Class SingletonClosure

  • All Implemented Interfaces:
    java.io.Serializable, SequenceIterable, ValueRepresentation

    public class SingletonClosure
    extends Closure
    A SingletonClosure represents a value that has not yet been evaluated: the value is represented by an expression, together with saved values of all the context variables that the expression depends on. The value of a SingletonClosure is always either a single item or an empty sequence.

    The expression may depend on local variables and on the context item; these values are held in the saved XPathContext object that is kept as part of the Closure, and they will always be read from that object. The expression may also depend on global variables; these are unchanging, so they can be read from the Bindery in the normal way. Expressions that depend on other contextual information, for example the values of position(), last(), current(), current-group(), should not be evaluated using this mechanism: they should always be evaluated eagerly. This means that the Closure does not need to keep a copy of these context variables.

    See Also:
    Serialized Form
    • Constructor Detail

      • SingletonClosure

        public SingletonClosure​(Expression exp,
                                XPathContext context)
                         throws XPathException
        Constructor should not be called directly, instances should be made using the make() method.
        Parameters:
        exp - the expression to be lazily evaluated
        context - the context in which the expression should be evaluated
        Throws:
        XPathException
    • Method Detail

      • iterate

        public SequenceIterator iterate()
                                 throws XPathException
        Evaluate the expression in a given context to return an iterator over a sequence
        Overrides:
        iterate in class Closure
        Returns:
        an iterator over the sequence of items
        Throws:
        XPathException - if a dynamic error occurs. This is possible only in the case of values that are materialized lazily, that is, where the iterate() method leads to computation of an expression that delivers the values.
      • process

        public void process​(XPathContext context)
                     throws XPathException
        Process the expression by writing the value to the current Receiver
        Overrides:
        process in class Closure
        Parameters:
        context - The dynamic context, giving access to the current node, the current variables, etc.
        Throws:
        XPathException
      • itemAt

        public Item itemAt​(int n)
                    throws XPathException
        Get the n'th item in the sequence (starting from 0). This is defined for all SequenceValues, but its real benefits come for a SequenceValue stored extensionally
        Overrides:
        itemAt in class Value
        Parameters:
        n - position of the required item, counting from zero.
        Returns:
        the n'th item in the sequence, where the first item in the sequence is numbered zero. If n is negative or >= the length of the sequence, returns null.
        Throws:
        XPathException
      • materialize

        public Value materialize()
                          throws XPathException
        Return a value containing all the items in the sequence returned by this SequenceIterator
        Returns:
        the corresponding value
        Throws:
        XPathException