java-primitive-array-element-type returns the element type (as a keyword as listed in Types and conversion between Lisp and Java) if object is an array with primitive element type. If object is some other type of Java object,
nil. Otherwise it signals an error.
java-primitive-array-element-typeis designed to be fast, so you can use it to test whether a Java object is an array of primitive element type.
LispWorks User Guide and Reference Manual - 13 Feb 2015