The diverse inheritance mechanisms provided by Smalltalk, Beta, and CLOS are interpreted as different uses of a single underlying construct. Smalltalk a.nd Beta differ primarily in the direction of class hierarchy growth. These inheritance mechanisms are subsumed in a new inheritance model ba.sed on composition of mixins, or abstract subclasses. This form of inheritance can also encode a CLOS multiple-inherita.nce hierarchy, although changes to the encoded hierarchy that would violate encapsulation are difficult. Practical applica.tion of mixin-based inheritance is illustrated in a sketch of an extension to Modula-3.

Class loaders are a powerful mechanism for dynamically loading software components on the Java platform.They are unusual in supporting all of the following features:laziness, type-q? linkage, user-defined extensibility, and multiple communicating namespaces.We present the notion of class loaders and demonstrate some of their interesting uses. In addition, we discuss how to maintain type safety in the presence of user-defined dynamic class loading.

Abstract. We describe support for modularity in Newspeak, a programming language descended from Smalltalk [33] and Self [69]. Like Self, all computation -even an object's own access to its internal structureis performed by invoking methods on objects. However, like Smalltalk, Newspeak is class-based. Classes can be nested arbitrarily, as in Beta [44]. Since all names denote method invocations, all classes are virtual; in particular, superclasses are virtual, so all classes act as mixins. Unlike its predecessors, there is no static state in Newspeak, nor is there a global namespace. Modularity in Newspeak is based exclusively on class nesting. There are no separate modularity constructs such as packages. Top level classes act as module definitions, which are independent, immutable, self-contained parametric namespaces. They can be instantiated into modules which may be stateful and mutually recursive.

This paper describes wildcards, a new language construct designed to increase the flexibility of object-oriented type systems with parameterized classes. Based on the notion of use-site variance, wildcards provide a type safe abstraction over different instantiations of parameterized classes, by using '?' to denote unspecified type arguments. Thus they essentially unify the distinct families of classes often introduced by parametric polymorphism. Wildcards are implemented as part of the upcoming addition of generics to the Java TM programming language, and will thus be deployed world-wide as part of the reference implementation of the Java compiler javac available from Sun Microsystems, Inc. By providing a richer type system, wildcards allow for an improved type inference scheme for polymorphic method calls. Moreover, by means of a novel notion of wildcard capture, polymorphic methods can be used to give symbolic names to unspecified types, in a manner similar to the "open" construct known from existential types. Wildcards show up in numerous places in the Java Platform APIs of the upcoming release, and some of the examples in this paper are taken from these APIs.

This paper describes wildcards, a new language construct designed to increase the flexibility of object-oriented type systems with parameterized classes. Based on the notion of use-site variance, wildcards provide type safe abstraction over different instantiations of parameterized classes, by using '?' to denote unspecified type arguments. Thus they essentially unify the distinct families of classes that parametric polymorphism introduces. Wildcards are implemented as part of the addition of generics to the Java TM programming language, and is thus deployed world-wide as part of the reference implementation of the Java compiler javac available from Sun Microsystems, Inc. By providing a richer type system, wildcards allow for an improved type inference scheme for polymorphic method calls. Moreover, by means of a novel notion of wildcard capture, polymorphic methods can be used to give symbolic names to unspecified types, in a manner similar to the "open" construct known from existential types. Wildcards show up in numerous places in the Java Platform APIs of the newest release, and some of the examples in this paper are taken from these APIs.