net.sf.saxon.value

Class EmptySequence

java.lang.Object

net.sf.saxon.value.Value

net.sf.saxon.value.EmptySequence

All Implemented Interfaces:

java.io.Serializable, Expression, ValueRepresentation

public final class EmptySequence
extends Value

An EmptySequence object represents a sequence containing no members.

See Also:

Serialized Form

Field Summary

Fields inherited from class net.sf.saxon.value.Value

EMPTY_CLASS_ARRAY

Fields inherited from interface net.sf.saxon.expr.Expression

EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD

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

EMPTY_VALUE_ARRAY

Method Summary

Modifier and Type	Method and Description
void	display(int level, java.io.PrintStream out, Configuration config) Diagnostic print of expression structure
boolean	effectiveBooleanValue(XPathContext context) Get the effective boolean value - always false
boolean	equals(java.lang.Object other) Is this expression the same as another expression?
int	getCardinality() Determine the static cardinality
int	getImplementationMethod() An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
static EmptySequence	getInstance() Get the implicit instance of this class
ItemType	getItemType(TypeHierarchy th) Determine the item type
int	getLength() Get the length of the sequence
int	getSpecialProperties() Get the static properties of this expression (other than its type).
int	hashCode() Return a hash code to support the equals() function
SequenceIterator	iterate(XPathContext context) Return an iteration over the sequence

Methods inherited from class net.sf.saxon.value.Value

asItem, asIterator, asValue, checkPermittedContents, convert, convertJavaObjectToXPath, convertToJava, evaluateAsString, evaluateItem, getDependencies, getIterator, getParentExpression, getStringValue, getStringValueCS, itemAt, iterateSubExpressions, makeQNameValue, optimize, process, promote, reduce, simplify, stringToNumber, toString, typeCheck

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Method Detail

getInstance

public static EmptySequence getInstance()

Get the implicit instance of this class

getImplementationMethod

public int getImplementationMethod()

An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process(). This method indicates which of these methods is prefered.

iterate

public SequenceIterator iterate(XPathContext context)

Return an iteration over the sequence

Parameters:

context - The context in which the expression is to be evaluated. Note that this context must contain a stackframe with sufficient slots to allow evaluation of any variables contained in the expression (including variables allocated internally by the optimizer)

Returns:

a SequenceIterator that can be used to iterate over the result of the expression

getItemType

public ItemType getItemType(TypeHierarchy th)

Determine the item type

Specified by:

getItemType in interface Expression

Overrides:

getItemType in class Value

Parameters:

th -

Returns:

for the default implementation: AnyItemType (not known)

getCardinality

public int getCardinality()

Determine the static cardinality

Specified by:

getCardinality in interface Expression

Overrides:

getCardinality in class Value

Returns:

one of the values StaticProperty.ALLOWS_ONE, StaticProperty.ALLOWS_ZERO_OR_MORE, StaticProperty.ALLOWS_ZERO_OR_ONE, StaticProperty.ALLOWS_ONE_OR_MORE, StaticProperty.EMPTY. This default implementation returns ZERO_OR_MORE (which effectively gives no information).

getSpecialProperties

public int getSpecialProperties()

Get the static properties of this expression (other than its type). The result is bit-signficant. These properties are used for optimizations. In general, if property bit is set, it is true, but if it is unset, the value is unknown.

Specified by:

getSpecialProperties in interface Expression

Overrides:

getSpecialProperties in class Value

Returns:

StaticProperty.NON_CREATIVE

getLength

public final int getLength()

Get the length of the sequence

Overrides:

getLength in class Value

Returns:

always 0 for an empty sequence

equals

public boolean equals(java.lang.Object other)

Is this expression the same as another expression?

Overrides:

equals in class Value

Throws:

java.lang.ClassCastException - if the values are not comparable

hashCode

public int hashCode()

Description copied from class: Value

Return a hash code to support the equals() function

Overrides:

hashCode in class Value

effectiveBooleanValue

public boolean effectiveBooleanValue(XPathContext context)

Get the effective boolean value - always false

Specified by:

effectiveBooleanValue in interface Expression

Overrides:

effectiveBooleanValue in class Value

Parameters:

context - The context in which the expression is to be evaluated

Returns:

the effective boolean value

display

public void display(int level,
                    java.io.PrintStream out,
                    Configuration config)

Diagnostic print of expression structure

Specified by:

display in interface Expression

Overrides:

display in class Value

Parameters:

level - indentation level for this expression

out - Output destination