[Chapter 7] 7.5 Properties

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| Chapter 7 Basic Utility Classes |

## 7.5 Properties

The `java.util.Properties` class is a specialized hashtable for strings. Java uses the `Properties` object to replace the environment variables used in other programming environments. You can use a `Properties` table to hold arbitrary configuration information for an application in an easily accessible format. The `Properties` object can also load and store information using streams (see Chapter 8, Input/Output Facilities 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, as is done with X resources on UNIX systems.[4] The `java.lang.System` class provides system-environment information in this way, through a system `Properties` table I'll describe shortly.

> [4] Unfortunately, this is just a naming convention right now, so you can't access logical groups of properties as you can with X resources.

Create an empty `Properties` table and add `String` key/value pairs just as with any `Hashtable`:

    Properties props = new Properties();
    props.put("myApp.xsize", "52");
    props.put("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; ) {
        String name = e.nextElement();
        ...
    }

### Default Values

When you create a `Properties` table, you can specify a second table for default property values:

    Properties defaults;
    ...
    Properties props = new Properties( defaults );

Now when you call `getProperty()`, the method searches the default table if it doesn't find the named property in the current table. An alternative version of `getProperty()` also accepts a default value; this value is returned if the property is not found in the current list or in the default list:

    String xsize = props.getProperty( "myApp.xsize", "50" );

### Loading and Storing

You can save a `Properties` table to an `OutputStream` using the `save()` method. The property information is output in flat ASCII format. Continuing with the above example, output the property information to `System.out` as follows:

    props.save( System.out, "Application Parameters" );

As we'll discuss in Chapter 8, Input/Output Facilities, `System.out` is a standard output stream similar to C's `stdout`. We could also save the information to a file by using a `FileOutputStream` as the first argument to `save()`. The second argument to `save()` is a `String` that is used as a header for the data. The above code outputs something like the following to `System.out`:

    #Application Parameters
    #Mon Feb 12 09:24:23 CST 1997
    myApp.ysize=79
    myApp.xsize=52

The `load()` method reads the previously saved contents of a `Properties` object from an `InputStream`:

    FileInputStream fin;
    ...
    Properties props = new Properties()
    props.load( fin );

The `list()` method is useful for debugging. It prints the contents to an `OutputStream` in a format that is more human-readable but not retrievable by `load()`.

### System Properties

The `java.lang.System` class provides access to basic system environment information through the `static System.getProperty()` method. This method returns a `Properties` table that contains system properties. System properties take the place of environment variables in other programming environments.

Table 7.7 summarizes system properties that are guaranteed to be defined in any Java environment.

Table 7.7: System Properties
System Property | Meaning
--- | ---
`java.vendor` | Vendor-specific string
`java.vendor.url` | URL of vendor
`java.version` | Java version
`java.home` | Java installation directory
`java.class.version` | Java class version
`java.class.path` | The class path
`os.name` | Operating-system name
`os.arch` | Operating-system architecture
`os.version` | Operating-system version
`file.separator` | File separator (such as "/" or " \")
`path.separator` | Path separator (such as ":" or ";")
`line.separator` | Line separator (such as "\n" or "\r\n")
`user.name` | User account name
`user.home` | User's home directory
`user.dir` | Current working directory

Applets are, by current Web browser conventions, prevented from reading the following properties: `java.home`, `java.class.path`, `user.name`, `user.home`, and `user.dir`. As you'll see in the next section, these restrictions are implemented by a `SecurityManager` object.

Vectors and Hashtables |  | The Security Manager