java code 39 barcode 3: Navigating and Manipulating Structured Documents in Java

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3: Navigating and Manipulating Structured Documents
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NodeFilters have the following interface definition:
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interface NodeFilter { // Constants returned by acceptNode const short FILTER_ACCEPT const short FILTER_REJECT const short FILTER_SKIP // Constants for whatToShow const unsigned long SHOW_ALL const unsigned long SHOW_ELEMENT const unsigned long SHOW_ATTRIBUTE const unsigned long SHOW_TEXT const unsigned long SHOW_CDATA_SECTION const unsigned long SHOW_ENTITY_REFERENCE const unsigned long SHOW_ENTITY const unsigned long SHOW_PROCESSING_INSTRUCTION const unsigned long SHOW_COMMENT const unsigned long SHOW_DOCUMENT const unsigned long SHOW_DOCUMENT_TYPE const unsigned long SHOW_DOCUMENT_FRAGMENT const unsigned long SHOW_NOTATION short }; acceptNode(in Node n);
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The first three constants are returned from a NodeFilter s acceptNode() method If acceptNode() returns FILTER_ACCEPT, the navigation methods for a NodeIterator or TreeWalker will return the node If acceptNode() returns FILTER_REJECT, the node is skipped If the traversal object is a TreeWalker, the child nodes of the given node are also skipped If acceptNode() returns FILTER_ SKIP, the node is skipped; but if the traversal object is a TreeWalker, child nodes of the given node will still be considered NodeIterators treat FILTER_REJECT and FILTER_SKIP the same The next 13 constants are used to determine what types of nodes are returned by NodeIterators and TreeWalkers These constant values and combinations of them can be used as values for the whatToShow parameter used in TreeWalkers and NodeIterators, and they represent a bit position in the whatToShow parameter If the corresponding bit position in the whatToShow parameter is set to false for the given node type, nodes of that type will be skipped over
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The Document Object Model: Processing Structured Documents
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The values SHOW_ATTRIBUTE, SHOW_ENTITY, and SHOW_NOTATION are only useful when the NodeIterator or TreeWalker has a node of that type set as its root; attributes, entities, and notations are not considered part of the document tree, and thus will not normally be encountered when traversing it
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The acceptNode() method tests whether a given node should be returned by the associated traversal object It is typically called by NodeIterator and TreeWalker objects under two separate circumstances: When a traversal method of the NodeIterator or the TreeWalker is executed When the node that a NodeIterator is using as its reference node is removed from the tree and a new one must be selected
NOTE
The exact timing of the two events just described may be different among DOM implementations The DOM specification explicitly states that NodeFilters should not try to maintain any state information from past invocations because the resulting behavior may not be portable
Implementing Your Own NodeFilters The DOM does not provide any built-in NodeFilters To use a NodeFilter, you must create your own and pass it to a NodeIterator or TreeWalker when it is created For example, suppose you wanted to create a NodeFilter that only accepted images that have an ALT attribute A Java implementation of such a filter might look like this:
class ImageWithAltFilter implements NodeFilter { short acceptNode(Node theNode) { // first, see if we're dealing with an element if (ngetNodeType() == NodeELEMENT_NODE) { // cast the node to an element so we can call // getAttributeNode() more easily Element el = (Element)theNode; if (!elgetNodeName()equals("IMG")) return FILTER_SKIP; if (elgetAttributeNode("ALT") != null) return FILTER_ACCEPT;
3: Navigating and Manipulating Structured Documents
} return FILTER_SKIP; } }
To use this filter, you would create it as a new object and pass it to a NodeIterator or TreeWalker, like this:
ImageWithAltFilter iwaf = new ImageWithAltFilter(); NodeIterator iterator = ((DocumentTraversal)document)createNodeIterator( rootNode, NodeFilterSHOW_ALL, iwaf );
Important Notes About NodeFilters There are some important things to keep in mind when creating and using NodeFilters The first is that you should avoid writing code for a NodeFilter that can cause exceptions to be thrown The DOM can t prevent this from happening, so NodeFilter behavior during an exception must be well defined The DOM specification states that NodeIterators and TreeWalkers do not catch exceptions that are thrown by NodeFilters Instead, they allow the exception to propagate up to the user s application code For NodeIterators, the nextNode() and previousNode() methods will cause a NodeFilter to execute and will propagate any exceptions that are thrown in the filter For TreeWalkers, the firstChild(), lastChild(), nextSibling(), previousSibling(), nextNode(), previousNode(), and parentNode() methods will invoke a NodeFilter and propagate any exceptions thrown inside the filter The second important thing to remember is that NodeFilters should not modify the contents of a document Again, the DOM can t prevent this and does not provide any special processing to handle this case, though it goes against the spirit of what NodeFilters are intended to do It is interesting to note that NodeFilters can still accept a node even if it is removed from the document, which will cause the node to be returned to the caller The caller might then try to operate on the node, which is no longer in the document tree This can easily lead to confusing behavior, so the DOM specification urges programmers to leave all tree processing to the code that is outside the filter The third thing to remember is that NodeIterators and TreeWalkers apply their whatToShow arguments before applying the filter If a node is skipped over by the whatToShow flag settings, the filter will not be called for the node The type of behavior that is used to skip over nodes using the whatToShow mask is similar to that of FILTER_SKIP: the node is skipped, but its child nodes will still be considered This is true even if the associated filter would have returned FILTER_REJECT for
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