## net.sf.saxon.expr
Class Expression

    java.lang.Object
      net.sf.saxon.expr.Expression

All Implemented Interfaces:
  Serializable, SourceLocator, LocationProvider, SaxonLocator, EvaluableItem, SequenceIterable, InstructionInfo, Locator

Direct Known Subclasses:
  Assignation, AxisExpression, BasicUpdatingExpression, BinaryExpression, CompareToIntegerConstant, ConditionalSorter, ContextItemExpression, CopyModifyExpression, ErrorExpression, EvaluateInstr, FilterExpression, Fork, FunctionCall, FunctionItemExpression, IndexedFilterExpression, IndexedLookupExpression, Instruction, IntegerRangeTest, IsLastExpression, Literal, NumberInstruction, PatternMatchExpression, PatternSponsor, SimpleContentConstructor, SimpleExpression, SingleNodeExpression, SlashExpression, SortExpression, StreamingCopy, SuppliedParameterReference, SwitchExpression, TailExpression, TryCatch, TupleExpression, TupleSorter, UnaryExpression, VariableReference

    public abstract class Expression

extends Object
implements SequenceIterable, EvaluableItem, Serializable, SourceLocator, InstructionInfo

Interface supported by an XPath expression. This includes both compile-time and run-time methods.

Two expressions are considered equal if they return the same result when evaluated in the same context.

See Also:
  Serialized Form

Field Summary
`static int` `EVALUATE_METHOD`
`static int` `EVENT_FEED_METHOD`
`static int` `ITEM_FEED_METHOD`
`static int` `ITERATE_METHOD`
`protected int` `locationId`
`static int` `PROCESS_METHOD`
`static int` `PUSH_SELECTION`
`protected int` `staticProperties`
`static int` `WATCH_METHOD`

