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

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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;
    }