Generics Type parameters The definition of ArrayApplier in applier java and applier2 java allows any function from int to int But suppose you want to write a single array applier for any function from type T1 to T2 Dynamic method binding allows the type of self to be variable but there s no way to get 2 variable types Generics in Ada Essentially a structured macro generic type Item is private type ItemArray is array Integer range of Item with function f X Item return Item procedure ArrayApplier A B in out ItemArray begin for I in 1 A Last loop B I f A I end loop end Instantiating an Ada generic function incr I Integer return Integer is begin return I 1 end type IntArray is array Integer range of Integer function ApplyIncr is new ArrayApplier Integer IntArray incr Implementation Each instantiation of the generic is built by a preprocessor at compile time and compiles to a separate routine Like a macro C templates template typename T int compare const T v1 const T v2 if v1 v2 return 1 if v2 v1 return 1 return 0 int I J compare I J int compare double X Y compare X Y double compare compare I X compiler error Java generics example interface UnaryFun T S public S f T x class IntSqrt implements UnaryFun Integer Float public Float f Integer i return new Float Math sqrt i Example continued class Applier T S public void ApplyToArray UnaryFun T S Q T A S B for int i 0 i A length i B i Q f A i Example continued main Integer A new Integer 1 2 3 Integer C new Integer 3 IntSqrt ISQ new IntSqrt Applier Integer Float IFApp new Applier Integer Float IFApp ApplyToArray ISQ A C Generic types LinkedList list new LinkedList list add new Integer 1 Integer num Integer list get 0 LinkedList Integer L new LinkedList Integer L add new Integer 1 Integer num list get 0 Subtypes of generic types LinkedList Integer LI new LinkedList Integer LinkedList Object LO LI Type error LO add new String Hello Difference between extensional and abstraction view of types Therefore if a method has a parameter of type LinkedList Object you can t pass an argument of type LinkedList Integer Wildcards void printCollection Collection c for Object e c System out println e LinkedList Integer L printCollection L Restricted type parameters and wildcards T extends shape T super shape extends shape super shape void drawAll List extends shape L for shape S L draw S Type definition for max c simplified public static T extends Comparable super T T max Collection T coll Embedded Type Parametrization LinkedList LinkedList Integer static List List extends Shape history new ArrayList List extends Shape public void drawAll List extends Shape shapes history addLast shapes for Shape s shapes s draw this Implementation Type erasure No record of the parameter value at run time If L is of type LinkedList Integer all the interpreter knows at runtime is that it is of type LinkedList One subroutine for ApplyToArray not a separate routine for each different argument type as in Ada and C Possible because parameters are reference types and all the same size Bug not a feature Done for compatibility with legacy code In complex cases leads to need for casts runtime type checking type unsafe code Morals Backward compatibility is the source of much grief Forward compatibility is a myth No type theory of complex entities collections functions etc is both adequate and simple
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