Chain

java.lang.Object
- net.sf.saxon.om.Chain

All Implemented Interfaces:

GroundedValue, Sequence
```
public class Chain
extends java.lang.Object
implements GroundedValue
```
A chain is an implementation of Sequence that represents the concatenation of a number of subsequences.

The most common use case is a recursive function that appends one item to a sequence each time it is called. Each call of this function will create a Chain with two subsequences, the first being a Chain and the second an individual item. The design of the class is constrained by the need to handle this extreme case.

Firstly, the iterator for the class cannot use simple recursion to navigate the tree because it will often be too deep, causing StackOverflow. So it maintains its own Stack (on the Java heap).

Secondly, even using the heap will run out of space at about a million entries. To prevent this, any Chains of size less than thirty items are amalgamated into chunks of 30. Building larger chunks than this would cause insertion operations to have linear performance (and thus the total cost of sequence construction would be quadratic). The figure of 30 was chosen because elapsed time is almost as good as with smaller chunks, and memory use during navigation is substantially reduced.

A Chain has two phases in its life-cycle. In the first phase, the Chain is mutable; it can be extended using append() calls. In the second phase, the Chain is immutable; further append() operations are not allowed. The transition from the first to the second phase occurs when any of the methods itemAt(), reduce(), or subsequence() is called.

Constructor Summary

Constructors
Constructor and Description

Chain(java.util.List<GroundedValue> children)

Constructors
Constructor and Description
`Chain(java.util.List<GroundedValue> children)`

Method Summary

All Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`void`	`append(Item item)` Add a single item to the end of this sequence.
`boolean`	`effectiveBooleanValue()` Get the effective boolean value of this sequence
`int`	`getLength()` Get the size of the value (the number of items)
`java.lang.String`	`getStringValue()` Get the string value of this sequence.
`java.lang.CharSequence`	`getStringValueCS()` Get the string value of this sequence.
`Item`	`head()` Get the first item of the sequence.
`Item`	`itemAt(int n)` Get the n'th item in the value, counting from 0
`UnfailingIterator`	`iterate()` Get an iterator over all the items in the sequence.
`GroundedValue`	`reduce()` Reduce the sequence to its simplest form.
`GroundedValue`	`subsequence(int start, int length)` Get a subsequence of the value

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

- Constructor Detail
  - Chain
```
public Chain(java.util.List<GroundedValue> children)
```
- Method Detail
  - head
```
public Item head()
```
    Description copied from interface: GroundedValue
    
    Get the first item of the sequence. This differs from the parent interface in not allowing an exception
    
    Specified by:
    
    head in interface GroundedValue
    
    Specified by:
    
    head in interface Sequence
    
    Returns:
    
    the first item of the sequence, or null if the sequence is empty
  - iterate
```
public UnfailingIterator iterate()
```
    Description copied from interface: GroundedValue
    
    Get an iterator over all the items in the sequence. This differs from the superclass method in not allowing an exception.
    
    Specified by:
    
    iterate in interface GroundedValue
    
    Specified by:
    
    iterate in interface Sequence
    
    Returns:
    
    an iterator over all the items
  - append
```
public void append(Item item)
```
    Add a single item to the end of this sequence. This method must only be called while the value is being constructed, since the sequence thereafter is immutable.
    
    Parameters:
    
    item - the item to be added
  - itemAt
```
public Item itemAt(int n)
```
    Get the n'th item in the value, counting from 0
    
    Specified by:
    
    itemAt in interface GroundedValue
    
    Parameters:
    
    n - the index of the required item, with 0 representing the first item in the sequence
    
    Returns:
    
    the n'th item if it exists, or null otherwise
  - subsequence
```
public GroundedValue subsequence(int start,
                                 int length)
```
    Get a subsequence of the value
    
    Specified by:
    
    subsequence in interface GroundedValue
    
    Parameters:
    
    start - the index of the first item to be included in the result, counting from zero. A negative value is taken as zero. If the value is beyond the end of the sequence, an empty sequence is returned
    
    length - the number of items to be included in the result. Specify Integer.MAX_VALUE to get the subsequence up to the end of the base sequence. If the value is negative, an empty sequence is returned. If the value goes off the end of the sequence, the result returns items up to the end of the sequence
    
    Returns:
    
    the required subsequence.
  - getLength
```
public int getLength()
```
    Get the size of the value (the number of items)
    
    Specified by:
    
    getLength in interface GroundedValue
    
    Returns:
    
    the number of items in the sequence
  - effectiveBooleanValue
```
public boolean effectiveBooleanValue()
                              throws XPathException
```
    Get the effective boolean value of this sequence
    
    Specified by:
    
    effectiveBooleanValue in interface GroundedValue
    
    Returns:
    
    the effective boolean value
    
    Throws:
    
    XPathException - if the sequence has no effective boolean value (for example a sequence of two integers)
  - getStringValue
```
public java.lang.String getStringValue()
                                throws XPathException
```
    Get the string value of this sequence. The string value of an item is the result of applying the string() function. The string value of a sequence is the space-separated result of applying the string-join() function using a single space as the separator
    
    Specified by:
    
    getStringValue in interface GroundedValue
    
    Returns:
    
    the string value of the sequence.
    
    Throws:
    
    XPathException - if the sequence contains items that have no string value (for example, function items)
  - getStringValueCS
```
public java.lang.CharSequence getStringValueCS()
                                        throws XPathException
```
    Get the string value of this sequence. The string value of an item is the result of applying the string() function. The string value of a sequence is the space-separated result of applying the string-join() function using a single space as the separator
    
    Specified by:
    
    getStringValueCS in interface GroundedValue
    
    Returns:
    
    the string value of the sequence.
    
    Throws:
    
    XPathException - if the sequence contains items that have no string value (for example, function items)
  - reduce
```
public GroundedValue reduce()
```
    Reduce the sequence to its simplest form. If the value is an empty sequence, the result will be EmptySequence.getInstance(). If the value is a single atomic value, the result will be an instance of AtomicValue. If the value is a single item of any other kind, the result will be an instance of SingletonItem. Otherwise, the result will typically be unchanged.
    
    Specified by:
    
    reduce in interface GroundedValue
    
    Returns:
    
    the simplified sequence

Class Chain

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Constructor Detail

Chain

Method Detail

head

iterate

append

itemAt

subsequence

getLength

effectiveBooleanValue

getStringValue

getStringValueCS

reduce