net.sf.saxon.instruct
Class ExprInstruction

java.lang.Object
  net.sf.saxon.instruct.Instruction
      net.sf.saxon.instruct.ExprInstruction

All Implemented Interfaces:

Serializable, SourceLocator, Expression

Direct Known Subclasses:

CopyOf, DocumentInstr, ElementCreator, Sequence, SimpleNodeConstructor

public abstract class ExprInstruction
extends Instruction
implements Expression

This class is an abstract superclass for Instructions that also behave as Expressions. Currently this is restricted to those instructions used when an XQuery query is compiled. Eventually it is intended that all instructions should support the Expression interface. Note that the Expression interfaces (e.g. iterate()) may only be used on an ExprInstruction if all its child instructions are also instances of ExprInstruction.

See Also:

Serialized Form

Field Summary

Fields inherited from class net.sf.saxon.instruct.Instruction

children

Constructor Summary

ExprInstruction()

Method Summary

boolean	effectiveBooleanValue(XPathContext context) Get the effective boolean value of the expression.
String	evaluateAsString(XPathContext context) Evaluate an expression as a String.
Item	evaluateItem(XPathContext context) Evaluate an expression as a single item.
int	getCardinality() Determine the static cardinality of the expression.
int	getDependencies() Determine which aspects of the context the expression depends on.
ItemType	getItemType() Determine the data type of the expression, if possible.
int	getSpecialProperties() Get the static properties of this expression (other than its type).
Expression[]	getSubExpressions() Get the immediate sub-expressions of this expression.
protected abstract void	getXPathExpressions(List list)
SequenceIterator	iterate(XPathContext context) Return an Iterator to iterate over the values of a sequence.
Expression	promote(PromotionOffer offer) Offer promotion for this subexpression.
protected abstract void	promoteInst(PromotionOffer offer)
Expression	simplify() Simplify an expression.

Methods inherited from class net.sf.saxon.instruct.Instruction

assembleParams, assembleTunnelParams, getChildren, getColumnNumber, getInstructionDetails, getInstructionName, getLineNumber, getPublicId, getResultType, getSystemId, getSystemId, process, processChildren, processChildrenLeavingTail, processLeavingTail, recoverableError, setChildren, setInstructionDetails, setSourceLocation, styleError, styleError

Methods inherited from class java.lang.Object

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

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

analyze, display

Constructor Detail

ExprInstruction

public ExprInstruction()

Method Detail

simplify

public Expression simplify()
                    throws XPathException

Description copied from interface: Expression

Simplify an expression. This performs any static optimization (by rewriting the expression as a different expression). The default implementation does nothing.

Specified by:

simplify in interface Expression

Returns:

the simplified expression

Throws:

XPathException - if an error is discovered during expression rewriting

getItemType

public ItemType getItemType()

Description copied from interface: Expression

Determine the data type of the expression, if possible. All expression return sequences, in general; this method determines the type of the items within the sequence, assuming that (a) this is known in advance, and (b) it is the same for all items in the sequence.

This method should always return a result, though it may be the best approximation that is available at the time.

Specified by:

getItemType in interface Expression

Returns:

a value such as Type.STRING, Type.BOOLEAN, Type.NUMBER, Type.NODE, or Type.ITEM (meaning not known at compile time)

getCardinality

public int getCardinality()

Description copied from interface: Expression

Determine the static cardinality of the expression. This establishes how many items there will be in the result of the expression, at compile time (i.e., without actually evaluating the result.

This method should always return a result, though it may be the best approximation that is available at the time.

Specified by:

getCardinality in interface Expression

Returns:

one of the values Cardinality.ONE_OR_MORE, Cardinality.ZERO_OR_MORE, Cardinality.EXACTLY_ONE, Cardinality.ZERO_OR_ONE, Cardinality.EMPTY. This default implementation returns ZERO_OR_MORE (which effectively gives no information).

getDependencies

public int getDependencies()

Description copied from interface: Expression

Determine which aspects of the context the expression depends on. The result is a bitwise-or'ed value composed from constants such as XPathContext.VARIABLES and XPathContext.CURRENT_NODE. The default implementation combines the intrinsic dependencies of this expression with the dependencies of the subexpressions, computed recursively. This is overridden for expressions such as FilterExpression where a subexpression's dependencies are not necessarily inherited by the parent expression.

