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Trail: Essential Java Classes
Lesson: Setting Program Attributes

Using Properties to Manage Program Attributes

An attribute has two parts: a name and a value. For example, "" is the name for one of the Java platform's system attributes; its value contains the name of the current operating system, such as "Solaris".

The Properties (in the API reference documentation) class in the java.util package manages a set of key/value pairs. A key/value pair is like a dictionary entry: The key is the word, and the value is the definition. This is a perfect match for managing the names and values of attributes. Each Properties key contains the name of a system attribute, and its corresponding Properties value is the current value of that attribute.

The System class uses a Properties object for managing system properties. Any Java program can use a Properties object to manage its program attributes. The Properties class itself provides methods for the following:

Properties extends the HashtableProperties (in the API reference documentation) class and inherits methods from it for doing the following:

Security Considerations:  Access to properties is subject to approval by the current security manager. The example code segments in this section are assumed to be in standalone applications, which, by default, have no security manager. If you attempt to use this code in an applet, it may not work, depending on the browser or viewer in which it is running. See Security Restrictions (in the Essential Java Classes trail) for information about security restrictions on applets.

The Life Cycle of a Program's Properties

The following figure illustrates how a typical program might manage its attributes with a Properties object over the course of its execution.

Starting Up
The actions given in the first three boxes occur when the program is starting up. First, the program loads the default properties from a well-known location into a Properties object. Normally, the default properties are stored in a file on disk along with the .class and other resource files for the program.

Next, the program creates another Properties object and loads the properties that were saved from the last time the program was run. Many applications store properties on a per-user basis, so the properties loaded in this step are usually in a specific file in a particular directory maintained by this application in the user's home directory. Finally, the program uses the default and remembered properties to initialize itself.

The key here is consistency. The application must always load and save properties to the same location so that it can find them the next time it's executed.

During the execution of the program, the user may change some settings, perhaps in a Preferences window, and the Properties object is updated to reflect these changes. For them to have a permanent effect, they must be saved.
Upon exiting, the program saves the properties to its well-known location, to be loaded again when the program is next started up.

Setting Up Your Properties Object

The following Java code performs the first two steps described in the previous section: loading the default properties and loading the remembered properties:
. . .
// create and load default properties
Properties defaultProps = new Properties();
FileInputStream in = new FileInputStream("defaultProperties");

// create program properties with default
Properties applicationProps = new Properties(defaultProps);

// now load properties from last invocation
in = new FileInputStream("appProperties");
. . .
First, the application sets up a default Properties object. This object contains the set of properties to use if values are not explicitly set elsewhere. Then the load method reads the default values from a file on disk named defaultProperties.

Next, the application uses a different constructor to create a second Properties object, applicationProps, whose default values are contained in defaultProps. The defaults come into play when a property is being retrieved. If the property can't be found in applicationProps, then its default list is searched.

Finally, the code loads a set of properties into applicationProps from a file named appProperties. The properties in this file are those that were saved from the program the last time it was invoked (the next section shows you how this was done).

Saving Properties

The following example writes out the application properties from the previous example using Properties's store method. The default properties don't need to be saved each time because they never change.
FileOutputStream out = new FileOutputStream("appProperties");, "---No Comment---");
The store method needs a stream to write to, as well as a string that it uses as a comment at the top of the output.

Note:  The store method was introduced to the Properties class in JDK 1.2. If you are using an earlier release, use the save method instead.

Getting Property Information

Once you've set up your Properties object, you can query it for information about various keys/values that it contains. An application gets information from a Properties object after start up so that it can initialize itself based on choices made by the user. The Properties class has several methods for getting property information:
contains(Object value)
containsKey(Object key)
Returns true if the value or the key is in the Properties object. Properties inherits these methods from Hashtable. Thus they accept Object arguments. You should pass in Strings.
getProperty(String key)
getProperty(String key, String default)
Returns the value for the specified property. The second version allows you to provide a default value. If the key is not found, the default is returned.
list(PrintStream s)
list(PrintWriter w)
Writes all of the properties to the specified stream or writer. This is useful for debugging.
Returns an Enumeration containing the keys or values (as indicated by the method name) contained in the Properties object.
Returns the current number of key/value pairs.

Setting Properties

A user's interaction with a program during its execution may impact property settings. These changes should be reflected in the Properties object so that they are saved when the program exits (and calls the store method). You can use the following methods to change the properties in a Properties object:
put(Object key, Object value)
Puts the key/value pair in the Properties object.
remove(Object key)
Removes the key/value pair associated with key.
Both put and removeHashtable and thus take Objects. You should pass in Strings.

A Real-Life Example

BINGO! (in the Essential Java Classes trail), shows and documents a complete client/server application that implements the game of BINGO. Both the client and the server application in that example use Properties to maintain program attributes.

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