net.sf.saxon.expr
Class Literal

java.lang.Object
  extended by net.sf.saxon.expr.Expression
      extended by net.sf.saxon.expr.Literal
All Implemented Interfaces:
Serializable, SourceLocator, EvaluableItem, SequenceIterable, InstructionInfoProvider
Direct Known Subclasses:
StringLiteral

public class Literal
extends Expression

A Literal is an expression whose value is constant: it is a class that implements the Expression interface as a wrapper around a Value. This may derive from an actual literal in an XPath expression or query, or it may be the result of evaluating a constant subexpression such as true() or xs:date('2007-01-16')

See Also:
Serialized Form

Field Summary
 
Fields inherited from class net.sf.saxon.expr.Expression
EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD
 
Constructor Summary
Literal(Value value)
          Create a literal as a wrapper around a Value
 
Method Summary
 PathMap.PathMapNodeSet addToPathMap(PathMap pathMap, PathMap.PathMapNodeSet pathMapNodeSet)
          Add a representation of this expression to a PathMap.
 int computeCardinality()
          Determine the cardinality
 int computeSpecialProperties()
          Compute the static properties of this expression (other than its type).
 Expression copy()
          Copy an expression.
 boolean effectiveBooleanValue(XPathContext context)
          Get the effective boolean value of the expression.
 boolean equals(Object obj)
          Determine whether two literals are equal, when considered as expressions.
 CharSequence evaluateAsString(XPathContext context)
          Evaluate an expression as a String.
 Item evaluateItem(XPathContext context)
          Evaluate as a singleton item (or empty sequence).
 void explain(ExpressionPresenter out)
          Diagnostic print of expression structure.
 int getDependencies()
          Determine which aspects of the context the expression depends on.
 ItemType getItemType(TypeHierarchy th)
          Determine the data type of the items in the expression, if possible
 Value getValue()
          Get the value represented by this Literal
 int hashCode()
          Return a hash code to support the equals() function
static boolean isAtomic(Expression exp)
          Test whether the literal wraps an atomic value.
static boolean isConstantBoolean(Expression exp, boolean value)
          Test if a literal represents the boolean value true
static boolean isConstantOne(Expression exp)
          Test if a literal represents the integer value 1
static boolean isEmptySequence(Expression exp)
          Test whether the literal explicitly wraps an empty sequence.
 SequenceIterator iterate(XPathContext context)
          Return an Iterator to iterate over the values of a sequence.
static Literal makeEmptySequence()
          Make an empty-sequence literal
static Literal makeLiteral(Value value)
          Create a literal as a wrapper around a Value (factory method)
 Expression optimize(ExpressionVisitor visitor, ItemType contextItemType)
          Optimize an expression
 void process(XPathContext context)
          Process the value as an instruction, without returning any tail calls
 Expression simplify(ExpressionVisitor visitor)
          Simplify an expression
 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)
          TypeCheck an expression
 
Methods inherited from class net.sf.saxon.expr.Expression
adoptChildExpression, checkPermittedContents, computeDependencies, computeStaticProperties, display, doPromotion, explain, findParentOf, getCardinality, getColumnNumber, getContainer, getExecutable, getHostLanguage, getImplementationMethod, getInstructionInfo, getIntrinsicDependencies, getLineNumber, getLocationId, getLocationProvider, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasLoopingSubexpression, implementsStaticTypeCheck, iterateEvents, iterateSubExpressions, markTailFunctionCalls, promote, replaceSubExpression, setContainer, setFiltered, setFlattened, setLocationId, staticTypeCheck, suppressValidation
 
Methods inherited from class java.lang.Object
getClass, notify, notifyAll, wait, wait, wait
 

Constructor Detail

Literal

public Literal(Value value)
Create a literal as a wrapper around a Value

Parameters:
value - the value of this literal
Method Detail

makeLiteral

public static Literal makeLiteral(Value value)
Create a literal as a wrapper around a Value (factory method)

Parameters:
value - the value of this literal
Returns:
the Literal

makeEmptySequence

public static Literal makeEmptySequence()
Make an empty-sequence literal

Returns:
a literal whose value is the empty sequence

getValue

public Value getValue()
Get the value represented by this Literal

Returns:
the constant value

simplify

public final Expression simplify(ExpressionVisitor visitor)
Simplify an expression

