Converting Method Arguments - General Rules
Having decided which method to call, Saxon has to convert the supplied XPath argument values to the Java objects required by this method.
The conversion is done as follows:
If there is an equivalent XPath type to the declared type of the Java method or constructor, then the supplied value is first converted to this equivalent XPath type using the standard function conversion rules used in all XPath function calls (without 1.0 backwards compatibility mode): for example an untyped attribute node can be supplied to a function that expects a string. The conversion from the equivalent XPath type to the actual Java type is then trivial, for example an
xs:stringvalue is converted to the corresponding
The equivalent XPath type is determined as follows:
If the Java class is an array of X, then the equivalent XPath type is T*, where T is the equivalent type to X. (For example
If the Java class is one of the Saxon classes
ZeroOrMore<X>, then the equivalent XPath type is the equivalent type to X, with an appropriate occurrence indicator. For example
If the Java class is a Saxon class used to implement items, such as
net.sf.saxon.om.NodeInfo, then the equivalent XPath type is the XPath item type that this Java class implements.
If the Java class is one of those listed in the table below, the equivalent XPath type is taken from this table:
Java class Equivalent XPath type
Where there is no equivalent XPath type, the following rules apply:
If the expected type is
Object, the supplied value must either be a singleton, or an empty sequence. If it is an empty sequence, null will be passed. If it is a singleton node, an instance of net.sf.saxon.om.NodeInfo will be passed. If it is a wrapped Java object, that Java object will be passed. If it is a singleton atomic value, the value will be converted to the nearest equivalent Java object: for example an
java.lang.String, and so on. An untyped atomic value is treated as a string. An
xs:integer(even if it belongs to a subtype such as
xs:short) is converted to a Java
BigInteger. The more specialized XML Schema primitive types such as
xs:durationare passed in their native Saxon representation (a subclass of net.sf.saxon.value.AtomicValue).
If the expected type is one of the Saxon-specific classes (SequenceIterator, Sequence, Item, AtomicValue, SequenceExtent), then the value is passed unchanged. An error occurs if the supplied value contains more than one item and the expected type does not allow this.
The types ZeroOrMore, OneOrMore, ZeroOrOne, and One can be used to specify the permitted cardinality, and can also be parameterized to define the permitted item type. For example, an argument defined as
One<StringValue>is equivalent to the XPath SequenceType
string, and allows a single string only. When such declarations are used, the conversion of the supplied argument to the required type follows the standard XPath conversion rules precisely.
If the expected type implements
java.util.Collection, Saxon attempts to convert each value in the supplied sequence to the most appropriate Java class, following the same rules as when converting a singleton to
java.lang.Object. This process takes no account of parameterized collection types (such as
List<String>). If the required collection type accepts an
java.util.ArrayList, Saxon will create an
ArrayListto hold the values; otherwise it will attempt to instantiate the required type of collection, which will only work if it is a concrete class with a zero-argument public constructor (so it will fail, for example, if the required type is
java.util.Set). If an empty sequence is supplied as the argument value, this is converted to an empty
If the required type is an array, Saxon will attempt to create an array of the required type. This will not always succeed, for example if the array has type
Xis an interface rather than a concrete class. If it is an array of items or nodes, the nodes in the supplied sequence will be inserted into the array directly; if it is an array of a type such as integer or double, the sequence will first be atomized.