Package com.sun.msv.grammar

Class Expression

java.lang.Object
com.sun.msv.grammar.Expression
All Implemented Interfaces:
Serializable
Direct Known Subclasses:
AttributeExp, BinaryExp, DataExp, ElementExp, OtherExp, ReferenceExp, UnaryExp, ValueExp
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:

  • Field Details

    • 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.

  • Constructor Details

    • Expression

      protected Expression(int hashCode)

    • Expression

      protected Expression()
      this constructor can be used for the ununified expressions. the only reason there are two parameters is to prevent unintentional use of the default constructor.

  • Method Details

    • 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.

    • calcEpsilonReducibility

      protected abstract boolean calcEpsilonReducibility()
      computes epsilon reducibility

    • getExpandedExp

      public Expression getExpandedExp(ExpressionPool pool)
      Gets the expression after removing all ReferenceExps, until child AttributeExp or ElementExp.

    • peelOccurence

      public final Expression peelOccurence()
      Peels the occurence expressions from this expression.

      In AGM, 'X?','X+' and 'X*' are represented by using other primitives. This method returns the 'X' part by removing occurence related expressions.

    • 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

    • calcHashCode

      protected abstract int calcHashCode()
      Computes the hashCode again.

      This method and the parameter to the constructor has to be the same. This method is used when the object is being read from the stream.

    • equals

      public abstract boolean equals(Object o)
      Overrides:
      equals in class Object

    • hashCode

      protected static int hashCode(Object o1, Object o2, int hashKey)

    • hashCode

      protected static int hashCode(Object o, int hashKey)

    • readResolve

      protected Object readResolve()