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ri cs1.5 va e Ja ent in Gnen a po av Com J ey K A 432 C MS C C pts once Ge ralizing C ction C s ne olle lasse Using Ge rics with othe Java 1.5 Fe ne r ature s I nte gration of Ge rics with Pre ne vious Re ase le s Use built ge ric classe r ne s 2 What AreGe rics? ne ne r s Ge rics abstract ove Type lasse rface and Me s thods can be C s, Inte Param te d by Type e rize s ne ase adability and typesafe ty Ge rics provideincre d re 3 Java Ge ric Program ing ne m ct Java has class Obje upe ct s S rtypeof all obje type orphism " This allows "subtypepolym an ration on class T to any subclass S<: T C apply ope m s Java 1.0 1.4 do not havete plate e orphism No param tric polym r st ficie Many conside this thebigge de ncy of Java m s t at Java typesyste doe not le you che an rtypeto subtype C cast fromsupe ast cke e C is che d at run tim 4 Exam ge ric construct: S ple ne tack tacks possiblefor any typeof obje ct S For any typet, can havetypestack_of_t rations push, pop work for any type Ope ++, ne I n C would writege ric stack class template <type t> class Stack { private: t data; Stack<t> * next; public: void push (t* x) { ... } t* pop ( ) { ... } }; What can wedo in Java? 5 S pleExam Using I nte im ple rface s inte rfaceList<E> { void add(E x); I te rator<E> ite rator(); } inte rfaceIte rator<E> { E ne xt(); boole hasNe an xt(); } 6 What Ge rics AreNot ne ne m s Ge rics arenot te plate ++, ne clarations are UnlikeC ge ric de che d pile e Type cke at com tim ne pile Ge rics arecom d onceand for all ne xpose to use d r Ge ric sourcecodenot e No bloat Thetypeche cking with Java 1.5 Ge rics ne change theway oneprogram s s (as thene fe slide show) xt w s 7 Java 1.0 class S tack { void push(Obje o) { ... } ct Obje pop() { ... } ct ...} S tring s = "He llo"; S tack st = ne S w tack(); ... st.push(s); ... s = (S tring) st.pop(); vs Ge rics ne class S tack<A> { void push(A a) { ... } A pop() { ... } ...} S tring s = "He llo"; S tack<S tring> st = ne S w tack<S tring>(); st.push(s); ... s = st.pop(); 8 Java ge rics aretypeche d ne cke ne ay rations on obje of a param te cts e r A ge ric class m useope type ple ue Exam : PriorityQue <T> ... if x.le ss(y) the ... n Two possiblesolutions ++: e rations can bere solve d C Link and se if all ope ck pile ne Java: Typeche and com ge rics w/o linking quire program e to giveinform s mr ation about typeparam te e r This re ple ue xte Exam : PriorityQue <T e nds ...> 9 How to UseGe rics ne List<I nte r> xs = ne Linke ge w dList<I nte r>(); ge xs.add(ne I nte r(0)); w ge I nte r x = xs.ite ge rator.ne xt(); C parewith om List xs = ne Linke w dList(); xs.add(ne I nte r(0)); w ge I nte r x = (I nte r)xs.ite ge ge rator.ne xt(); 10 C ction C Exam olle lass ple HashMap<I ntge Double hm= r, > ne HashMap<I ntge Double (); w r, > // NoteAuto-boxing from15. hm (1,2.0); .put doublecoe = hm t(1); ff .ge 11 List Usage Without Ge rics : ne List ys = ne Linke w dList(); ys.add("ze ro"); List yss; yss = ne Linke w dList(); yss.add(ys); S tring y = (S tring) ((List)yss.ite rator().ne xt()).ite rator().ne xt(); // Evil run-tim e e rror I nte r z = (I nte r)ys.ite ge ge rator().ne xt(); 12 List Usage With Ge rics : ne List<S tring> ys = ne Linke w dList<S tring>(); ys.add("ze ro"); List<List<S tring>> yss; yss = ne Linke w dList<List<S tring>>(); yss.add(ys); S tring y = yss.ite rator().ne xt().ite rator().ne xt(); // C pile ee m be r! om -tim rror uch tte I nte r z = ys.ite ge rator().ne xt(); 13 List I m m ntation w/o Ge rics ple e ne class Linke dList im m nts List { ple e I nne C r lass prote d class Node{ cte Re e be the ? m m r se Object elt; Nodene xt; Node (Object elt){e = e ne = null;} lt ; xt } prote d Nodeh, t; cte public