Constructor Summary
`Expression()`

Method Summary
`PathMap.PathMapNodeSet` `addToPathMap(PathMap pathMap, PathMap.PathMapNodeSet pathMapNodeSet)` Add a representation of this expression to a PathMap.
`void` `adoptChildExpression(Expression child)` Set up a parent-child relationship between this expression and a given child expression.
`void` `checkForUpdatingSubexpressions()` Check to ensure that this expression does not contain any inappropriate updating subexpressions.
`void` `checkPermittedContents(SchemaType parentType, StaticContext env, boolean whole)` Check that any elements and attributes constructed or returned by this expression are acceptable in the content model of a given complex type.
`protected abstract int` `computeCardinality()` Compute the static cardinality of this expression
`int` `computeDependencies()` Compute the dependencies of an expression, as the union of the dependencies of its subexpressions.
`protected int` `computeSpecialProperties()` Compute the special properties of this expression.
`void` `computeStaticProperties()` Compute the static properties.
`abstract Expression` `copy()` Copy an expression.
`Expression` `doPromotion(Expression subexpression, PromotionOffer offer)` Promote a subexpression if possible, and if the expression was changed, carry out housekeeping to reset the static properties and correct the parent pointers in the tree
`protected void` `dynamicError(String message, String code, XPathContext context)` Method used in subclasses to signal a dynamic error
`boolean` `effectiveBooleanValue(XPathContext context)` Get the effective boolean value of the expression.
`CharSequence` `evaluateAsString(XPathContext context)` Evaluate an expression as a String.
`Item` `evaluateItem(XPathContext context)` Evaluate an expression as a single item.
`void` `evaluatePendingUpdates(XPathContext context, PendingUpdateList pul)` Evaluate an updating expression, adding the results to a Pending Update List.
`abstract void` `explain(ExpressionPresenter out)` Diagnostic print of expression structure.
`void` `explain(OutputStream out)` Diagnostic print of expression structure.
`int` `getCardinality()` Determine the static cardinality of the expression.
`int` `getColumnNumber()` Get the column number of the expression
`int` `getColumnNumber(long locationId)` Get the column number within the document, entity, or module containing a particular location
`int` `getConstructType()` Get the type of this expression for use in tracing and diagnostics
`Container` `getContainer()` Get the container in which this expression is located.
`int` `getDependencies()` Determine which aspects of the context the expression depends on.
`int` `getEvaluationMethod()`
`Executable` `getExecutable()` Get the executable containing this expression
`String` `getExpressionName()` Get a name identifying the kind of expression, in terms meaningful to a user.
`int` `getHostLanguage()` Get the host language (XSLT, XQuery, XPath) used to implement the code in this container
`int` `getImplementationMethod()` An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
`int` `getIntrinsicDependencies()` Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions.
`abstract ItemType` `getItemType(TypeHierarchy th)` Determine the data type of the expression, if possible.
`int` `getLineNumber()` Get the line number of the expression
`int` `getLineNumber(long locationId)` Get the line number within the document or module containing a particular location
`int` `getLocationId()` Get the location ID of the expression
`LocationProvider` `getLocationProvider()` Get the LocationProvider allowing location identifiers to be resolved.
`StructuredQName` `getObjectName()` Get a name identifying the object of the expression, for example a function name, template name, variable name, key name, element name, etc.
`Iterator` `getProperties()` Get an iterator over all the properties available.
`Object` `getProperty(String name)` Get the value of a particular property of the instruction.
`String` `getPublicId()` Get the publicId of the module containing the expression (to satisfy the SourceLocator interface)
`int[]` `getSlotsUsed()` Get the local variables (identified by their slot numbers) on which this expression depends.
`int` `getSpecialProperties()` Get the static properties of this expression (other than its type).
`String` `getSystemId()` Get the systemId of the module containing the expression
`String` `getSystemId(long locationId)` Get the URI of the document or module containing a particular location
`boolean` `hasLoopingSubexpression(Expression child)` Given an expression that is an immediate child of this expression, test whether the evaluation of the parent expression causes the child expression to be evaluated repeatedly
`boolean` `implementsStaticTypeCheck()` Determine whether this expression implements its own method for static type checking
`boolean` `isSubtreeExpression()` Determine whether the expression can be evaluated without reference to the part of the context document outside the subtree rooted at the context node.
`boolean` `isUpdatingExpression()` Determine whether this is an updating expression as defined in the XQuery update specification
`boolean` `isVacuousExpression()` Determine whether this is a vacuous expression as defined in the XQuery update specification
`SequenceIterator` `iterate(XPathContext context)` Return an Iterator to iterate over the values of a sequence.
`EventIterator` `iterateEvents(XPathContext context)` Deliver the result of the expression as a sequence of events.
`Iterator<Expression>` `iterateSameFocusSubExpressions()` Get the immediate sub-expressions of this expression that are evaluated with the same focus (context item, position, and size) as the parent expression.
`Iterator<Expression>` `iterateSubExpressions()` Get the immediate sub-expressions of this expression.
`int` `markTailFunctionCalls(StructuredQName qName, int arity)` Mark tail-recursive calls on stylesheet functions.
`Expression` `optimize(ExpressionVisitor visitor, ItemType contextItemType)` Perform optimisation of an expression and its subexpressions.
`void` `process(XPathContext context)` Process the instruction, without returning any tail calls
`Expression` `promote(PromotionOffer offer, Expression parent)` Offer promotion for this subexpression.
`boolean` `replaceSubExpression(Expression original, Expression replacement)` Replace one subexpression by a replacement subexpression
`protected void` `resetLocalStaticProperties()` Reset the static properties of the expression to -1, so that they have to be recomputed next time they are used.
`void` `setContainer(Container container)` Mark an expression as being in a given Container.
`void` `setEvaluationMethod(int method)`
`void` `setFiltered(boolean filtered)` Mark an expression as filtered: that is, it appears as the base expression in a filter expression.
`void` `setFlattened(boolean flattened)` Mark an expression as being "flattened".
`void` `setLocationId(int id)` Set the location ID on an expression.
`Expression` `simplify(ExpressionVisitor visitor)` Simplify an expression.
`Expression` `staticTypeCheck(SequenceType req, boolean backwardsCompatible, RoleLocator role, TypeCheckerEnvironment visitor)` Static type checking of some expressions is delegated to the expression itself, by calling this method.
`void` `suppressValidation(int validationMode)` Suppress validation on contained element constructors, on the grounds that the parent element is already performing validation.
`String` `toString()` The toString() method for an expression attempts to give a representation of the expression in an XPath-like form, but there is no guarantee that the syntax will actually be true XPath.
`Expression` `typeCheck(ExpressionVisitor visitor, ItemType contextItemType)` Perform type checking of an expression and its subexpressions.
`protected void` `typeError(String message, String errorCode, XPathContext context)` Method used in subclasses to signal a runtime type error

