e415. determining if a preference node exists.txt

这里面包含了一百多个JAVA源文件

A preference node is automatically created whenever Preferences.node(), Preferences.userNodeForPackage(), or Preferences.systemNodeForPackage() is called. To avoid creating a node, you should first check to see if it exists. 
    try {
        // Check if a node exists
        boolean exists = Preferences.userRoot().nodeExists("/foo"); // false
    
        // Get the node
        Preferences.userRoot().node("/foo");
    
        // Getting a non-existent node automatically creates it
        exists = Preferences.userRoot().nodeExists("/foo"); // true
    
        // Remove the node
        Preferences prefs = Preferences.userRoot().node("/foo");
        prefs.removeNode();
    
        // The following would cause an IllegalStateException
        //exists = prefs.nodeExists("/foo");
    
        // Use the following to determine if the node has been removed
        exists = prefs.nodeExists(""); // false
    } catch (BackingStoreException e) {
    }

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