net.sf.saxon.om

Class FocusTrackingIterator

java.lang.Object

net.sf.saxon.om.FocusTrackingIterator

All Implemented Interfaces:

LastPositionFinder, FocusIterator, SequenceIterator, GroundedIterator, LookaheadIterator

public class FocusTrackingIterator
extends java.lang.Object
implements FocusIterator, LookaheadIterator, GroundedIterator, LastPositionFinder

An iterator that maintains the values of position() and current(), as a wrapper over an iterator which does not maintain these values itself.

Note that when a FocusTrackingIterator is used to wrap a SequenceIterator in order to track the values of position() and current(), it is important to ensure (a) that the SequenceIterator is initially positioned at the start of the sequence, and (b) that all calls on next() to advance the iterator are directed at the FocusTrackingIterator, and not at the wrapped SequenceIterator.

The methods on this class are synchronized in resolution of bug 3927, concerning calls to last() when executing asynchronous calls on xsl:result-document. These do not make the iterator fully thread-safe; they merely protect it against calls on getLength() made in a thread other than the thread that is making successive calls on next().

Since:

9.6

Field Summary

Fields

Modifier and Type	Field and Description
static boolean	MEMOIZABLE
static boolean	MEMOIZE

Fields inherited from interface net.sf.saxon.om.SequenceIterator

ATOMIZING, GROUNDED, LAST_POSITION_FINDER, LOOKAHEAD

Constructor Summary

Constructors

Constructor and Description
FocusTrackingIterator(SequenceIterator base)

Method Summary

Modifier and Type	Method and Description
void	close() Close the iterator.
Item	current() Get the current value in the sequence (the one returned by the most recent call on next()).
int	getLength() Get the position of the last item in the sequence.
int	getProperties() Get properties of this iterator, as a bit-significant integer.
GroundedValue	getResidue() Return a GroundedValue containing all the remaining items in the sequence returned by this SequenceIterator, starting at the current position.
int	getSiblingPosition(NodeInfo node, NodeTest nodeTest, int max) Get the sibling position of a node: specifically, count how many preceding siblings of a node satisfy the nodetest.
SequenceIterator	getUnderlyingIterator() Get the underlying iterator
boolean	hasNext() Determine whether there are more items to come.
GroundedValue	materialize() Return a GroundedValue containing all the items in the sequence returned by this SequenceIterator.
Item	next() Get the next item in the sequence.
int	position() Get the current position.

Methods inherited from class java.lang.Object

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

Field Detail

MEMOIZABLE

public static final boolean MEMOIZABLE

See Also:

Constant Field Values

MEMOIZE

public static boolean MEMOIZE

Constructor Detail

FocusTrackingIterator

public FocusTrackingIterator(SequenceIterator base)

Method Detail

getUnderlyingIterator

public SequenceIterator getUnderlyingIterator()

Get the underlying iterator

Returns:

the iterator underlying this FocusIterator

next

public Item next()
          throws XPathException

Get the next item in the sequence. This method changes the state of the iterator, in particular it affects the result of subsequent calls of position() and current().

Specified by:

next in interface SequenceIterator

Returns:

the next item, or null if there are no more items. Once a call on next() has returned null, no further calls should be made. The preferred action for an iterator if subsequent calls on next() are made is to return null again, and all implementations within Saxon follow this rule.

Throws:

XPathException - if an error occurs retrieving the next item

Since:

8.4

current

public Item current()

Get the current value in the sequence (the one returned by the most recent call on next()). This will be null before the first call of next(). This method does not change the state of the iterator.

Specified by:

current in interface FocusIterator

Returns:

the current item, the one most recently returned by a call on next(). Returns null if next() has not been called, or if the end of the sequence has been reached.

Since:

8.4

position

public int position()

Get the current position. This will usually be zero before the first call on next(), otherwise it will be the number of times that next() has been called. Once next() has returned null, the preferred action is for subsequent calls on position() to return -1, but not all existing implementations follow this practice. (In particular, the EmptyIterator is stateless, and always returns 0 as the value of position(), whether or not next() has been called.)

This method does not change the state of the iterator.

