com.sun.msv.grammar
public abstract class Expression extends Object implements Serializable
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.
Field Summary | |
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static Expression | anyString
special expression object that represents "any string".
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static Expression | epsilon
Special expression object that represents epsilon (ε).
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static Expression | nullSet
special expression object that represents the empty set (Φ).
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Object | verifierTag
this field can be used by Verifier implementation to speed up
validation. |
Method Summary | |
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abstract boolean | equals(Object o) |
Expression | getExpandedExp(ExpressionPool pool)
Gets the expression after removing all ReferenceExps, until child
AttributeExp or ElementExp. |
int | hashCode() |
boolean | isEpsilonReducible() returns true if this expression accepts empty sequence.
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Expression | peelOccurence()
Peels the occurence expressions from this expression.
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abstract Object | visit(ExpressionVisitor visitor) |
abstract Expression | visit(ExpressionVisitorExpression visitor) |
abstract boolean | visit(ExpressionVisitorBoolean visitor) |
abstract void | visit(ExpressionVisitorVoid visitor) |
Object | visit(RELAXExpressionVisitor visitor) |
Expression | visit(RELAXExpressionVisitorExpression visitor) |
boolean | visit(RELAXExpressionVisitorBoolean visitor) |
void | visit(RELAXExpressionVisitorVoid visitor) |
This object is used as <anyString/> pattern of TREX and <text/> pattern of RELAX NG.
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.
In AGM, 'X?','X+' and 'X*' are represented by using other primitives. This method returns the 'X' part by removing occurence related expressions.