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## net.sf.saxon.expr
Class TailExpression

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

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

    public class TailExpression

extends Expression

A TailExpression represents a FilterExpression of the form EXPR[position() > n] Here n is usually 2, but we allow other values

See Also:
  Serialized Form

Field Summary

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

Constructor Summary
`TailExpression(Expression base, int start)` Construct a TailExpression, representing a filter expression of the form $base[position() >= $start]

Method Summary
`int` | `computeCardinality()` Compute the static cardinality of this expression
`int` | `computeSpecialProperties()` Compute the special properties of this expression.
`Expression` | `copy()` Copy an expression.
`boolean` | `equals(Object other)` Compare two expressions to see if they are equal
`void` | `explain(ExpressionPresenter destination)` Diagnostic print of expression structure.
`Expression` | `getBaseExpression()` Get the base expression (of which this expression returns the tail part of the value)
`ItemType` | `getItemType(TypeHierarchy th)` Determine the data type of the expression, if possible.
`int` | `getStart()` Get the start offset
`int` | `hashCode()`
`SequenceIterator` | `iterate(XPathContext context)` Return an Iterator to iterate over the values of a sequence.
`Iterator<Expression>` | `iterateSubExpressions()` Get the immediate sub-expressions of this expression.
`Expression` | `optimize(ExpressionVisitor visitor, ItemType contextItemType)` Perform optimisation of an expression and its subexpressions.
`Expression` | `promote(PromotionOffer offer, Expression parent)` Offer promotion for this subexpression.
`boolean` | `replaceSubExpression(Expression original, Expression replacement)` Replace one subexpression by a replacement subexpression
`Expression` | `typeCheck(ExpressionVisitor visitor, ItemType contextItemType)` Perform type checking of an expression and its subexpressions.

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

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

Constructor Detail

### TailExpression

    public TailExpression(Expression base,
                          int start)

  Construct a TailExpression, representing a filter expression of the form $base[position() >= $start]

  Parameters:
    `base` - the expression to be filtered
    `start` - the position (1-based) of the first item to be included

Method Detail

### typeCheck

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

  Description copied from class: `Expression`
  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.

  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:
    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: `Expression`
  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.

  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:
    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

  Description copied from class: `Expression`
  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.

  Overrides:
    `promote` in class `Expression`
  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
  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

### computeSpecialProperties

    public int computeSpecialProperties()

  Description copied from class: `Expression`
  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.

  Overrides:
    `computeSpecialProperties` in class `Expression`
  Returns:
    the special properties, as a bit-significant integer

### 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

### getItemType

    public ItemType getItemType(TypeHierarchy th)

  Description copied from class: `Expression`
  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.

  Specified by:
    `getItemType` in class `Expression`
  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)

### computeCardinality

    public int computeCardinality()

  Description copied from class: `Expression`
  Compute the static cardinality of this expression

  Specified by:
    `computeCardinality` in class `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`

### iterateSubExpressions

    public Iterator<Expression> iterateSubExpressions()

  Description copied from class: `Expression`
  Get the immediate sub-expressions of this expression. Default implementation returns a zero-length array, appropriate for an expression that has no sub-expressions.

  Overrides:
    `iterateSubExpressions` in class `Expression`
  Returns:
    an iterator containing the sub-expressions of this expression

### replaceSubExpression

    public boolean replaceSubExpression(Expression original,
                                        Expression replacement)

  Replace one subexpression by a replacement subexpression

  Overrides:
    `replaceSubExpression` in class `Expression`
  Parameters:
    `original` - the original subexpression
    `replacement` - the replacement subexpression
  Returns:
    true if the original subexpression is found

### getBaseExpression

    public Expression getBaseExpression()

  Get the base expression (of which this expression returns the tail part of the value)

  Returns:
    the base expression

### getStart

    public int getStart()

  Get the start offset

  Returns:
    the one-based start offset (returns 2 if all but the first item is being selected)

### equals

    public boolean equals(Object other)

  Compare two expressions to see if they are equal

  Overrides:
    `equals` in class `Object`
  Parameters:
    `other` - the other expression
  Returns:
    true if the expressions are equivalent

### hashCode

    public int hashCode()

  Overrides:
    `hashCode` in class `Object`

### iterate

    public SequenceIterator iterate(XPathContext context)
                             throws XPathException

  Description copied from class: `Expression`
  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

### explain

    public void explain(ExpressionPresenter destination)

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

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

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