Specified by:

getDependencies in interface Expression

Returns:

a set of bit-significant flags identifying the dependencies of the expression

getSpecialProperties

public int getSpecialProperties()

Description copied from interface: Expression

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

Returns:

a set of flags indicating static properties of this expression

promoteInst

protected abstract void promoteInst(PromotionOffer offer)
                             throws XPathException

Throws:

XPathException

promote

public Expression promote(PromotionOffer offer)
                   throws XPathException

Description copied from interface: Expression

Offer promotion for this subexpression. The offer will be accepted if the subexpression is not dependent on the factors (e.g. the context item) identified in the PromotionOffer. By default the offer is not accepted - this is appropriate in the case of simple expressions such as constant values and variable references where promotion would give no performance advantage. This method is always called at compile time.

Specified by:

promote in interface Expression

Parameters:

offer - details of the offer, for example the offer to move expressions that don't depend on the context to an outer level in the containing expression

Returns:

if the offer is not accepted, return this expression unchanged. Otherwise return the result of rewriting the expression to promote this subexpression

Throws:

XPathException - if any error is detected

getXPathExpressions

protected abstract void getXPathExpressions(List list)

getSubExpressions

public Expression[] getSubExpressions()

Description copied from interface: Expression

Get the immediate sub-expressions of this expression. Default implementation returns a zero-length array, appropriate for an expression that has no sub-expressions.

Specified by:

getSubExpressions in interface Expression

Returns:

an array containing the sub-expressions of this expression

evaluateItem

public Item evaluateItem(XPathContext context)
                  throws XPathException

Description copied from interface: Expression

Evaluate an expression as a single item. This always returns either a single Item or null (denoting the empty sequence). No conversion is done. This method should not be used unless the static type of the expression is a subtype of "item" or "item?": that is, it should not be called if the expression may return a sequence. There is no guarantee that this condition will be detected.

Specified by:

evaluateItem in interface Expression

Parameters:

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

Returns:

the node or atomic value that results from evaluating the expression; or null to indicate that the result is an empty sequence

Throws:

XPathException - if any dynamic error occurs evaluating the expression

evaluateAsString

public final String evaluateAsString(XPathContext context)
                              throws XPathException

Description copied from interface: Expression

Evaluate an expression as a String. This function must only be called in contexts where it is known that the expression will return a single string (or where an empty sequence is to be treated as a zero-length string). Implementations should not attempt to convert the result to a string, other than converting () to "". This method is used mainly to evaluate expressions produced by compiling an attribute value template.

Specified by:

evaluateAsString in interface Expression

Parameters:

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

Returns:

the value of the expression, evaluated in the current context. The expression must return a string or (); if the value of the expression is (), this method returns "".

Throws:

XPathException - if any dynamic error occurs evaluating the expression

effectiveBooleanValue

public boolean effectiveBooleanValue(XPathContext context)
                              throws XPathException

Description copied from interface: Expression

Get the effective boolean value of the expression. This returns false if the value is the empty sequence, a zero-length string, a number equal to zero, or the boolean false. Otherwise it returns true.

Specified by:

effectiveBooleanValue in interface Expression

Parameters:

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

Returns:

the effective boolean value

Throws:

XPathException - if any dynamic error occurs evaluating the expression

iterate

public SequenceIterator iterate(XPathContext context)
                         throws XPathException

Description copied from interface: Expression

Return an Iterator to iterate over the values of a sequence. The value of every expression can be regarded as a sequence, so this method is supported for all expressions. This default implementation handles iteration for expressions that return singleton values: for non-singleton expressions, the subclass must provide its own implementation.

Specified by:

iterate in interface Expression

Parameters:

context - supplies the context for evaluation

Returns:

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

Throws:

XPathException - if any dynamic error occurs evaluating the expression