- All Implemented Interfaces:
public class MappingIterator extends java.lang.Object implements SequenceIteratorMappingIterator 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.
All Methods Static Methods Instance Methods Concrete Methods Modifier and Type Method Description
close()Close the iterator.
map(SequenceIterator base, SequenceMapper.Lambda mappingExpression)Static factory method designed to handle the case where the mapping function is supplied as a lambda expression.
next()Get the next item in the sequence.
public MappingIterator(SequenceIterator base, MappingFunction action)Construct a MappingIterator that will apply a specified MappingFunction to each Item returned by the base iterator.
base- the base iterator
action- the mapping function to be applied: a function from items to SequenceIterators.
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#).
base- the base iterator
mappingExpression- the mapping function to be applied: a function from items to SequenceIterators.
- a flattened iterator
public Item next()Description copied from interface:
SequenceIteratorGet the next item in the sequence. This method changes the state of the iterator.
- Specified by:
- 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.
public void close()Description copied from interface:
SequenceIteratorClose 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.
It is not possible to guarantee that an iterator that is not read to completion or will be closed. For example, if a lazy-evaluated variable
$varis passed to a user-written function, the function may access
$varonly; we have no way of knowing whether further items will be read. For this reason, any
SequenceIteratorthat holds resources which need to be closed should use the
Cleaner, and resources held by a
SequenceIteratorshould be registered with the
Cleaner; if the
SequenceIteratoris then garbage-collected without being closed, the
Cleanerwill ensure that the underlying resources are closed. (An example of a
SequenceIteratorthat uses this mechanism is the