com.sun.msv.grammar
Class Expression

java.lang.Object
  |
  +--com.sun.msv.grammar.Expression

All Implemented Interfaces:

Serializable

Direct Known Subclasses:

AttributeExp, BinaryExp, ElementExp, OtherExp, ReferenceExp, TypedStringExp, UnaryExp

public abstract class Expression

extends Object

implements Serializable

Primitive of the tree regular expression. most of the derived class is immutable (except ReferenceExp, ElementExp and OtherExp).

By making it immutable, it becomes possible to share subexpressions among expressions. This is very important for regular-expression-derivation based validation algorithm, as well as for smaller memory footprint. This sharing is automatically achieved by ExpressionPool.

ReferebceExp, ElementExp, and OtherExp are also placed in the pool, but these are not unified. Since they are not unified, application can derive classes from these expressions and mix them into AGM. This technique is heavily used to introduce schema language specific primitives into AGM. See various sub-packages of this package for examples.

The equals method must be implemented by the derived type. equals method will be used to unify the expressions. equals method can safely assume that its children are already unified (therefore == can be used to test the equality, rather than equals method).

To achieve unification, we overload the equals method so that o1.equals(o2) is true if o1 and o2 are identical. There, those two objects must return the same hash code. For this purpose, the hash code is calculated statically and cached internally.

Author:

Kohsuke KAWAGUCHI

See Also:

Serialized Form

Field Summary

static Expression	anyString special expression object that represents "any string".
static Expression	epsilon Special expression object that represents epsilon (ε).
static Expression	nullSet special expression object that represents the empty set (Φ).
Object	verifierTag this field can be used by Verifier implementation to speed up validation.

Method Summary

abstract boolean	equals(Object o)
int	hashCode()
boolean	isEpsilonReducible() returns true if this expression accepts empty sequence.
abstract Object	visit(ExpressionVisitor visitor)
abstract boolean	visit(ExpressionVisitorBoolean visitor)
abstract Expression	visit(ExpressionVisitorExpression visitor)
abstract void	visit(ExpressionVisitorVoid visitor)
Object	visit(RELAXExpressionVisitor visitor)
boolean	visit(RELAXExpressionVisitorBoolean visitor)
Expression	visit(RELAXExpressionVisitorExpression visitor)
void	visit(RELAXExpressionVisitorVoid visitor)

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, toString, wait, wait, wait

Field Detail

verifierTag

public transient Object verifierTag

this field can be used by Verifier implementation to speed up validation. Due to its nature, this field is not serialized. TODO: revisit this decision of not to serialize this field.

epsilon

public static final Expression epsilon

Special expression object that represents epsilon (ε). This expression matches to "empty". Epsilon can be thought as an empty sequence.

nullSet

public static final Expression nullSet

special expression object that represents the empty set (Φ). This expression doesn't match to anything. NullSet can be thought as an empty choice.

anyString

public static final Expression anyString

special expression object that represents "any string". It is close to xsd:string datatype, but they have different semantics in several things.

This object is used as <anyString/> pattern of TREX and <text/> pattern of RELAX NG.

Method Detail

isEpsilonReducible

public boolean isEpsilonReducible()

returns true if this expression accepts empty sequence.

If this method is called while creating Expressions, then this method may return approximated value. When this method is used while validation, this method is guaranteed to return the correct value.

visit

public abstract Object visit(ExpressionVisitor visitor)

visit

public abstract Expression visit(ExpressionVisitorExpression visitor)

visit

public abstract boolean visit(ExpressionVisitorBoolean visitor)

visit

public abstract void visit(ExpressionVisitorVoid visitor)

visit

public Object visit(RELAXExpressionVisitor visitor)

visit

public Expression visit(RELAXExpressionVisitorExpression visitor)

visit

public boolean visit(RELAXExpressionVisitorBoolean visitor)

visit

public void visit(RELAXExpressionVisitorVoid visitor)

hashCode

public final int hashCode()

Overrides:

hashCode in class Object

equals

public abstract boolean equals(Object o)

Overrides:

equals in class Object