Package org.apache.commons.jxpath.ri.compiler

Class Expression

java.lang.Object

org.apache.commons.jxpath.ri.compiler.Expression

Direct Known Subclasses:

Constant, Operation, Path, VariableReference

public abstract class Expression
extends Object

Common superclass for several types of nodes in the parse tree. Provides APIs for optimization of evaluation of expressions. Specifically, an expression only needs to executed once during the evaluation of an XPath if that expression is context-independent. Expression.isContextDependent() provides that hint.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static class	Expression.PointerIterator	Pointer iterator
static class	Expression.ValueIterator	Value Iterator

Field Summary

Fields

Modifier and Type	Field	Description
protected static final Double	NOT_A_NUMBER	NaN
protected static final Double	ONE	One
protected static final Double	ZERO	Zero

Constructor Summary

Constructors

Constructor	Description
Expression()	Constructs a new instance.

Method Summary

Modifier and Type	Method	Description
abstract Object	compute(EvalContext context)	Evaluates the expression.
abstract boolean	computeContextDependent()	Implemented by subclasses and result is cached by isContextDependent()
abstract Object	computeValue(EvalContext context)	Evaluates the expression.
boolean	isContextDependent()	Returns true if this expression should be re-evaluated each time the current position in the context changes.
Iterator	iterate(EvalContext context)	Iterate over the values from the specified context.
Iterator	iteratePointers(EvalContext context)	Iterate over the pointers from the specified context.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Details

ZERO

protected static final Double ZERO

Zero

ONE

protected static final Double ONE

One

NOT_A_NUMBER

protected static final Double NOT_A_NUMBER

NaN

Constructor Details

Expression

public Expression()

Constructs a new instance.

Method Details

compute

public abstract Object compute(EvalContext context)

Evaluates the expression. If the result is a node set, returns the first element of the node set.

Parameters:

context - evaluation context

Returns:

Object

computeContextDependent

public abstract boolean computeContextDependent()

Implemented by subclasses and result is cached by isContextDependent()

Returns:

calculated context-dependentness as boolean

computeValue

public abstract Object computeValue(EvalContext context)

Evaluates the expression. If the result is a node set, returns the first element of the node set.

Parameters:

context - evaluation context

Returns:

Object

isContextDependent

public boolean isContextDependent()

Returns true if this expression should be re-evaluated each time the current position in the context changes.

Returns:

boolean

iterate

public Iterator iterate(EvalContext context)

Iterate over the values from the specified context.

Parameters:

context - evaluation context

Returns:

value Iterator

iteratePointers

public Iterator iteratePointers(EvalContext context)

Iterate over the pointers from the specified context.

Parameters:

context - evaluation context

Returns:

pointer Iterator