A Generic Java Language Extension
How GJ and Pizza differ
GJ is based on previous work on
but there are a number of differences
between the two.
Generally, the design requirements for Pizza and GJ are
different. Pizza extends the Java programming language with
several ideas from the functional language community, while GJ
adds just one important idea -- generic types -- and provides
a much closer fit to the Java programming language.
In addition to generic types, Pizza introduces first-class
functions and algebraic data types with pattern matching. We omitted
first-class functions from GJ because they have already been added to
Java in the form of inner classes. (Pizza was designed before inner
classes first appeared, in Java 1.1.) We omitted algebraic types from
GJ because they are only important for manipulating trees, and other
techniques like visitor patterns work almost as well.
Generics in GJ only apply to reference types. In GJ, you cannot use
a base type such as int as a type parameter, while in Pizza
base types are legal as type parameters.
The GJ compiler performs a very simple mapping into Java
bytecodes, making it easy to interface GJ programs with Java programs
and to apply reflection to GJ programs. The Pizza compiler performs a
more involved translation which makes it harder to call Pizza from
Java, or to use reflection on Pizza programs.
The GJ translation scheme makes it possible to use existing
Java library code as if it were generic.
Since the Pizza translation scheme can affect method names, these
tricks are impossible for Pizza.
The GJ design lifts some of the language restrictions in Pizza. It is now
possible to cast to a parameterized type and to create a generic
array, but both operations will generate unchecked warnings.
See the documention
for a detailed explanation.
The GJ compiler gjc is a closer match of the standard
Java compiler javac than the Pizza compiler is. In
particular, gjc can dynamically compile missing library
Comments and bug reports to the Pizza Group,
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1996, 1997 by the respective authors as attributed on each.