Linke dList() {h = ne Node w (null); t = h;} public void add(Object elt){ t.ne = ne Node lt); xt w (e t = t.ne xt; } } 14 List I m m ntation With Ge rics ple e ne class Linke dList<E >im m nts List<E> ple e prote d class Node{ cte Ee lt; Nodene xt; Node e lt = e ne = null;} } (E lt){e ; xt prote d Nodeh, t; cte public Linke dList() {h = ne Node w (null); t = h;} public void add(E e lt){ t.ne = ne Node lt); xt w (e t = t.ne xt; } // ... } 15 Re theI nte call rator I nte rface class Linke dList<E >im m nts List<E> ple e // ... public Ite rator<E> ite rator(){ re turn ne I te w rator<E>(){ prote d Nodep = h.ne cte xt; public boole hasNe an xt(){re turn p != null;} public E ne xt(){ E e= p.e lt; p = p.ne xt; re turn e ; } } } } Anonym I nne C ous r lass (se S e wing scribblee ple xam ) 16 Me thods C BeGe ric Also an ne inte rfaceFunction<A,B>{ B value arg);} (A inte rfaceNtuple <T> { <S Ntuple > m > <S ap(Function<T,S f); > } Ntuple nte r> nti = ....; <I ge nti.m (ne Function<I nte r, I nte r> { ap w ge ge I nte r value nte r i) { ge (I ge re turn ne I nte r(i.intValue w ge ()*2); } } ); 17 Exam : Ge rics and I nhe nce ple ne rite What dange lurk he ? rs re 1 conside this codew/o Ge rics r ne 18 C onside what happe (and whe r ns n) List xs = ne Linke w dList(); xs.add(ne Byte w (0)); Bytex = (Byte ListUtilitie ax(xs); ) s.m List ys = ne Linke w dList(); ys.add(ne Boole w an(false )); // Evil run-tim e e rror Boole y = (Boole ListUtilitie ax(ys); an an) s.m 19 With Ge rics WeGe C pileC ck ne t om he List<Byte xs = ne Linke > w dList<Byte >(); xs.add(ne Byte w (0)); Bytex = ListUtilitie ax(xs); s.m List<Boole ys = ne Linke an> w dList<Boole an>(); ys.add(ne Boole w an(false )); // C pile ee om -tim rror Boole y = ListUtilitie ax(ys); an s.m 20 Ge rics and I nhe nce ne rite upposeyou want to re strict thetypeparam te to e ss e r xpre S som re striction e on thetypeparam te e r s This can bedonewith a notion of subtype ubtype (we s akly construe can bee sse in Java using d) xpre d S inhe nce rite o bination to com inhe ncewith bine rite S it's a natural com ge rics ne w xam s A fe e ple follow 21 Priority Que Exam ue ple inte rfaceC parable > { boole le om <I an ssThan(I ); } class PriorityQue <T e nds C parable ue xte om <T>> { T que [ ] ; ... ue void inse t) { rt(T ... if ( t.le ssThan(que [i]) ) ... ue } T re ove { ... } m () S to bebounde aid d ... } 22 Bounde Param te d Type d e rize s xte be e The<E e nds Num r> syntax m ans that thetype param te of MathBox m bea subclass of theNum r e r ust be class e r d Wesay that thetypeparam te is bounde new MathBox<Integer>(5); //Legal new MathBox<Double>(32.1); //Legal new MathBox<String>("No good!");//Illegal 23 Bounde Param te d Type d e rize s e rize e r rve . I nsidea param te d class, thetypeparam te se s as a valid type S thefollowing is valid. o public class OuterClass<T> { private class InnerClass<E extends T> { ... } ... } Syntax note: The <A extends B> syntax is valid even if B is an interface. 24 Bounde Param te d Type d e rize s ultipleinhe ritancein theformof im m nting m ple e ultiple Java allows m inte rface so m s, ultiplebounds m bene ssary to spe a type ay ce cify param te Thefollowing syntax is use the e r. d n: <T extends A & B & C & ...> Example interface A {...} interface B {...