net.sf.saxon.om

Class LazySequence

java.lang.Object

net.sf.saxon.om.LazySequence

All Implemented Interfaces:

Sequence

public class LazySequence
extends java.lang.Object
implements Sequence

A sequence that wraps an iterator, without being materialized. It can only be used once.

Constructor Summary

Constructors

Constructor and Description
LazySequence(SequenceIterator iterator) Create a virtual sequence backed by an iterator

Method Summary

Modifier and Type	Method and Description
Item	head() Get the first item in the sequence.
SequenceIterator	iterate() Iterate over all the items in the sequence.
Sequence	makeRepeatable() Ensure that the sequence is in a form where it can be evaluated more than once.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface net.sf.saxon.om.Sequence

materialize

Constructor Detail

LazySequence

public LazySequence(SequenceIterator iterator)

Create a virtual sequence backed by an iterator

Parameters:

iterator - the iterator that delivers the items in the sequence

Method Detail

head

public Item head()
          throws XPathException

Get the first item in the sequence. This calls iterate() internally, which means it can only be called once; the method should only be used if the client requires only the first item in the sequence (or knows that the sequence has length one).

Specified by:

head in interface Sequence

Returns:

the first item, or null if the sequence is empty

Throws:

XPathException - if an error occurs evaluating the underlying expression

iterate

public SequenceIterator iterate()
                         throws XPathException

Iterate over all the items in the sequence. The first time this is called it returns the original iterator backing the sequence. On subsequent calls it clones the original iterator.

Specified by:

iterate in interface Sequence

Returns:

an iterator over the items in the sequence

Throws:

XPathException - if evaluation of the iterator fails.

java.lang.IllegalStateException - if iterate() has already been called

makeRepeatable

public Sequence makeRepeatable()
                        throws XPathException

Description copied from interface: Sequence

Ensure that the sequence is in a form where it can be evaluated more than once. Some sequences (for example LazySequence and Closure can only be evaluated once, and this operation causes these to be grounded. However, making it repeatable is not the same as making it grounded; it does not flush out all errors. Indeed, lazy evaluation relies on this property, because an expression that has been lifted out of a loop must not be evaluated unless the loop is executed at least once, to prevent spurious errors.

Specified by:

makeRepeatable in interface Sequence

Returns:

An equivalent sequence that can be repeatedly evaluated

Throws:

XPathException - if evaluation fails