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

Field Detail

### EVALUATE_METHOD

    public static final int EVALUATE_METHOD

See Also:
  Constant Field Values

### ITERATE_METHOD

    public static final int ITERATE_METHOD

See Also:
  Constant Field Values

### PROCESS_METHOD

    public static final int PROCESS_METHOD

See Also:
  Constant Field Values

### WATCH_METHOD

    public static final int WATCH_METHOD

See Also:
  Constant Field Values

### ITEM_FEED_METHOD

    public static final int ITEM_FEED_METHOD

See Also:
  Constant Field Values

### EVENT_FEED_METHOD

    public static final int EVENT_FEED_METHOD

See Also:
  Constant Field Values

### PUSH_SELECTION

    public static final int PUSH_SELECTION

See Also:
  Constant Field Values

### staticProperties

    protected int staticProperties

### locationId

    protected int locationId

Constructor Detail

### Expression

    public Expression()

Method Detail

### getExpressionName

    public String getExpressionName()

  Get a name identifying the kind of expression, in terms meaningful to a user.

  Returns:
    a name identifying the kind of expression, in terms meaningful to a user. The name will always be in the form of a lexical XML QName, and should match the name used in explain() output displaying the expression.

### 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 provided directly. The other methods will always be available indirectly, using an implementation that relies on one of the other methods.

  Returns:
    the implementation method, for example `ITERATE_METHOD` or `EVALUATE_METHOD` or `PROCESS_METHOD`

### implementsStaticTypeCheck

    public boolean implementsStaticTypeCheck()

  Determine whether this expression implements its own method for static type checking

  Returns:
    true if this expression has a non-trivial implementation of the staticTypeCheck() method

### simplify

    public Expression simplify(ExpressionVisitor visitor)
                        throws XPathException

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

  Parameters:
    `visitor` - an expression visitor
  Returns:
    the simplified expression
  Throws:
    `XPathException` - if an error is discovered during expression rewriting

### typeCheck

    public Expression typeCheck(ExpressionVisitor visitor,
                                ItemType contextItemType)
                         throws XPathException

  Perform type checking of an expression and its subexpressions. This is the second phase of static optimization.

  This checks statically that the operands of the expression have the correct type; if necessary it generates code to do run-time type checking or type conversion. A static type error is reported only if execution cannot possibly succeed, that is, if a run-time type error is inevitable. The call may return a modified form of the expression.

  This method is called after all references to functions and variables have been resolved to the declaration of the function or variable. However, the types of such functions and variables may not be accurately known if they have not been explicitly declared.

  If the implementation returns a value other than "this", then it is required to ensure that the location information in the returned expression have been set up correctly. It should not rely on the caller to do this, although for historical reasons many callers do so.