} class MultiBounds<T extends A & B> { ... } 25 Anothe e ple... r xam inte rfaceLe ssAndEqual<I > { boole le an ssThan(I); boole e an qual(I); } class Re lations<Ce nds Le xte ssAndEqual<C e nds C{ >> xte boole gre rThan(Re an ate lations<C a) { > re turn a.le ssThan(this); } boole gre rEqual(Re an ate lations<C> a) { re turn gre rThan(a) | | e ate qual(a); } boole notEqual(Re an lations<C> a) { ... } boole le an ssEqual(Re lations<C> a) { ... } ... } 26 Generics and Subtyping t gal? Is thefollowing codesnippe le List<S tring> ls = ne ArrayList<S w tring>(); //1 List<Obje lo = ls; //2 ct> Line1 is ce rtainly le What about line2? I s a List of S gal. trings a List of Obje ct. Intuitiveanswe for m is "sure But wait! r ost !". Thefollowing code(if line2 we allowe would atte pt to assign an Obje to a re d) m ct S tring! lo.add(ne Obje w ct()); // 3 S tring s = ls.ge // 4: t(0); For all type P and C (i.eCis a subtypeof P) s S ubtype ) !=> S (P,C ubtype ne (Ge ric<P>,Ge ric<C>) ne 27 S ubclassing a ge ric class ne im java.awt.C port olor; public class S ubclass e nds MyC xte lass<C olor> { // You alm always ne d to supply a constructor ost e public S ubclass(C color) { olor supe r(color); } public static void m ain(S tring[ ] args) { S ubclass sc = ne S w ubclass(C olor.GREEN); sc.print(C olor.WHI TE); } 28 } Wildcards onside theproble of writing codethat prints out all the r m C e m nts in a colle le e ction be 1.5. fore void printC ction(C ction c) { olle olle I te rator i = c.ite rator(); for (k = 0; k < c.size k++) { S m (); yste .out.println(i.ne xt()); } } 29 1st Na veTry w/ Ge rics ne void printC ction(C ction<Obje c) { olle olle ct> for (Obje e: c) { ct S m yste .out.println(e ); } } m w rsion is m le use than theold uch ss ful Theproble is that this ne ve one . d ction as a param te e r, Theold codecould becalle with any kind of colle w s olle ct>, Thene codeonly take C ction<Obje which, as isnot a supe rtype all kinds of colle of ctions! 30 C ct way UseWildcards orre S what is thesupe o rtypeof all kinds of colle ctions? Pronounce "colle d ction of unknown" and de d C ction<?>, note olle A colle ction whosee m nt typem le e atche anything. s I t's calle a wildcard typefor obvious re d asons. void printC ction(C ction<?> c) { olle olle for (Obje e: c) { ct S m yste .out.println(e ); } } 31 Using Wildcards Again public class C nsus { e public static void addRe gistry(Map<S tring, ? extends Pe rson> re gistry) { ...} }... // Assum Drive area subtypeof Pe ing rs rson Map<S tring, Drive allDrive = ...; r> rs C nsus.addRe e gistry(allDrive rs); 32 I m m nting Ge rics ple e ne rasure Typee om -tim cking use ge rics s ne C pile etypeche om r lim s ne rasing the m C pile e inate ge rics by e om ct, rt C pileList<T> to List, T to Obje inse casts ne m s" "Ge rics arenot te plate ne clarations aretype cke che d Ge ric de ne pile Ge rics arecom d onceand for all No instantiation No "codebloat" ... 33 How Do Ge rics Affe My C ? ne ct ode y xce ! The don't e pt for theway you'll code ne ne s; Non-ge ric codecan usege ric librarie xam , xisting codewill run unchange with d For e ple e ge ric C ction library ne olle 34 Erasure rase ne e ation at Erasuree s all ge rics typeargum nt inform com pilation phase . tring> is conve d to List rte E.g. List<S tring t = stringlist.ite rator().ne is conve d to S xt() rte tring t = E.g. S (S tring) stringlist.ite rator().ne xt() pile ap ve As part of its translation, a com r will m e ry param te d typeto its typee e rize rasure . 35 Erasure tring> l1 = ne ArrayList<S w tring>(); List <S ge w ge List<I nte r> l2 = ne ArrayList<I nte r>(); yste .out.println(l1.ge lass() == tC S m l2.ge lass()); tC 36 Que stions? Wrap Up Re re s fe nce du/class/cs242/slide s/2004/java.ppt http://www.stanford.e ne m , Adding Ge rics to theJavaTM Program ing Language Gilad Bracha.Talk TS -2733Java OneC re 2001 onfe nce ne m . Gilad Bracha. Ge rics in theJava Program ing Language http://java.sun.com /1.5/pdf/ge rics-tutorial.pdf /j2se ne bstuff/Me taProgram ingJavaKalvot.pdf m http://www.cs.tut.fi/~kk/we .it/corsi/lsp1/dispe /LS nse P1-S ucci-3.ppt http://www.lips.dist.unige 38

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