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## net.sf.saxon.expr
Class SingletonAtomizer

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

All Implemented Interfaces:
  Container, Expression, InstructionInfoProvider, java.io.Serializable, javax.xml.transform.SourceLocator

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.ComputedExpression
`locationId, staticProperties`

Fields inherited from interface 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
`protected java.lang.String` | `displayOperator(NamePool pool)` Give a string representation of the operator 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
`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`

Methods inherited from class net.sf.saxon.expr.ComputedExpression
`adoptChildExpression, checkPermittedContents, computeDependencies, computeStaticProperties, doPromotion, dynamicError, dynamicError, effectiveBooleanValue, evaluateAsString, getCardinality, getColumnNumber, getConstructType, getDependencies, getExecutable, getImplementationMethod, getInstructionInfo, getIntrinsicDependencies, getLineNumber, getLocationId, getLocationProvider, getParentExpression, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasBadParentPointer, iterate, markTailFunctionCalls, process, resetStaticProperties, setLocationId, setParentExpression, suppressValidation, typeError, typeError`

Methods inherited from class java.lang.Object
`clone, finalize, 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

  Specified by:
    `simplify` in interface `Expression`
  Overrides:
    `simplify` in class `UnaryExpression`
  Returns:
    the simplified expression
  Throws:
    `XPathException`

### typeCheck

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

  Type-check the expression

  Specified by:
    `typeCheck` in interface `Expression`
  Overrides:
    `typeCheck` in class `UnaryExpression`
  Throws:
    `XPathException`

### computeSpecialProperties

    public int computeSpecialProperties()

  Determine the special properties of this expression

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

### evaluateItem

    public Item evaluateItem(XPathContext context)
                      throws XPathException

  Evaluate as an Item. This should only be called if the Atomizer has cardinality zero-or-one, which will only be the case if the underlying expression has cardinality zero-or-one.

  Specified by:
    `evaluateItem` in interface `Expression`
  Overrides:
    `evaluateItem` in class `ComputedExpression`
  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

  Specified by:
    `getItemType` in interface `Expression`
  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`

### displayOperator

    protected java.lang.String displayOperator(NamePool pool)

  Give a string representation of the operator for use in diagnostics

  Specified by:
    `displayOperator` in class `UnaryExpression`
  Returns:
    the operator, as a string

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