Check an expression against a required type, modifying it if necessary.
This method takes the supplied expression and checks to see whether it is
known statically to conform to the specified type. There are three possible
outcomes. If the static type of the expression is a subtype of the required
type, the method returns the expression unchanged. If the static type of
the expression is incompatible with the required type (for example, if the
supplied type is integer and the required type is string) the method throws
an exception (this results in a compile-time type error being reported). If
the static type is a supertype of the required type, then a new expression
is constructed that evaluates the original expression and checks the dynamic
type of the result; this new expression is returned as the result of the
method.
The rules applied are those for function calling in XPath, that is, the rules
that the argument of a function call must obey in relation to the signature of
the function. Some contexts require slightly different rules (for example,
operands of polymorphic operators such as "+"). In such cases this method cannot
be used.
Note that this method does not do recursive type-checking of the
sub-expressions.
- Parameters:
supplied
- The expression to be type-checkedreq
- The required type for the context in which the expression is usedbackwardsCompatible
- True if XPath 1.0 backwards compatibility mode is applicable
- Returns:
- The original expression if it is type-safe, or the expression
wrapped in a run-time type checking expression if not.
- Throws:
XPathException
- if the supplied type is statically inconsistent with the
required type (that is, if they have no common subtype)