# Extensibility

The Saxon XQuery implementation allows you to call Java methods as external functions. The function does not need to be declared. Use a namespace declaration such as `declare namespace math=java:java.lang.Math`, and invoke the method as `math:sqrt(2)`.

More details of this mechanism are found in Writing Extension Functions; note however that for XQuery the only form of namespace URI accepted is `java:full.class.Name`.

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