Specified by:

position in interface FocusIterator

Returns:

the current position, the position of the item returned by the most recent call of next(). This is 1 after next() has been successfully called once, 2 after it has been called twice, and so on. If next() has never been called, the method returns zero. If the end of the sequence has been reached, the value returned will always be <= 0; the preferred value is -1.

Since:

8.4

getLength

public int getLength()
              throws XPathException

Get the position of the last item in the sequence. The method is stateless in its external effect: that is, it does not change the values returned by position(), next(), hasNext(), etc. However, it creates a new base iterator which means that the result of calling getUnderlyingIterator() may change.

Specified by:

getLength in interface LastPositionFinder

Specified by:

getLength in interface FocusIterator

Returns:

the position of the last item

Throws:

XPathException - if a failure occurs reading the sequence

hasNext

public boolean hasNext()

Determine whether there are more items to come. Note that this operation is stateless and it is not necessary (or usual) to call it before calling next(). It is used only when there is an explicit need to tell if we are at the last element.

This method must not be called unless the result of getProperties() on the iterator includes the bit setting SequenceIterator.LOOKAHEAD

Specified by:

hasNext in interface LookaheadIterator

Returns:

true if there are more items in the sequence

Throws:

java.lang.ClassCastException - if the base iterator does not support lookahead processing

materialize

public GroundedValue materialize()
                          throws XPathException

Return a GroundedValue containing all the items in the sequence returned by this SequenceIterator. This should be an "in-memory" value, not a Closure. This method must not consume the iterator.

Specified by:

materialize in interface GroundedIterator

Returns:

the corresponding Value

Throws:

XPathException - in the cases of subclasses (such as the iterator over a MemoClosure) which cause evaluation of expressions while materializing the value.

getResidue

public GroundedValue getResidue()
                         throws XPathException

Return a GroundedValue containing all the remaining items in the sequence returned by this SequenceIterator, starting at the current position. This should be an "in-memory" value, not a Closure. This method must not consume the iterator.

Specified by:

getResidue in interface GroundedIterator

Returns:

the corresponding Value

Throws:

XPathException - in the cases of subclasses (such as the iterator over a MemoClosure) which cause evaluation of expressions while materializing the value.

close

public void close()

Close the iterator. This indicates to the supplier of the data that the client does not require any more items to be delivered by the iterator. This may enable the supplier to release resources. After calling close(), no further calls on the iterator should be made; if further calls are made, the effect of such calls is undefined.

(Currently, closing an iterator is important only when the data is being "pushed" in another thread. Closing the iterator terminates that thread and means that it needs to do no additional work. Indeed, failing to close the iterator may cause the push thread to hang waiting for the buffer to be emptied.)

Specified by:

close in interface SequenceIterator

Since:

9.1

getProperties

public int getProperties()

Get properties of this iterator, as a bit-significant integer.

Specified by:

getProperties in interface SequenceIterator

Returns:

the properties of this iterator. This will be some combination of properties such as SequenceIterator.GROUNDED, SequenceIterator.LAST_POSITION_FINDER, and SequenceIterator.LOOKAHEAD. It is always acceptable to return the value zero, indicating that there are no known special properties. It is acceptable for the properties of the iterator to change depending on its state.

Since:

8.6

getSiblingPosition

public int getSiblingPosition(NodeInfo node,
                              NodeTest nodeTest,
                              int max)

Get the sibling position of a node: specifically, count how many preceding siblings of a node satisfy the nodetest. This method appears here because it can potentially make use of cached information. When an instruction such as xsl:apply-templates select="*" (which selects a set of sibling nodes) is used in conjunction with patterns such as match="*[position() mod 2 = 1], then calculation of the position of one node in the sequence of siblings can take advantage of the fact that the position of the immediately preceding sibling is already known.

This optimization was suggested by the late Sebastian Rahtz, one of Saxon's earliest power users, and it is dedicated to his memory.

Parameters:

node - the starting node, which is assumed to satisfy the node test

nodeTest - the node test

max - the maximum number of nodes to be counted

Returns:

the number of preceding siblings that satisfy the node test, plus one, unless the number exceeds max, in which case return some number greater than or equal to max.