  Parameters:
    `visitor` - an expression visitor
    `contextItemType` - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to `Type.ITEM_TYPE`
  Returns:
    the original expression, rewritten to perform necessary run-time type checks, and to perform other type-related optimizations
  Throws:
    `XPathException` - if an error is discovered during this phase (typically a type error)

### staticTypeCheck

    public Expression staticTypeCheck(SequenceType req,
                                      boolean backwardsCompatible,
                                      RoleLocator role,
                                      TypeCheckerEnvironment visitor)
                               throws XPathException

  Static type checking of some expressions is delegated to the expression itself, by calling this method. The default implementation of the method throws UnsupportedOperationException. If there is a non-default implementation, then implementsStaticTypeCheck() will return true

  Parameters:
    `req` - the required type
    `backwardsCompatible` - true if backwards compatibility mode applies
    `role` - the role of the expression in relation to the required type
    `visitor` - an expression visitor
  Returns:
    the expression after type checking (perhaps augmented with dynamic type checking code)
  Throws:
    `XPathException` - if failures occur, for example if the static type of one branch of the conditional is incompatible with the required type

### optimize

    public Expression optimize(ExpressionVisitor visitor,
                               ItemType contextItemType)
                        throws XPathException

  Perform optimisation of an expression and its subexpressions. This is the third and final phase of static optimization.

  This method is called after all references to functions and variables have been resolved to the declaration of the function or variable, and after all type checking has been done.

  Parameters:
    `visitor` - an expression visitor
    `contextItemType` - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to `Type.ITEM_TYPE`
  Returns:
    the original expression, rewritten if appropriate to optimize execution
  Throws:
    `XPathException` - if an error is discovered during this phase (typically a type error)

### promote

    public Expression promote(PromotionOffer offer,
                              Expression parent)
                       throws XPathException

  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.

  This method must be overridden for any Expression that has subexpressions.

  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
    `parent` -
  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

### getSpecialProperties

    public final 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.

  Returns:
    a set of flags indicating static properties of this expression

### getCardinality

    public int getCardinality()

  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.

  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).

### getItemType

    public abstract ItemType getItemType(TypeHierarchy th)

  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.

  Parameters:
    `th` - the type hierarchy cache
  Returns:
    a value such as Type.STRING, Type.BOOLEAN, Type.NUMBER, Type.NODE, or Type.ITEM (meaning not known at compile time)

### getDependencies

    public int getDependencies()

  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.

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

### iterateSubExpressions

    public Iterator<Expression> iterateSubExpressions()

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

  Returns:
    an iterator containing the sub-expressions of this expression

### iterateSameFocusSubExpressions

    public Iterator<Expression> iterateSameFocusSubExpressions()

  Get the immediate sub-expressions of this expression that are evaluated with the same focus (context item, position, and size) as the parent expression. The default implementation returns all the sub-expressions of this expression

  Returns:
    an iterator containing those sub-expressions of this expression that are evaluated with the same focus as the parent expression

### hasLoopingSubexpression

    public boolean hasLoopingSubexpression(Expression child)

  Given an expression that is an immediate child of this expression, test whether the evaluation of the parent expression causes the child expression to be evaluated repeatedly

  Parameters:
    `child` - the immediate subexpression
  Returns:
    true if the child expression is evaluated repeatedly

### setFlattened

    public void setFlattened(boolean flattened)

  Mark an expression as being "flattened". This is a collective term that includes extracting the string value or typed value, or operations such as simple value construction that concatenate text nodes before atomizing. The implication of all of these is that although the expression might return nodes, the identity of the nodes has no significance. This is called during type checking of the parent expression.

  Parameters:
    `flattened` - set to true if the result of the expression is atomized or otherwise turned into an atomic value

### setFiltered

    public void setFiltered(boolean filtered)

  Mark an expression as filtered: that is, it appears as the base expression in a filter expression. This notification currently has no effect except when the expression is a variable reference.

  Parameters:
    `filtered` - if true, marks this expression as the base of a filter expression

### evaluateItem

    public Item evaluateItem(XPathContext context)
                      throws XPathException

  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 `EvaluableItem`
  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

### iterate

    public SequenceIterator iterate(XPathContext context)
                             throws XPathException

  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 `SequenceIterable`
  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

### iterateEvents

    public EventIterator iterateEvents(XPathContext context)
                                throws XPathException

  Deliver the result of the expression as a sequence of events.

