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## org.apache.xpath
Class Expression

    java.lang.Object
      |
      +--org.apache.xpath.Expression

Direct Known Subclasses:
  Function, NodeTest, Operation, UnaryOperation, UnionPattern, Variable, XObject

public abstract class Expression
extends java.lang.Object
implements java.io.Serializable, ExpressionNode, XPathVisitable

This abstract class serves as the base for all expression objects. An Expression can be executed to return a `XObject`, normally has a location within a document or DOM, can send error and warning events, and normally do not hold state and are meant to be immutable once construction has completed. An exception to the immutibility rule is iterators and walkers, which must be cloned in order to be used -- the original must still be immutable.

See Also:
  Serialized Form

Constructor Summary
`Expression()`

Method Summary
`DTMIterator` | `asIterator(XPathContext xctxt, int contextNode)` Given an select expression and a context, evaluate the XPath and return the resulting iterator.
`DTMIterator` | `asIteratorRaw(XPathContext xctxt, int contextNode)` Given an select expression and a context, evaluate the XPath and return the resulting iterator, but do not clone.
`int` | `asNode(XPathContext xctxt)` Return the first node out of the nodeset, if this expression is a nodeset expression.
`void` | `assertion(boolean b, java.lang.String msg)` Tell the user of an assertion error, and probably throw an exception.
`boolean` | `bool(XPathContext xctxt)` Evaluate expression to a boolean.
`boolean` | `canTraverseOutsideSubtree()` Tell if this expression or it's subexpressions can traverse outside the current subtree.
`abstract boolean` | `deepEquals(Expression expr)` Compare this object with another object and see if they are equal, include the sub heararchy.
`void` | `error(XPathContext xctxt, java.lang.String msg, java.lang.Object[] args)` Tell the user of an error, and probably throw an exception.
`abstract XObject` | `execute(XPathContext xctxt)` Execute an expression in the XPath runtime context, and return the result of the expression.
`XObject` | `execute(XPathContext xctxt, boolean destructiveOK)` Execute an expression in the XPath runtime context, and return the result of the expression, but tell that a "safe" object doesn't have to be returned.
`XObject` | `execute(XPathContext xctxt, int currentNode)` Execute an expression in the XPath runtime context, and return the result of the expression.
`XObject` | `execute(XPathContext xctxt, int currentNode, DTM dtm, int expType)` Execute an expression in the XPath runtime context, and return the result of the expression.
`void` | `executeCharsToContentHandler(XPathContext xctxt, ContentHandler handler)` Execute an expression in the XPath runtime context, and return the result of the expression.
`void` | `exprAddChild(ExpressionNode n, int i)` This method tells the node to add its argument to the node's list of children.
`ExpressionNode` | `exprGetChild(int i)` This method returns a child node.
`int` | `exprGetNumChildren()` Return the number of children the node has.
`ExpressionNode` | `exprGetParent()`
`void` | `exprSetParent(ExpressionNode n)` This pair of methods are used to inform the node of its parent.
`abstract void` | `fixupVariables(java.util.Vector vars, int globalsSize)` This function is used to fixup variables from QNames to stack frame indexes at stylesheet build time.
`int` | `getColumnNumber()` Return the character position where the current document event ends.
`ExpressionNode` | `getExpressionOwner()` Get the first non-Expression parent of this node.
`int` | `getLineNumber()` Return the line number where the current document event ends.
`java.lang.String` | `getPublicId()` Return the public identifier for the current document event.
`java.lang.String` | `getSystemId()` Return the system identifier for the current document event.
`boolean` | `isNodesetExpr()` Tell if the expression is a nodeset expression.
`boolean` | `isStableNumber()` Tell if this expression returns a stable number that will not change during iterations within the expression.
`double` | `num(XPathContext xctxt)` Evaluate expression to a number.
`void` | `warn(XPathContext xctxt, java.lang.String msg, java.lang.Object[] args)` Warn the user of an problem.
`XMLString` | `xstr(XPathContext xctxt)` Cast result object to a string.

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

Constructor Detail

### Expression

    public Expression()

Method Detail

### canTraverseOutsideSubtree

    public boolean canTraverseOutsideSubtree()

  Tell if this expression or it's subexpressions can traverse outside the current subtree.
  Returns:
    true if traversal outside the context node's subtree can occur.

### execute

    public XObject execute(XPathContext xctxt,
                           int currentNode)
                    throws TransformerException

  Execute an expression in the XPath runtime context, and return the result of the expression.
  Parameters:
    `xctxt` - The XPath runtime context.
    `currentNode` - The currentNode.
  Returns:
    The result of the expression in the form of a `XObject`.
  Throws:
    TransformerException - if a runtime exception occurs.

