Source Code Cross Referenced for LazyExpression.java in  » XML » saxonb » net » sf » saxon » expr » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » XML » saxonb » net.sf.saxon.expr 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        package net.sf.saxon.expr;
002:
003:        import net.sf.saxon.om.NamePool;
004:        import net.sf.saxon.om.Item;
005:        import net.sf.saxon.om.SequenceIterator;
006:        import net.sf.saxon.trans.XPathException;
007:        import net.sf.saxon.value.Value;
008:
009:        /**
010:         * A LazyExpression is an expression that forces lazy evaluation: it must not be evaluated eagerly,
011:         * because a failure must not be reported unless the value is actually referenced. This is used
012:         * for an expression that has been moved out of a loop. If the loop iterates zero times, the expression
013:         * will not be evaluated, and in particular, it will not cause a dynamic error.
014:         */
015:
016:        public class LazyExpression extends UnaryExpression {
017:
018:            private LazyExpression(Expression operand) {
019:                super (operand);
020:            }
021:
022:            public static Expression makeLazyExpression(Expression operand) {
023:                if (operand instanceof  LazyExpression
024:                        || operand instanceof  Value) {
025:                    return operand;
026:                } else {
027:                    return new LazyExpression(operand);
028:                }
029:            }
030:
031:            /**
032:             * Evaluate an expression as a single item. This always returns either a single Item or
033:             * null (denoting the empty sequence). No conversion is done. This method should not be
034:             * used unless the static type of the expression is a subtype of "item" or "item?": that is,
035:             * it should not be called if the expression may return a sequence. There is no guarantee that
036:             * this condition will be detected.
037:             *
038:             * @param context The context in which the expression is to be evaluated
039:             * @return the node or atomic value that results from evaluating the
040:             *         expression; or null to indicate that the result is an empty
041:             *         sequence
042:             * @throws net.sf.saxon.trans.XPathException
043:             *          if any dynamic error occurs evaluating the
044:             *          expression
045:             */
046:
047:            public Item evaluateItem(XPathContext context)
048:                    throws XPathException {
049:                return operand.evaluateItem(context);
050:            }
051:
052:            /**
053:             * Return an Iterator to iterate over the values of a sequence. The value of every
054:             * expression can be regarded as a sequence, so this method is supported for all
055:             * expressions. This default implementation handles iteration for expressions that
056:             * return singleton values: for non-singleton expressions, the subclass must
057:             * provide its own implementation.
058:             *
059:             * @param context supplies the context for evaluation
060:             * @return a SequenceIterator that can be used to iterate over the result
061:             *         of the expression
062:             * @throws net.sf.saxon.trans.XPathException
063:             *          if any dynamic error occurs evaluating the
064:             *          expression
065:             */
066:
067:            public SequenceIterator iterate(XPathContext context)
068:                    throws XPathException {
069:                return operand.iterate(context);
070:            }
071:
072:            /**
073:             * Process the instruction, without returning any tail calls
074:             *
075:             * @param context The dynamic context, giving access to the current node,
076:             *                the current variables, etc.
077:             */
078:
079:            public void process(XPathContext context) throws XPathException {
080:                operand.process(context);
081:            }
082:
083:            protected String displayOperator(NamePool pool) {
084:                return "lazy";
085:            }
086:
087:        }
088:
089:        //
090:        // The contents of this file are subject to the Mozilla Public License Version 1.0 (the "License");
091:        // you may not use this file except in compliance with the License. You may obtain a copy of the
092:        // License at http://www.mozilla.org/MPL/
093:        //
094:        // Software distributed under the License is distributed on an "AS IS" basis,
095:        // WITHOUT WARRANTY OF ANY KIND, either express or implied.
096:        // See the License for the specific language governing rights and limitations under the License.
097:        //
098:        // The Original Code is: all this file.
099:        //
100:        // The Initial Developer of the Original Code is Michael H. Kay
101:        //
102:        // Portions created by (your name) are Copyright (C) (your legal entity). All Rights Reserved.
103:        //
104:        // Contributor(s): none.
105:        //
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