  The events (of class `PullEvent`) are either complete items, or one of startElement, endElement, startDocument, or endDocument, known as semi-nodes. The stream of events may also include a nested EventIterator. If a start-end pair exists in the sequence, then the events between this pair represent the content of the document or element. The content sequence will have been processed to the extent that any attribute and namespace nodes in the content sequence will have been merged into the startElement event. Namespace fixup will have been performed: that is, unique prefixes will have been allocated to element and attribute nodes, and all namespaces will be declared by means of a namespace node in the startElement event or in an outer startElement forming part of the sequence. However, duplicate namespaces may appear in the sequence.

  The content of an element or document may include adjacent or zero-length text nodes, atomic values, and nodes represented as nodes rather than broken down into events.

  Parameters:
    `context` - The dynamic evaluation context
  Returns:
    the result of the expression as an iterator over a sequence of PullEvent objects
  Throws:
    `XPathException` - if a dynamic error occurs during expression evaluation

### effectiveBooleanValue

    public boolean effectiveBooleanValue(XPathContext context)
                                  throws XPathException

  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.

  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

### evaluateAsString

    public CharSequence evaluateAsString(XPathContext context)
                                  throws XPathException

  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.

  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
    `ClassCastException` - if the result type of the expression is not xs:string?

### process

    public void process(XPathContext context)
                 throws XPathException

  Process the instruction, without returning any tail calls

  Parameters:
    `context` - The dynamic context, giving access to the current node, the current variables, etc.
  Throws:
    `XPathException`

### evaluatePendingUpdates

    public void evaluatePendingUpdates(XPathContext context,
                                       PendingUpdateList pul)
                                throws XPathException

  Evaluate an updating expression, adding the results to a Pending Update List. The default implementation of this method, which is used for non-updating expressions, throws an UnsupportedOperationException

  Parameters:
    `context` - the XPath dynamic evaluation context
    `pul` - the pending update list to which the results should be written
  Throws:
    `XPathException`

### toString

    public String toString()

  The toString() method for an expression attempts to give a representation of the expression in an XPath-like form, but there is no guarantee that the syntax will actually be true XPath. In the case of XSLT instructions, the toString() method gives an abstracted view of the syntax

  Overrides:
    `toString` in class `Object`
  Returns:
    a representation of the expression as a string

### explain

    public abstract void explain(ExpressionPresenter out)

  Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

  Parameters:
    `out` - the expression presenter used to display the structure

### explain

    public final void explain(OutputStream out)

  Diagnostic print of expression structure. The abstract expression tree is written to the supplied outputstream.

  Parameters:
    `out` - the expression presenter used to display the structure

### checkPermittedContents

    public void checkPermittedContents(SchemaType parentType,
                                       StaticContext env,
                                       boolean whole)
                                throws XPathException

  Check that any elements and attributes constructed or returned by this expression are acceptable in the content model of a given complex type. It's always OK to say yes, since the check will be repeated at run-time. The process of checking element and attribute constructors against the content model of a complex type also registers the type of content expected of those constructors, so the static validation can continue recursively.

  Parameters:
    `parentType` - the "given complex type": the method is checking that the nodes returned by this expression are acceptable members of the content model of this type
    `env` - the static context
    `whole` - if true, we want to check that the value of this expression satisfies the content model as a whole; if false we want to check that the value of the expression is acceptable as one part of the content
  Throws:
    `XPathException` - if the value delivered by this expression cannot be part of the content model of the given type

### setContainer

    public void setContainer(Container container)

  Mark an expression as being in a given Container. This link is used primarily for diagnostics: the container links to the location map held in the executable.

  This affects the expression and all its subexpressions. Any subexpressions that are not in the same container are marked with the new container, and this proceeds recursively. However, any subexpression that is already in the correct container is not modified.

