Programming Tutorials

Properties example in Java

By: Jagan in Java Tutorials on 2007-09-14  

Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key is a String and the value is also a String. The Properties class is used by many other Java classes. For example, it is the type of object returned by System.getProperties() when obtaining environmental values.

Properties defines the following instance variable:

Properties defaults;

This variable holds a default property list associated with a Properties object. Properties defines these constructors:

Properties()
Properties(Properties propDefault)

The first version creates a Properties object that has no default values. The second creates an object that uses propDefault for its default values. In both cases, the property list is empty.

Properties also contains one deprecated method: save(). This was replaced by store() because save() did not handle errors correctly.

One useful capability of the Properties class is that you can specify a default property that will be returned if no value is associated with a certain key. For example, a default value can be specified along with the key in the getProperty() method-such as getProperty("name", "default value"). If the "name" value is not found, then "default value" is returned. When you construct a Properties object, you can pass another instance of Properties to be used as the default properties for the new instance. In this case, if you call getProperty("foo") on a given Properties object, and "foo" does not exist, Java looks for "foo" in the default Properties object. This allows for arbitrary nesting of levels of default properties.

The following example demonstrates Properties. It creates a property list in which the keys are the names of states and the values are the names of their capitals. Notice that the attempt to find the capital for Florida includes a default value.

// Demonstrate a Property list.
import java.util.*;

class PropDemo {
    public static void main(String args[]) {
        Properties capitals = new Properties();
        Set<Object> states;
        String str;
        capitals.put("Illinois", "Springfield");
        capitals.put("Missouri", "Jefferson City");
        capitals.put("Washington", "Olympia");
        capitals.put("California", "Sacramento");
        capitals.put("Indiana", "Indianapolis");
        // Show all states and capitals in hashtable.
        states = capitals.keySet(); // get set-view of keys
        Iterator<Object> itr = states.iterator();
        while (itr.hasNext()) {
            str = (String) itr.next();
            System.out.println("The capital of " +
                    str + " is " +
                    capitals.getProperty(str)
                    + ".");
        }
        System.out.println();
        // look for state not in list - specify default
        str = capitals.getProperty("Florida", "Not Found");
        System.out.println("The capital of Florida is "
                + str + ".");
    }
}

The output from this program is shown here:

The capital of California is Sacramento.
The capital of Washington is Olympia.
The capital of Missouri is Jefferson City.
The capital of Indiana is Indianapolis.
The capital of Illinois is Springfield.
The capital of Florida is Not Found.

Since Florida is not in the list, the default value is used.

Although it is perfectly valid to use a default value when you call getProperty(), as the preceding example shows, there is a better way of handling default values for most applications of property lists. For greater flexibility, specify a default property list when constructing a Properties object. The default list will be searched if the desired key is not found in the main list. For example, the following is a slightly reworked version of the preceding program, with a default list of states specified. Now, when Florida is sought, it will be found in the default list:

// Use a default property list.
import java.util.*;

class PropDemoDef {
    public static void main(String args[]) {
        Properties defList = new Properties();
        defList.put("Florida", "Tallahassee");
        defList.put("Wisconsin", "Madison");
        Properties capitals = new Properties(defList);
        Set<Object> states;
        String str;
        capitals.put("Illinois", "Springfield");
        capitals.put("Missouri", "Jefferson City");
        capitals.put("Washington", "Olympia");
        capitals.put("California", "Sacramento");
        capitals.put("Indiana", "Indianapolis");
        // Show all states and capitals in hashtable.
        states = capitals.keySet(); // get set-view of keys
        Iterator<Object> itr = states.iterator();
        while (itr.hasNext()) {
            str = (String) itr.next();
            System.out.println("The capital of " +
                    str + " is " +
                    capitals.getProperty(str)
                    + ".");
        }
        System.out.println();
        // Florida will now be found in the default list.
        str = capitals.getProperty("Florida");
        System.out.println("The capital of Florida is "
                + str + ".");
    }
}

The output:

The capital of Indiana is Indianapolis.
The capital of Illinois is Springfield. 
The capital of Missouri is Jefferson City.
The capital of California is Sacramento. 
The capital of Washington is Olympia. 
The capital of Florida is Tallahassee.