompanying discussion forum. (You can also click Discuss at the top or bottom of the article to access the forum.)

J2SE versions prior to 1.5 required you to work directly with the XML parser to load a configuration file and store settings. While it was never difficult, and while the parser is a standard part of the platform, the extra work was a bit of an annoyance. The newly updated java.util.Properties class now offers an easier way to load and store settings for a program: the loadFromXML(InputStream is) and storeToXML(OutputStream os, String comment) methods.

Properties basics

If you aren't familiar with the java.util.Properties class, you use it to store a set of key-value pairs in a file, where the key and value are separated by an equal sign, as shown in Listing 1.

foo=bar
fu=baz

Had Listing 1 been loaded into a Properties object, you would then find two keys (foo and fu) and two values (bar for foo and baz for fu). The class supports embedding Unicode strings with \u, but the important thing here is that everything is treated as a String.

Listing 2 shows how to load the properties file and list its current set of keys and values. You just pass an InputStream for the file to the load() method, and each key-value pair is added to the Properties instance. You would then use list() to list all the properties or getProperty() to retrieve an individual one.

import java.util.*;
import java.io.*;

public class LoadSample {
  public static void main(String args[]) throws Exception {
    Properties prop = new Properties();
    FileInputStream fis = 
      new FileInputStream("sample.properties");
    prop.load(fis);
    prop.list(System.out);
    System.out.println("\nThe foo property: " +
        prop.getProperty("foo"));
  }
}

Running the LoadSample program produces the output in Listing 3. Notice that the output of the list() method does not produce the list of key-value pairs in the same order they were in the input file. The Properties class stores the set of pairs in a hashtable (in fact, it is a Hashtable subclass), so there is no guarantee for order.

-- listing properties --
fu=baz
foo=bar

The foo property: bar