  Parameters:
    `container` - The container of this expression.

### getContainer

    public Container getContainer()

  Get the container in which this expression is located. This will usually be a top-level construct such as a function or global variable, and XSLT template, or an XQueryExpression. In the case of free-standing XPath expressions it will be the StaticContext object

  Returns:
    the expression's container

### adoptChildExpression

    public void adoptChildExpression(Expression child)

  Set up a parent-child relationship between this expression and a given child expression.

  Note: many calls on this method are now redundant, but are kept in place for "belt-and-braces" reasons. The rule is that an implementation of simplify(), typeCheck(), or optimize() that returns a value other than "this" is required to set the location information and parent pointer in the new child expression. However, in the past this was often left to the caller, which did it by calling this method, either unconditionally on return from one of these methods, or after testing that the returned object was not the same as the original.

  Parameters:
    `child` - the child expression

### setLocationId

    public void setLocationId(int id)

  Set the location ID on an expression.

  Parameters:
    `id` - the location id

### getLocationId

    public final int getLocationId()

  Get the location ID of the expression

  Returns:
    a location identifier, which can be turned into real location information by reference to a location provider

### getLineNumber

    public int getLineNumber()

  Get the line number of the expression

  Specified by:
    `getLineNumber` in interface `SourceLocator`
  Specified by:
    `getLineNumber` in interface `InstructionInfo`
  Specified by:
    `getLineNumber` in interface `Locator`
  Returns:
    the line number of the expression within the containing module

### getColumnNumber

    public int getColumnNumber()

  Get the column number of the expression

  Specified by:
    `getColumnNumber` in interface `SourceLocator`
  Specified by:
    `getColumnNumber` in interface `Locator`

### getSystemId

    public String getSystemId()

  Get the systemId of the module containing the expression

  Specified by:
    `getSystemId` in interface `SourceLocator`
  Specified by:
    `getSystemId` in interface `InstructionInfo`
  Specified by:
    `getSystemId` in interface `Locator`
  Returns:
    the URI of the containing module

### getPublicId

    public final String getPublicId()

  Get the publicId of the module containing the expression (to satisfy the SourceLocator interface)

  Specified by:
    `getPublicId` in interface `SourceLocator`
  Specified by:
    `getPublicId` in interface `Locator`

### getExecutable

    public Executable getExecutable()

  Get the executable containing this expression

  Returns:
    the containing Executable

### getLocationProvider

    public LocationProvider getLocationProvider()

  Get the LocationProvider allowing location identifiers to be resolved.

  Returns:
    the LocationProvider used to turn the location id into real location information

### doPromotion

    public final Expression doPromotion(Expression subexpression,
                                        PromotionOffer offer)
                                 throws XPathException

  Promote a subexpression if possible, and if the expression was changed, carry out housekeeping to reset the static properties and correct the parent pointers in the tree

  Parameters:
    `subexpression` - the subexpression that is a candidate for promotion
    `offer` - details of the promotion being considered @return the result of the promotion. This will be the current expression if no promotion actions have taken place
  Throws:
    `XPathException`

### computeStaticProperties

    public final void computeStaticProperties()

  Compute the static properties. This should only be done once for each expression.

### resetLocalStaticProperties

    protected void resetLocalStaticProperties()

  Reset the static properties of the expression to -1, so that they have to be recomputed next time they are used.

### computeCardinality

    protected abstract int computeCardinality()

  Compute the static cardinality of this expression

  Returns:
    the computed cardinality, as one of the values `StaticProperty.ALLOWS_ZERO_OR_ONE`, `StaticProperty.EXACTLY_ONE`, `StaticProperty.ALLOWS_ONE_OR_MORE`, `StaticProperty.ALLOWS_ZERO_OR_MORE`

### computeSpecialProperties

    protected int computeSpecialProperties()

  Compute the special properties of this expression. These properties are denoted by a bit-significant integer, possible values are in class `StaticProperty`. The "special" properties are properties other than cardinality and dependencies, and most of them relate to properties of node sequences, for example whether the nodes are in document order.

