Joe and Java Data Types

Joe supports three kinds of data type:

• Standard Types
• Opaque Types
• Registered Types

Standard Types

Joe provides a number of standard data types, as described in the Types and Values section and the Joe Standard Library section. Joe represents these internally as their normal Java types.

When a script passes a value into Java, consequently,

• A Joe List is a Java List<Object>.
• A Joe string is a Java String.
• A Joe Number is a Java Double.

This greatly decreases the amount of work a binding needs to do when working with Joe values, compared to other embedded languages I have used.

A client binding should generally try to make the best use of these types.

Opaque Types

Every Java value¹ is a valid Joe value, and can be assigned to Joe variables, passed to Joe functions, and returned by Joe functions, whether Joe knows anything special about the value's type or not.

A Java type that Joe knows nothing special about is called an opaque type. But every such type is still a Java class and a subtype of java.lang.Object, and so "nothing special" is still quite a lot.
Given such a value, Joe can:

• Determine the name of the Java class to which it belongs
• Convert it to a string for output
• Compare it for equality with other values
• Use it as a key in a hash table
• If it's a Collection<?>, iterate over its members

That might be all that's needed for many domain-specific use cases. When more is wanted, the client can register a proxy type for the type, thus turning the opaque type into a registered type.

Registered Types

A registered type is a Java type for which the client has registered a proxy type. The proxy type can give the Joe interpreter the ability to:

• Create instances of the type
• Access fields of values of the type
• Call methods on values of the type
• Customize the string representation for the type
• Define static constants and methods for the type
• Make the type iterable, so that its contents can be iterated over via the foreach statement
• Convert values of the type into Nero Fact values

Note: Most of the standard Joe types are nothing more than registered types that are registered automatically with every Joe interpreter. There's no magic; or, rather, the magic is fully available for use in client bindings.