Overrides:
simplify in class Expression
Parameters:
visitor - an expression visitor
Returns:
for a Value, this always returns the value unchanged

typeCheck

public final Expression typeCheck(ExpressionVisitor visitor,
                                  ItemType contextItemType)
TypeCheck an expression

Overrides:
typeCheck in class Expression
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:
for a Value, this always returns the value unchanged

optimize

public final Expression optimize(ExpressionVisitor visitor,
                                 ItemType contextItemType)
Optimize an expression

Overrides:
optimize in class Expression
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:
for a Value, this always returns the value unchanged

getItemType

public ItemType getItemType(TypeHierarchy th)
Determine the data type of the items in the expression, if possible

Specified by:
getItemType in class Expression
Parameters:
th - The TypeHierarchy. Can be null if the target is an AtomicValue.
Returns:
for the default implementation: AnyItemType (not known)

computeCardinality

public int computeCardinality()
Determine the cardinality

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

public int computeSpecialProperties()
Compute the static properties of this expression (other than its type). For a Value, the only special property is StaticProperty.NON_CREATIVE.

Returns:
the value StaticProperty.NON_CREATIVE

copy

public Expression copy()
Copy an expression. This makes a deep copy.

Specified by:
copy in class Expression
Returns:
the copy of the original expression

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.

Overrides:
addToPathMap in class Expression
Parameters:
pathMap - the PathMap to which the expression should be added
pathMapNodeSet -
Returns:
the pathMapNode representing the focus established by this expression, in the case where this expression is the first operand of a path expression or filter expression

getDependencies

public final 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 StaticProperty.VARIABLES and StaticProperty.CURRENT_NODE

Overrides:
getDependencies in class Expression
Returns:
for a Value, this always returns zero.

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
Overrides:
iterate in class 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

evaluateItem

public Item evaluateItem(XPathContext context)
                  throws XPathException
Evaluate as a singleton item (or empty sequence). Note: this implementation returns the first item in the sequence. The method should not be used unless appropriate type-checking has been done to ensure that the value will be a singleton.

Specified by:
evaluateItem in interface EvaluableItem
Overrides:
evaluateItem in class 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

process

public void process(XPathContext context)
             throws XPathException
Process the value as an instruction, without returning any tail calls

Overrides:
process in class Expression
Parameters:
context - The dynamic context, giving access to the current node, the current variables, etc.
Throws:
XPathException

evaluateAsString

public CharSequence evaluateAsString(XPathContext context)
                              throws XPathException
Description copied from class: 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.

Overrides:
evaluateAsString in class 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
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.

Overrides:
effectiveBooleanValue in class 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

equals

public boolean equals(Object obj)
Determine whether two literals are equal, when considered as expressions.

Overrides:
equals in class Object
Parameters:
obj - the other expression
Returns:
true if the two literals are equal

hashCode

public int hashCode()
Return a hash code to support the equals() function

Overrides:
hashCode in class Object

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 Expression

explain

public void explain(ExpressionPresenter out)
Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

Specified by:
explain in class Expression
Parameters:
out - the expression presenter used to display the structure

isAtomic

public static boolean isAtomic(Expression exp)
Test whether the literal wraps an atomic value. (Note, if this method returns false, this still leaves the possibility that the literal wraps a sequence that happens to contain a single atomic value).

Parameters:
exp - an expression
Returns:
if the expression is a literal and the literal wraps an AtomicValue

isEmptySequence

public static boolean isEmptySequence(Expression exp)
Test whether the literal explicitly wraps an empty sequence. (Note, if this method returns false, this still leaves the possibility that the literal wraps a sequence that happens to be empty).

Parameters:
exp - an expression
Returns:
if the expression is a literal and the literal wraps an AtomicValue

isConstantBoolean

public static boolean isConstantBoolean(Expression exp,
                                        boolean value)
Test if a literal represents the boolean value true

Parameters:
exp - an expression
value - true or false
Returns:
if the expression is a literal and the literal represents the boolean value given in the second argument

isConstantOne

public static boolean isConstantOne(Expression exp)
Test if a literal represents the integer value 1

Parameters:
exp - an expression
Returns:
if the expression is a literal and the literal represents the integer value 1


Copyright (C) Michael H. Kay. All rights reserved.