net.sf.saxon.sort
Class GroupByIterator

java.lang.Object
  net.sf.saxon.sort.GroupByIterator

All Implemented Interfaces:

LastPositionFinder, LookaheadIterator, SequenceIterator, GroupIterator

public class GroupByIterator
extends Object
implements GroupIterator, LastPositionFinder, LookaheadIterator

A GroupByIterator iterates over a sequence of groups defined by xsl:for-each-group group-by="x". The groups are returned in order of first appearance. Note that an item can appear in several groups; indeed, an item may be the leading item of more than one group, which means that knowing the leading item is not enough to know the current group.

The GroupByIterator acts as a SequenceIterator, where successive calls of next() return the leading item of each group in turn. The current item of the iterator is therefore the leading item of the current group. To get access to all the members of the current group, the method iterateCurrentGroup() is used; this underpins the current-group() function in XSLT. The grouping key for the current group is available via the getCurrentGroupingKey() method.

Field Summary

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

GROUNDED, LAST_POSITION_FINDER, LOOKAHEAD

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

GROUNDED, LAST_POSITION_FINDER, LOOKAHEAD

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

GROUNDED, LAST_POSITION_FINDER, LOOKAHEAD

Constructor Summary

GroupByIterator(SequenceIterator population, Expression keyExpression, XPathContext keyContext, StringCollator collator)
Create a GroupByIterator

Method Summary

Item	current() Get the current value in the sequence (the one returned by the most recent call on next()).
SequenceIterator	getAnother() Get another SequenceIterator that iterates over the same items as the original, but which is repositioned at the start of the sequence.
List	getCurrentGroup() Get the contents of the current group as a java List
AtomicValue	getCurrentGroupingKey() Get the value of the grouping key for the current group
int	getLastPosition() Get the last position (that is, the number of groups)
int	getProperties() Get properties of this iterator, as a bit-significant integer.
boolean	hasNext() Determine whether there are more items to come.
SequenceIterator	iterateCurrentGroup() Get an iterator over the items in the current group
Item	next() Get the next item in the sequence.
int	position() Get the current position.

Methods inherited from class java.lang.Object

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

Constructor Detail

GroupByIterator

public GroupByIterator(SequenceIterator population,
                       Expression keyExpression,
                       XPathContext keyContext,
                       StringCollator collator)
                throws XPathException

Create a GroupByIterator

Parameters:

population - iterator over the population to be grouped

keyExpression - the expression used to calculate the grouping key

keyContext - dynamic context for calculating the grouping key

collator - Collation to be used for comparing grouping keys

Throws:

XPathException

Method Detail

getCurrentGroupingKey

public AtomicValue getCurrentGroupingKey()

Get the value of the grouping key for the current group

Specified by:

getCurrentGroupingKey in interface GroupIterator

Returns:

the grouping key

iterateCurrentGroup

public SequenceIterator iterateCurrentGroup()

Get an iterator over the items in the current group

Specified by:

iterateCurrentGroup in interface GroupIterator

Returns:

the iterator

getCurrentGroup

public List getCurrentGroup()

Get the contents of the current group as a java List

Returns:

the contents of the current group

hasNext

public boolean hasNext()

Description copied from interface: LookaheadIterator

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

next

public Item next()
          throws XPathException

Description copied from interface: SequenceIterator

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

current

public Item current()

Description copied from interface: SequenceIterator

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 SequenceIterator

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.

position

public int position()

Description copied from interface: SequenceIterator

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 SequenceIterator

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.

getAnother

public SequenceIterator getAnother()
                            throws XPathException

Description copied from interface: SequenceIterator

Get another SequenceIterator that iterates over the same items as the original, but which is repositioned at the start of the sequence.

This method allows access to all the items in the sequence without disturbing the current position of the iterator. Internally, its main use is in evaluating the last() function.

This method does not change the state of the iterator.

Specified by:

getAnother in interface SequenceIterator

Returns:

a SequenceIterator that iterates over the same items, positioned before the first item

Throws:

XPathException - if any error occurs

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.

getLastPosition

public int getLastPosition()
                    throws XPathException

Get the last position (that is, the number of groups)

Specified by:

getLastPosition in interface LastPositionFinder

Throws:

XPathException