Learning Java, 4th Edition

Properties

The java.util.Properties class is a specialized hash table for strings. Properties are generally used to hold textual configuration data. Examples of this are the Java System properties, which are passed to a Java application on the command line. We’ll cover those later in this section. More generally, you can use a Properties table to hold arbitrary configuration information for an application in an easily accessible format. The neat thing about a Properties object is that it can load and store its information in a plain text or XML text format using streams (see Chapter 12 for information on streams).

Any string values can be stored as key/value pairs in a Properties table. However, the convention is to use a dot-separated naming hierarchy to group property names into logical structures. (Unfortunately, this is just a convention, and you can’t really work with groups of properties in a hierarchical way as this might imply.) For example, you can create an empty Properties object and add String key/value pairs just as you could with a Map:

    Properties props = new Properties();
    props.setProperty("myApp.xsize", "52");
    props.setProperty("myApp.ysize", "79");

Thereafter, you can retrieve values with the getProperty() method:

    String xsize = props.getProperty( "myApp.xsize" );

If the named property doesn’t exist, getProperty() returns null. You can get an Enumeration of the property names with the propertyNames() method:

    for ( Enumeration e = props.propertyNames(); e.hasMoreElements ...