e406. determining if a preference node contains a specific key.txt

这里面包含了一百多个JAVA源文件

You cannot save null as a preference value. Therefore, if you retrieve a value using a default value of null, and null is returned, you can assume that the key is not present. 
    // Returns true if node contains the specified key; false otherwise.
    public static boolean contains(Preferences node, String key) {
        return node.get(key, null) != null;
    }