CSC 520 – Advanced Object Oriented Programming, Fall, 2010 Thursday, September 30 Week 5, Generics and Inheritance Techniques, Meyer Ch. 10 & 16

Generic Interfaces / Classes We have been using generic interfaces and classes from the library. java.util.Set, java.util.HashSet java.util.Map, java.util.HashMap They allow containers to contain different types (classes) of objects. Generic parameters such as E (element type), K (key type) and V (value type) above are types bound by client code at compile time.

Using Generic Classes Client code supplies a binding for each generic parameter that must be an interface or class name. It cannot be a primitive type. A generic parameter may constrain the type of class or interface that client code can bind using a wildcard. ?, ? extends T, ? super T – details follow. These are Meyer’s constrained generics (16.4).

Building Generics A class or interface specifies its generic type. HashSet, HashMap, etc. ? extends T is a bounded wildcard, T or a subclass ? super T is a lower-bounded wildcard, T or a superclass ? is an unbounded wildcard, same as ? Extends Object The bound constrains client code type binding. The writer of a generic class must ensure type consistency within the class. The user can safely assume type compatibility if the compiler does not flag an error.

Erasure and Restrictions Unlike C++ template classes, the compiler generates only 1 copy of the class code for a generic class, erasing the generic type T information at run time. Every generic type T is treated as a java.lang.Object at run time. Writers of generic classes may need to cast to generic type T, E, K, V etc. in places. Writers of generic classes must ensure that those casts are valid. The compiler cannot ensure this.

Restrictions 1. No constructor – no “new T()” for generic T. 2. No “new T[]” – you must cast an “Object []” to a “T []” within the generic class code. This cast often fails at run time with a wildcard T. Make sure to test if you use arrays of wildcard generic types. 3. No generic T in a static field, static method or static initialization block of code. 4. Exception classes cannot be generic. parson/JavaLang/mapgame/Mymap.java

C++ Template classes A template class is a macro-like cookie cutter. The compiler (and possibly linker) create a distinct set of source methods in memory for every binding of the generic type (create all used methods for list, list, etc.) Cyclic cooperation between compiler and linker in the old days, went out the window with Microsoft. See STL reference.

Polymorphism Subclass polymorphism O-O inheritance allows a base interface or base class to take many forms in derived interfaces or classes Explicit parametric polymorphism Java, C++ or similar generic classes with type parameters. Implicit parametric polymorphism Python and other dynamically-typed languages allow multiple types to bind to operations and function parameters, return results, variables and fields.

Some Inheritance Issues from Meyer Ch. 16 Parents’ Invariant Rule The invariants of all the parents of a class apply to the class itself. Assertion Redeclaration Rule (1) A routine (method) redeclaration may only replace the original precondition by one equal or weaker, and the original postcondition by one equal or stronger. Universal Class Rule is Java’s java.lang.Object. Meyer’s frozen is Java final modifier, C++ const. Java and C++ provide class static methods and fields. Meyer also covers RTTI (run-time type identification) as related to guarded down casting.