### execute

    public XObject execute(XPathContext xctxt,
                           int currentNode,
                           DTM dtm,
                           int expType)
                    throws TransformerException

  Execute an expression in the XPath runtime context, and return the result of the expression.
  Parameters:
    `xctxt` - The XPath runtime context.
    `currentNode` - The currentNode.
    `dtm` - The DTM of the current node.
    `expType` - The expanded type ID of the current node.
  Returns:
    The result of the expression in the form of a `XObject`.
  Throws:
    TransformerException - if a runtime exception occurs.

### execute

    public abstract XObject execute(XPathContext xctxt)
                             throws TransformerException

  Execute an expression in the XPath runtime context, and return the result of the expression.
  Parameters:
    `xctxt` - The XPath runtime context.
  Returns:
    The result of the expression in the form of a `XObject`.
  Throws:
    TransformerException - if a runtime exception occurs.

### execute

    public XObject execute(XPathContext xctxt,
                           boolean destructiveOK)
                    throws TransformerException

  Execute an expression in the XPath runtime context, and return the result of the expression, but tell that a "safe" object doesn't have to be returned. The default implementation just calls execute(xctxt).
  Parameters:
    `xctxt` - The XPath runtime context.
    `destructiveOK` - true if a "safe" object doesn't need to be returned.
  Returns:
    The result of the expression in the form of a `XObject`.
  Throws:
    TransformerException - if a runtime exception occurs.

### num

    public double num(XPathContext xctxt)
               throws TransformerException

  Evaluate expression to a number.
  Parameters:
    `xctxt` - The XPath runtime context.
  Returns:
    The expression evaluated as a double.
  Throws:
    TransformerException -

### bool

    public boolean bool(XPathContext xctxt)
                 throws TransformerException

  Evaluate expression to a boolean.
  Parameters:
    `xctxt` - The XPath runtime context.
  Returns:
    false
  Throws:
    TransformerException -

### xstr

    public XMLString xstr(XPathContext xctxt)
                   throws TransformerException

  Cast result object to a string.
  Parameters:
    `xctxt` - The XPath runtime context.
  Returns:
    The string this wraps or the empty string if null
  Throws:
    TransformerException -

### isNodesetExpr

    public boolean isNodesetExpr()

  Tell if the expression is a nodeset expression. In other words, tell if you can execute asNode without an exception.
  Returns:
    true if the expression can be represented as a nodeset.

### asNode

    public int asNode(XPathContext xctxt)
               throws TransformerException

  Return the first node out of the nodeset, if this expression is a nodeset expression.
  Parameters:
    `xctxt` - The XPath runtime context.
  Returns:
    the first node out of the nodeset, or DTM.NULL.
  Throws:
    TransformerException -

### asIterator

    public DTMIterator asIterator(XPathContext xctxt,
                                  int contextNode)
                           throws TransformerException

  Given an select expression and a context, evaluate the XPath and return the resulting iterator.
  Parameters:
    `xctxt` - The execution context.
    `contextNode` - The node that "." expresses.
  Returns:
    A valid DTMIterator.
  Throws:
    TransformerException - thrown if the active ProblemListener decides the error condition is severe enough to halt processing.
    TransformerException -
  Usage:
    **Experimental**

### asIteratorRaw

    public DTMIterator asIteratorRaw(XPathContext xctxt,
                                     int contextNode)
                              throws TransformerException

  Given an select expression and a context, evaluate the XPath and return the resulting iterator, but do not clone.
  Parameters:
    `xctxt` - The execution context.
    `contextNode` - The node that "." expresses.
  Returns:
    A valid DTMIterator.
  Throws:
    TransformerException - thrown if the active ProblemListener decides the error condition is severe enough to halt processing.
    TransformerException -
  Usage:
    **Experimental**

### executeCharsToContentHandler

    public void executeCharsToContentHandler(XPathContext xctxt,
                                             ContentHandler handler)
                                      throws TransformerException,
                                             SAXException

  Execute an expression in the XPath runtime context, and return the result of the expression.
  Parameters:
    `xctxt` - The XPath runtime context. NEEDSDOC @param handler
  Returns:
    The result of the expression in the form of a `XObject`.
  Throws:
    TransformerException - if a runtime exception occurs.
    SAXException -

### isStableNumber

    public boolean isStableNumber()

  Tell if this expression returns a stable number that will not change during iterations within the expression. This is used to determine if a proximity position predicate can indicate that no more searching has to occur.
  Returns:
    true if the expression represents a stable number.

