1 Genericity Parameterizing by Type

2 Generic Class One that is parameterized by type  Works when feature semantics is common to a set of types On object declaration the parameter is assigned a type  For example catalogue : ARRAY [ PERSON ]  We want an array of references to persons  All the array operations for catalogue are customized to use persons

3 Common Generic Classes Collection classes – classes that are collections of objects  Strong typing requires specifying a type  But feature semantics is independent of type Examples  Sets, Stacks, Arrays, Queues, Sequences a : ARRAY [ MATRIX_ELEMENT ] a : ARRAY [ INTEGER ] a : ARRAY [ STACK [ ELEPHANTS ] ]

4 Your Generic Classes You can write generic classes Why is this useful?  reuse  reliability

5 Understanding Genericity We need  lists of INTEGER, lists of BOOK, lists of STRING etc. Without genericity we have  INTEGER_LIST, BOOK_LIST, STRING_LIST Problem — Reusability.  The basic operations (e.g. extend)are essentially the same.  Have to re-write essentially the same code over and over again.  Violation of the Single Choice Principle — changing STACK requires multiple changes, instead of a change at a single point.

6 Generic Stack class STACK [ G ] feature count : INTEGER -- number of elements empty : BOOLEAN is do... end full : BOOLEAN is do... end item : G is do... end put ( x : G ) is do... end remove is do... end end -- STACK Can use parameter G wherever a type is expected

7 Generic Array class ARRAY [ P ] creation make feature make ( minIndex, maxIndex : INTEGER ) is do... end lower, upper, count : INTEGER put ( value : P ; index : INTEGER ) is do... end infix item ( index : INTEGER ) : P is do... end end -- ARRAY

8 Using the Generic Array circus : ARRAY [ STACK [ ELEPHANTS ] ] create circus.make ( 1, 200 ) st_el : STACK [ ELEPHANTS ] -- element to put in the array create st_el circus.put ( st_el, 30 ) -- put an element into the array st_el2 : STACK [ ELEPHANTS ] st_el2 := get an element from the array

9 Types of Genericity Types  Unconstrained  Constrained The previous examples showed unconstrained genericity  Any type could be passed as a parameter

10 Constrained Genericity Used when the generic type parameters must satisfy some conditions The following makes sense only if P has the feature ≥ class VECTOR [ P ] feature... minimum ( x, y : P ) : P is do if x ≥ y then Result := y else Result := x end... end How we enforce constraints is discussed in Inheritance Techniques

11 Discussion on Genericity What programming languages offer genericity that you know of? Java? C++? Other? C++ has the template: Set s ; Java has no genericity – can it be faked? What is the effect of genericity on  compile time  size of the generated code  execution time  execution space Warning: generics cheap in Eiffel – expensive in C++

12 Java Stack public class Stack { private int count; private int capacity; private int capacityIncrement; private Object[] itemArray; // instead of ARRAY[G] Stack() {//constructor limited to class name count= 0; capacity= 10; capacityIncrement= 5; itemArray= new Object[capacity]; // no pre/postconditions/invariants } public boolean empty() { return (count == 0); }

13 Java Stack (cont.) public void push(Object x) { if(count == capacity) { capacity += capacityIncrement; Object[] tempArray= new Object[capacity]; for(int i=0; i < count; i++) tempArray[i]= itemArray[i]; itemArray= tempArray; } itemArray[count++]= x; } public Object pop() { if(count == 0) // no preconditions; // hence defensive programming return null; else return itemArray[--count]; }

14 Java Stack (cont.) public Object peek() { if(count == 0) return null; else return itemArray[count-1]; } }// end Stack; no class invariant public class Point { public int x; public int y; }

15 Java Stack runtime error class testStack { public static void main(String[] args) { Stack s = new Stack(); Point upperRight = new Point(); upperRight.x= 1280;// breaks information hiding upperRight.y= 1024;// cannot guarantee any contract s.push("red"); s.push("green"); // push some String s.push("blue"); // objects on the stack s.push(upperRight);// push a POINT object on the stack while(!s.empty()) {// cast all items from OBJECT String color = (String) s.pop(); // to STRING in order to print System.out.println(color); }}} // causes a run-time error in Java // ClassCastException: Point at testStack.main(testStack.java:18) // In Eiffel it is a compile time error

16 Does run-time vs. compile time matter? Principle: When flying a plane, run-time is too late to find out that you don’t have landing gear! Always better to catch errors at compile time! This is the main purpose of Strong Typing [OOSC2, Chapter 17]. Genericity helps to enforce Strong Typing, i.e. no run- time typing errors  LIST[INTEGER]  LIST[BOOK]  LIST[STRING]