  Returns:
    the special properties, as a bit-significant integer

### computeDependencies

    public int computeDependencies()

  Compute the dependencies of an expression, as the union of the dependencies of its subexpressions. (This is overridden for path expressions and filter expressions, where the dependencies of a subexpression are not all propogated). This method should be called only once, to compute the dependencies; after that, getDependencies should be used.

  Returns:
    the depencies, as a bit-mask

### getIntrinsicDependencies

    public int getIntrinsicDependencies()

  Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions. For example, position() has an intrinsic dependency on the context position, while (position()+1) does not. The default implementation of the method returns 0, indicating "no dependencies".

  Returns:
    a set of bit-significant flags identifying the "intrinsic" dependencies. The flags are documented in class net.sf.saxon.value.StaticProperty

### checkForUpdatingSubexpressions

    public void checkForUpdatingSubexpressions()
                                        throws XPathException

  Check to ensure that this expression does not contain any inappropriate updating subexpressions. This check is overridden for those expressions that permit updating subexpressions.

  Throws:
    `XPathException` - if the expression has a non-permitted updating subexpression

### isUpdatingExpression

    public boolean isUpdatingExpression()

  Determine whether this is an updating expression as defined in the XQuery update specification

  Returns:
    true if this is an updating expression

### isVacuousExpression

    public boolean isVacuousExpression()

  Determine whether this is a vacuous expression as defined in the XQuery update specification

  Returns:
    true if this expression is vacuous

### copy

    public abstract Expression copy()

  Copy an expression. This makes a deep copy.

  Returns:
    the copy of the original expression

### replaceSubExpression

    public boolean replaceSubExpression(Expression original,
                                        Expression replacement)

  Replace one subexpression by a replacement subexpression

  Parameters:
    `original` - the original subexpression
    `replacement` - the replacement subexpression
  Returns:
    true if the original subexpression is found

### suppressValidation

    public void suppressValidation(int validationMode)

  Suppress validation on contained element constructors, on the grounds that the parent element is already performing validation. The default implementation does nothing.

  Parameters:
    `validationMode` - the kind of validation being performed on the parent expression

### markTailFunctionCalls

    public int markTailFunctionCalls(StructuredQName qName,
                                     int arity)

  Mark tail-recursive calls on stylesheet functions. For most expressions, this does nothing.

  Parameters:
    `qName` - the name of the function
    `arity` - the arity (number of parameters) of the function
  Returns:
    0 if no tail call was found; 1 if a tail call on a different function was found; 2 if a tail recursive call was found and if this call accounts for the whole of the value.

### getSlotsUsed

    public final int[] getSlotsUsed()

  Get the local variables (identified by their slot numbers) on which this expression depends. Should only be called if the caller has established that there is a dependency on local variables.

  Returns:
    an array of integers giving the slot numbers of the local variables referenced in this expression.

### dynamicError

    protected void dynamicError(String message,
                                String code,
                                XPathContext context)
                         throws XPathException

  Method used in subclasses to signal a dynamic error

  Parameters:
    `message` - the error message
    `code` - the error code
    `context` - the XPath dynamic context
  Throws:
    `XPathException`

### typeError

    protected void typeError(String message,
                             String errorCode,
                             XPathContext context)
                      throws XPathException

  Method used in subclasses to signal a runtime type error

  Parameters:
    `message` - the error message
    `errorCode` - the error code
    `context` - the XPath dynamic context
  Throws:
    `XPathException`

### getConstructType

    public int getConstructType()

  Get the type of this expression for use in tracing and diagnostics

  Specified by:
    `getConstructType` in interface `InstructionInfo`
  Returns:
    the type of expression, as enumerated in class `Location`

### getObjectName

    public StructuredQName getObjectName()

  Description copied from interface: `InstructionInfo`
  Get a name identifying the object of the expression, for example a function name, template name, variable name, key name, element name, etc. This is used only where the name is known statically.