### fixupVariables

    public abstract void fixupVariables(java.util.Vector vars,
                                        int globalsSize)

  This function is used to fixup variables from QNames to stack frame indexes at stylesheet build time.
  Parameters:
    `vars` - List of QNames that correspond to variables. This list should be searched backwards for the first qualified name that corresponds to the variable reference qname. The position of the QName in the vector from the start of the vector will be its position in the stack frame (but variables above the globalsTop value will need to be offset to the current stack frame). NEEDSDOC @param globalsSize

### deepEquals

    public abstract boolean deepEquals(Expression expr)

  Compare this object with another object and see if they are equal, include the sub heararchy.
  Parameters:
    `expr` - Another expression object.
  Returns:
    true if this objects class and the expr object's class are the same, and the data contained within both objects are considered equal.

### warn

    public void warn(XPathContext xctxt,
                     java.lang.String msg,
                     java.lang.Object[] args)
              throws TransformerException

  Warn the user of an problem.
  Parameters:
    `xctxt` - The XPath runtime context.
    `msg` - An error msgkey that corresponds to one of the conststants found in `XPATHErrorResources`, which is a key for a format string.
    `args` - An array of arguments represented in the format string, which may be null.
  Throws:
    TransformerException - if the current ErrorListoner determines to throw an exception.
    TransformerException -

### assertion

    public void assertion(boolean b,
                          java.lang.String msg)

  Tell the user of an assertion error, and probably throw an exception.
  Parameters:
    `b` - If false, a runtime exception will be thrown.
    `msg` - The assertion message, which should be informative.
  Throws:
    java.lang.RuntimeException - if the b argument is false.
    TransformerException -

### error

    public void error(XPathContext xctxt,
                      java.lang.String msg,
                      java.lang.Object[] args)
               throws TransformerException

  Tell the user of an error, and probably throw an exception.
  Parameters:
    `xctxt` - The XPath runtime context.
    `msg` - An error msgkey that corresponds to one of the constants found in `XPATHErrorResources`, which is a key for a format string.
    `args` - An array of arguments represented in the format string, which may be null.
  Throws:
    TransformerException - if the current ErrorListoner determines to throw an exception.
    TransformerException -

### getExpressionOwner

    public ExpressionNode getExpressionOwner()

  Get the first non-Expression parent of this node.
  Returns:
    null or first ancestor that is not an Expression.

### exprSetParent

    public void exprSetParent(ExpressionNode n)

  This pair of methods are used to inform the node of its parent.
  Specified by:
    exprSetParent in interface ExpressionNode

### exprGetParent

    public ExpressionNode exprGetParent()

  Specified by:
    exprGetParent in interface ExpressionNode

### exprAddChild

    public void exprAddChild(ExpressionNode n,
                             int i)

  This method tells the node to add its argument to the node's list of children.
  Specified by:
    exprAddChild in interface ExpressionNode

### exprGetChild

    public ExpressionNode exprGetChild(int i)

  This method returns a child node. The children are numbered from zero, left to right.
  Specified by:
    exprGetChild in interface ExpressionNode

### exprGetNumChildren

    public int exprGetNumChildren()

  Return the number of children the node has.
  Specified by:
    exprGetNumChildren in interface ExpressionNode

### getPublicId

    public java.lang.String getPublicId()

  Return the public identifier for the current document event.

  The return value is the public identifier of the document entity or of the external parsed entity in which the markup that triggered the event appears.

  Returns:
    A string containing the public identifier, or null if none is available.
  See Also:
    `getSystemId()`

### getSystemId

    public java.lang.String getSystemId()

  Return the system identifier for the current document event.

  The return value is the system identifier of the document entity or of the external parsed entity in which the markup that triggered the event appears.

  If the system identifier is a URL, the parser must resolve it fully before passing it to the application.

  Returns:
    A string containing the system identifier, or null if none is available.
  See Also:
    `getPublicId()`

### getLineNumber

    public int getLineNumber()

  Return the line number where the current document event ends.

  Warning: The return value from the method is intended only as an approximation for the sake of error reporting; it is not intended to provide sufficient information to edit the character content of the original XML document.

  The return value is an approximation of the line number in the document entity or external parsed entity where the markup that triggered the event appears.

  Returns:
    The line number, or -1 if none is available.
  See Also:
    `getColumnNumber()`

### getColumnNumber

    public int getColumnNumber()

  Return the character position where the current document event ends.

  Warning: The return value from the method is intended only as an approximation for the sake of error reporting; it is not intended to provide sufficient information to edit the character content of the original XML document.

  The return value is an approximation of the column number in the document entity or external parsed entity where the markup that triggered the event appears.

  Returns:
    The column number, or -1 if none is available.
  See Also:
    `getLineNumber()`

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Copyright © 2006 Apache XML Project. All Rights Reserved.