## 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:
  Serializable, SourceLocator, Container, EvaluableItem, SequenceIterable, InstructionInfoProvider

    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.Expression
`EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD`

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

Method Summary
`int` `computeCardinality()` Determine the static cardinality of the expression
`int` `computeSpecialProperties()` Determine the special properties of this expression
`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` `simplify(StaticContext env)` Simplify an expression
`Expression` `typeCheck(StaticContext env, ItemType contextItemType)` Type-check the expression

Methods inherited from class net.sf.saxon.expr.UnaryExpression
`display, equals, getBaseExpression, hashCode, iterateSubExpressions, optimize, promote, replaceSubExpression`

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

`adoptChildExpression, checkPermittedContents, computeDependencies, computeStaticProperties, doPromotion, effectiveBooleanValue, evaluateAsString, getCardinality, getColumnNumber, getContainingProcedure, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getInstructionInfo, getIntrinsicDependencies, getLineNumber, getLocationId, getLocationProvider, getParentExpression, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasBadParentPointer, iterate, markTailFunctionCalls, process, resetStaticProperties, setLocationId, setParentExpression, setParentExpression, suppressValidation`

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

Constructor Detail

### SingletonAtomizer

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

  Constructor

  Parameters:
    `sequence` - the sequence to be atomized
    `allowEmpty` - true if the result sequence is allowed to be empty.

Method Detail

### simplify

    public Expression simplify(StaticContext env)
                        throws XPathException

  Simplify an expression

  Overrides:
    `simplify` in class `UnaryExpression`
  Returns:
    the simplified expression
  Throws:
    `XPathException` - if an error is discovered during expression rewriting

### typeCheck

    public Expression typeCheck(StaticContext env,
                                ItemType contextItemType)
                         throws XPathException

  Type-check the expression

  Overrides:
    `typeCheck` in class `UnaryExpression`
  Parameters:
    `env` - the static context of the expression
    `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:
    `StaticError` - if an error is discovered during this phase (typically a type error)
    `XPathException`

### computeSpecialProperties

    public int computeSpecialProperties()

  Determine the special properties of this expression

  Overrides:
    `computeSpecialProperties` in class `UnaryExpression`
  Returns:
    `StaticProperty.NON_CREATIVE`.

### getRole

    public RoleLocator getRole()

  Get the RoleLocator (used to construct error messages)

### 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` -
  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`

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