  Specified by:
    `getObjectName` in interface `InstructionInfo`
  Returns:
    the QName of the object declared or manipulated by this instruction or expression

### getProperty

    public Object getProperty(String name)

  Description copied from interface: `InstructionInfo`
  Get the value of a particular property of the instruction. Properties of XSLT instructions are generally known by the name of the stylesheet attribute that defines them.

  Specified by:
    `getProperty` in interface `InstructionInfo`
  Parameters:
    `name` - The name of the required property
  Returns:
    The value of the requested property, or null if the property is not available

### getLineNumber

    public int getLineNumber(long locationId)

  Get the line number within the document or module containing a particular location

  Specified by:
    `getLineNumber` in interface `LocationProvider`
  Parameters:
    `locationId` - identifier of the location in question (as passed down the Receiver pipeline)
  Returns:
    the line number within the document or module.

### getColumnNumber

    public int getColumnNumber(long locationId)

  Description copied from interface: `LocationProvider`
  Get the column number within the document, entity, or module containing a particular location

  Specified by:
    `getColumnNumber` in interface `LocationProvider`
  Parameters:
    `locationId` - identifier of the location in question (as passed down the Receiver pipeline)
  Returns:
    the column number within the document, entity, or module, or -1 if this is not available

### getSystemId

    public String getSystemId(long locationId)

  Get the URI of the document or module containing a particular location

  Specified by:
    `getSystemId` in interface `LocationProvider`
  Parameters:
    `locationId` - identifier of the location in question (as passed down the Receiver pipeline)
  Returns:
    the URI of the document or module.

### getProperties

    public Iterator getProperties()

  Get an iterator over all the properties available. The values returned by the iterator will be of type String, and each string can be supplied as input to the getProperty() method to retrieve the value of the property. The iterator may return properties whose value is null.

  Specified by:
    `getProperties` in interface `InstructionInfo`
  Returns:
    an iterator over the properties.

### getHostLanguage

    public int getHostLanguage()

  Get the host language (XSLT, XQuery, XPath) used to implement the code in this container

  Returns:
    typically `Configuration.XSLT` or `Configuration.XQUERY`

### addToPathMap

    public PathMap.PathMapNodeSet addToPathMap(PathMap pathMap,
                                               PathMap.PathMapNodeSet pathMapNodeSet)

  Add a representation of this expression to a PathMap. The PathMap captures a map of the nodes visited by an expression in a source tree.

  The default implementation of this method assumes that an expression does no navigation other than the navigation done by evaluating its subexpressions, and that the subexpressions are evaluated in the same context as the containing expression. The method must be overridden for any expression where these assumptions do not hold. For example, implementations exist for AxisExpression, ParentExpression, and RootExpression (because they perform navigation), and for the doc(), document(), and collection() functions because they create a new navigation root. Implementations also exist for PathExpression and FilterExpression because they have subexpressions that are evaluated in a different context from the calling expression.

  Parameters:
    `pathMap` - the PathMap to which the expression should be added
    `pathMapNodeSet` - the PathMapNodeSet to which the paths embodied in this expression should be added
  Returns:
    the pathMapNodeSet representing the points in the source document that are both reachable by this expression, and that represent possible results of this expression. For an expression that does navigation, it represents the end of the arc in the path map that describes the navigation route. For other expressions, it is the same as the input pathMapNode.

### isSubtreeExpression

    public boolean isSubtreeExpression()

  Determine whether the expression can be evaluated without reference to the part of the context document outside the subtree rooted at the context node.

  Returns:
    true if the expression has no dependencies on the context node, or if the only dependencies on the context node are downward selections using the self, child, descendant, attribute, and namespace axes.

### setEvaluationMethod

    public void setEvaluationMethod(int method)

### getEvaluationMethod

    public int getEvaluationMethod()

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