The "Wrapper" Classes: Integer, Float, Boolean, and Others

Java has a set of so-called "wrapper" classes that represent primitive types. These classes are Byte, Character, Short, Integer, Long, Float, Double, and Boolean. The wrapper classes hold single values of their respective primitive types, and are necessary to allow everything in Java to be represented as a subclass of Object. This lets various utility methods and data structures that deal with objects handle primitive types in a straightforward way. It is also necessary for Java's reflection capabilities.

If you have a Java method that returns one of these objects, it will arrive in Mathematica as an integer (for Byte, Character, Short, Integer, and Long), real number (for Float and Double), or the symbols True or False (for Boolean). Likewise, a Java method that takes one of these objects as an argument can be called from Mathematica with the appropriate raw Mathematica value. The same rules hold true for arrays of these objects, which are mapped to lists of values.

In the unlikely event that you want to defeat these automatic "pass by value" semantics, you can use the ReturnAsJavaObject and JavaObjectToExpression functions, discussed in Section 1.2.5.