Class SingletonClosure

All Implemented Interfaces:
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:
  • Constructor Details

    • 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 Details

    • 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
    • asItem

      public Item asItem() throws XPathException
      Return the value in the form of an Item
      Overrides:
      asItem in class Value
      Returns:
      the value in the form of an Item
      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
    • getLength

      public int getLength() throws XPathException
      Get the length of the sequence
      Overrides:
      getLength in class Value
      Returns:
      the number of items in the sequence
      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