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## net.sf.saxon.expr.sort
Class GroupByIterator

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

All Implemented Interfaces:
  LastPositionFinder, GroupIterator, SequenceIterator, LookaheadIterator

Direct Known Subclasses:
  GroupBySingletonIterator

    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
`protected AtomicComparer` | `comparer`
`protected List<AtomicValue>` | `groupKeys`
`protected List<List<Item>>` | `groups`
`protected Expression` | `keyExpression`

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
`void` | `close()` Close the iterator.
`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.
`protected void` | `processItem(HashMap<ComparisonKey,List<Item>> index, Item item, XPathContext c2)` Process one item in the population

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### keyExpression

    protected Expression keyExpression

### groups

    protected List<List<Item>> groups

### groupKeys

    protected List<AtomicValue> groupKeys

### comparer

    protected AtomicComparer comparer

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

### processItem

    protected void processItem(HashMap<ComparisonKey,List<Item>> index,
                               Item item,
                               XPathContext c2)
                        throws XPathException

  Process one item in the population

  Parameters:
    `index` - the index of items
    `item` - the item from the population to be processed
    `c2` - the XPath evaluation context
  Throws:
    `XPathException`

### 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, or null if the grouping key is an empty sequence

### 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.

### close

    public void close()

  Description copied from interface: `SequenceIterator`
  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`

### 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`

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