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## net.sf.saxon.expr
Class SingletonAtomizer

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

All Implemented Interfaces:
  java.io.Serializable, javax.xml.transform.SourceLocator, LocationProvider, SaxonLocator, EvaluableItem, SequenceIterable, InstructionInfo, org.xml.sax.Locator

    public final class SingletonAtomizer

extends UnaryExpression

A SingletonAtomizer combines the functions of an Atomizer and a CardinalityChecker: it is used to atomize a sequence of nodes, checking that the result of the atomization contains zero or one atomic values. Note that the input may be a sequence of nodes or atomic values, even though the result must contain at most one atomic value.

See Also:
  Serialized Form

Field Summary

Fields inherited from class net.sf.saxon.expr.UnaryExpression
`operand`

Fields inherited from class net.sf.saxon.expr.Expression
`EVALUATE_METHOD, ITERATE_METHOD, locationId, PROCESS_METHOD, staticProperties`

Constructor Summary
`SingletonAtomizer(Expression sequence, RoleLocator role, boolean allowEmpty)` Constructor

Method Summary
`PathMap.PathMapNodeSet` | `addToPathMap(PathMap pathMap, PathMap.PathMapNodeSet pathMapNodeSet)` Add a representation of this expression to a PathMap.
`int` | `computeCardinality()` Determine the static cardinality of the expression
`int` | `computeSpecialProperties()` Determine the special properties of this expression
`Expression` | `copy()` Copy an expression.
`protected java.lang.String` | `displayExpressionName()` Give a string representation of the expression name for use in diagnostics
`Item` | `evaluateItem(XPathContext context)` Evaluate as an Item.
`ItemType` | `getItemType(TypeHierarchy th)` Determine the data type of the items returned by the expression, if possible
`RoleLocator` | `getRole()` Get the RoleLocator (used to construct error messages)
`Expression` | `optimize(ExpressionVisitor visitor, ItemType contextItemType)` Perform optimisation of an expression and its subexpressions.
`Expression` | `simplify(ExpressionVisitor visitor)` Simplify an expression
`Expression` | `typeCheck(ExpressionVisitor visitor, ItemType contextItemType)` Type-check the expression

Methods inherited from class net.sf.saxon.expr.UnaryExpression
`displayOperator, equals, explain, getBaseExpression, hashCode, iterateSubExpressions, promote, replaceSubExpression, toString`

Methods inherited from class net.sf.saxon.expr.Expression
`adoptChildExpression, checkForUpdatingSubexpressions, checkPermittedContents, computeDependencies, computeStaticProperties, display, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, evaluatePendingUpdates, explain, findParentOf, getCardinality, getColumnNumber, getColumnNumber, getConstructType, getContainer, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getIntrinsicDependencies, getLineNumber, getLineNumber, getLocationId, getLocationProvider, getObjectName, getProperties, getProperty, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, getSystemId, hasLoopingSubexpression, implementsStaticTypeCheck, isUpdatingExpression, iterate, iterateEvents, markTailFunctionCalls, process, resetLocalStaticProperties, setContainer, setFiltered, setFlattened, setLocationId, staticTypeCheck, suppressValidation, typeError`

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

Constructor Detail

### SingletonAtomizer

    public SingletonAtomizer(Expression sequence,
                             RoleLocator role,
                             boolean allowEmpty)

  Constructor

  Parameters:
    `sequence` - the sequence to be atomized
    `role` - contains information about where the expression appears, for use in any error message
    `allowEmpty` - true if the result sequence is allowed to be empty.

Method Detail

### simplify

    public Expression simplify(ExpressionVisitor visitor)
                        throws XPathException

  Simplify an expression

  Overrides:
    `simplify` in class `UnaryExpression`
  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

  Type-check the expression

  Overrides:
    `typeCheck` in class `UnaryExpression`
  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)

### optimize

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

  Description copied from class: `UnaryExpression`
  Perform optimisation of an expression and its subexpressions.

  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.

  Overrides:
    `optimize` in class `UnaryExpression`
  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)

### computeSpecialProperties

    public int computeSpecialProperties()

  Determine the special properties of this expression

  Overrides:
    `computeSpecialProperties` in class `UnaryExpression`
  Returns:
    `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

### getRole

    public RoleLocator getRole()

  Get the RoleLocator (used to construct error messages)

  Returns:
    the role locator

### addToPathMap

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

  Description copied from class: `Expression`
  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.

  Overrides:
    `addToPathMap` in class `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.

### evaluateItem

    public Item evaluateItem(XPathContext context)
                      throws XPathException

  Evaluate as an Item. This should only be called if a singleton or empty sequence is required; it throws a type error if the underlying sequence is multi-valued.

  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

### getItemType

    public ItemType getItemType(TypeHierarchy th)

  Determine the data type of the items returned by the expression, if possible

  Overrides:
    `getItemType` in class `UnaryExpression`
  Parameters:
    `th` - the type hierarchy cache
  Returns:
    a value such as Type.STRING, Type.BOOLEAN, Type.NUMBER. For this class, the result is always an atomic type, but it might be more specific.

### computeCardinality

    public int computeCardinality()

  Determine the static cardinality of the expression

  Overrides:
    `computeCardinality` in class `UnaryExpression`
  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`

### displayExpressionName

    protected java.lang.String displayExpressionName()

  Give a string representation of the expression name for use in diagnostics

  Overrides:
    `displayExpressionName` in class `UnaryExpression`
  Returns:
    the expression name, as a string

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