This example enumerates the key/value pairs in a preference node and checks each value for a match. 
    // Returns the key of a preference whose value matches the specified value;
    // null is returned if no such key exists.
    public static String containsValue(Preferences node, String value) {
        try {
            // Get all the keys
            String[] keys = node.keys();
    
            // Scan the keys
            for (int i=0; i<keys.length; i++) {
                if (value.equals(node.get(keys[i], null))) {
                    return keys[i];
                }
            }
        } catch (BackingStoreException e) {
        }
        return null;
    }