net.sf.saxon.expr

Class MappingIterator

java.lang.Object

net.sf.saxon.expr.MappingIterator

All Implemented Interfaces:

java.io.Closeable, java.lang.AutoCloseable, SequenceIterator

public class MappingIterator
extends java.lang.Object
implements SequenceIterator

MappingIterator merges a sequence of sequences into a single flat sequence. It takes as inputs an iteration, and a mapping function to be applied to each Item returned by that iteration. The mapping function itself returns another iteration. The result is an iteration of the concatenation of all the iterations returned by the mapping function.

This is a powerful class. It is used, with different mapping functions, in a great variety of ways. It underpins the way that "for" expressions and path expressions are evaluated, as well as sequence expressions. It is also used in the implementation of the document(), key(), and id() functions.

In functional programming theory, this iterator implements a "flatMap" operation.

Constructor Summary

Constructors

Constructor and Description
MappingIterator(SequenceIterator base, MappingFunction action) Construct a MappingIterator that will apply a specified MappingFunction to each Item returned by the base iterator.

Method Summary

Modifier and Type	Method and Description
void	close() Close the iterator.
void	discharge() Calling this method instructs the iterator to release any resources it holds, while still remaining able to deliver the remaining items in the sequence.
static MappingIterator	map(SequenceIterator base, SequenceMapper.Lambda mappingExpression) Static factory method designed to handle the case where the mapping function is supplied as a lambda expression.
Item	next() Get the next item in the sequence.

Methods inherited from class java.lang.Object

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

Constructor Detail

MappingIterator

public MappingIterator(SequenceIterator base,
                       MappingFunction action)

Construct a MappingIterator that will apply a specified MappingFunction to each Item returned by the base iterator.

Parameters:

base - the base iterator

action - the mapping function to be applied: a function from items to SequenceIterators.

Method Detail

map

public static MappingIterator map(SequenceIterator base,
                                  SequenceMapper.Lambda mappingExpression)

Static factory method designed to handle the case where the mapping function is supplied as a lambda expression. (Although a lambda expression can be used directly as the mapping function in Java, this is not possible in C#).

Parameters:

base - the base iterator

mappingExpression - the mapping function to be applied: a function from items to SequenceIterators.

Returns:

a flattened iterator

next

public Item next()

Description copied from interface: SequenceIterator

Get the next item in the sequence. This method changes the state of the iterator.

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.

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.

For example, the iterator returned by the unparsed-text-lines() function has a close() method that causes the underlying input stream to be closed, whether or not the file has been read to completion.

Closing an iterator is important 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.

Closing an iterator is not necessary if the iterator is read to completion: if a call on SequenceIterator.next() returns null, the iterator will be closed automatically. An explicit call on SequenceIterator.close() is needed only when iteration is abandoned prematurely.

Specified by:

close in interface java.io.Closeable

Specified by:

close in interface java.lang.AutoCloseable

Specified by:

close in interface SequenceIterator

discharge

public void discharge()

Description copied from interface: SequenceIterator

Calling this method instructs the iterator to release any resources it holds, while still remaining able to deliver the remaining items in the sequence. This may require the iterator to calculate the rest of the sequence eagerly. The method is called by a client if it anticipates that it might not read the iterator to completion, but it cannot guarantee that SequenceIterator.close() will be called when no more items are needed.

Specified by:

discharge in interface SequenceIterator