JVM, Class Files, and the CLEmitter - Introduction to Compiler Construction in a Java World - page 321

D.3.2 Names and Descriptors
Class and interface names that appear in the ClassFile structure are always represented
in a fully qualified form, with identifiers making up the fully qualified name separated by
forward slashes (`/') 4 . This is the so-called internal form of class or interface names. For
example, the name of class Thread in internal form is java/lang/Thread .
The JVM expects the types of fields and methods to be specified in a certain format
called descriptors.
A field descriptor is a string representing the type of a class, instance, or local variable.
It is a series of characters generated by the grammar 5 :

FieldDescriptor ::= FieldType

ComponentType ::= FieldType

FieldType ::= BaseType j ObjectType j ArrayType

BaseType ::= B j C j D j F j I j J j S j Z

ObjectType ::= L<classname>; // class name is in internal form

ArrayType ::= [ ComponentType

The interpretation of the base types is shown in the table below:

Basic Type Character Type

B byte

C char

D double

F float

I int

J long

S short

Z boolean

For example, the table below indicates the field descriptors for various field declarations:

Field Descriptor

inti; I
Objecto; Ljava/lang/Object;
double[][][]d;[[[D
Long[][]l; [[Ljava/lang/Long;

A method descriptor is a string that represents the types of the parameters that the
method takes and the type of the value that it returns. It is a series of characters generated
by the grammar:

4 This is different from the familiar syntax of fully qualified names, where the identifiers are separated
by periods (`.').
5 This is the so-called EBNF (Extended Backus Naur Form) notation for describing the syntax of lan-
guages.