index.pdf March 20,

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1 ndex.pdf March 20, 20 ITI 2. Introducton to omputng II Impementaton -2-: usng an nner cass We use an nner cass to create the terators: Marce Turcotte Schoo of Informaton Technoogy and Engneerng Iterator (part II) Inner cass Impementaton: fa-fast Verson of March 20, 20 bstract These ecture notes are meant to be ooed at on a computer screen. o not prnt them uness t s necessary. n nstance of an nner cass cannot exst on ts own, t requres an nstance of the outer cass (the converse s not true); n nstance of an nner cass has access to the nstance varabes (and methods) of the (outer) obect that created t; cass Outer { cass Inner { Inner newinner() { return new Inner(); thn about nstance varabes and methods. Gettng n Touch wth your Inner ass (web resource) Inner cass Let s ca a non-statc nested cass an nner cass. n nstance of an nner cass cannot exst on ts own, t requres an nstance of the outer cass (but an nstance of the outer cass does not necessary requre an nstance of an nner cass); n nstance of an nner cass has access to the nstance varabes (and methods) of the (outer) obect that created t; cass Outer { cass Inner { Thn about nstance varabes and methods. pubc cass Outer { cass Inner { Inner() { System.out.prntn( "* new nstance of Inner cass *" ); pubc Outer() { System.out.prntn( "* new nstance of Outer cass *" ); cass Test { pubc statc vod man( Strng[] args ) { Outer o = new Outer(); ava Test smpy prnts * new nstance of Outer cass *,.e an nstance of the Outer cass can exst wthout the need to create an nstance of the nner cass (ths s not automatc). pubc cass Outer { cass Inner { Inner() { System.out.prntn( "* new nstance of Inner cass *" ); pubc Outer() { System.out.prntn( "* new nstance of Outer cass *" ); pubc statc vod doesnotwor() { Inner = new Inner(); Outer.ava:: non-statc varabe ths cannot be referenced from a statc context Inner = new Inner (); ^ n nstance of an nner cass can ony be created by an nstance of the outer cass!

2 ndex.pdf March 20, 20 2 pubc cass Outer { cass Inner { Inner() { System.out.prntn( "* new nstance of Inner cass *" ); Inner ; pubc Outer() { System.out.prntn( "* new nstance of Outer cass *" ); = new Inner(); cass Test { pubc statc vod man( Strng[] args ) { Outer o = new Outer(); > ava Test * new nstance of Outer cass * * new nstance of Inner cass * pubc cass Outer { prvate cass Inner { prvate Inner() { System.out.prntn( "* new nstance of Inner cass *" ); System.out.prntn( "vaue = " + vaue ); prvate Inner o; prvate Strng vaue; pubc Outer() { System.out.prntn( "* new nstance of Outer cass *" ); vaue = "from the nstance varabe of the outer obect"; o = new Inner(); cass Test { pubc statc vod man( Strng[] args ) { new Outer(); > ava Test * new nstance of Outer cass * * new nstance of Inner cass * vaue = from nstance varabe of the outer obect n nstance of an nner cass has access to the varabes and methods of ts outer cass! There are other subte detas about nner casses but our use w be mted to what has been descrbed n those sdes. statc nested cass needs to mae the reatonshp nner/outer cass expct! pubc cass Outer { prvate statc cass Inner { prvate Outer parent; // <-- prvate Inner( Outer parent ) { ths.parent = parent; // <-- System.out.prntn( "* new nstance of Inner cass *" ); System.out.prntn( "vaue = " + parent.vaue ); // <-- prvate Inner o; prvate Strng vaue; pubc Outer() { System.out.prntn( "* new nstance of Outer cass *" ); vaue = "from the nstance varabe of the outer obect"; o = new Inner( ths ); // <-- Inner cass: summary n nner cass s a nested cass that s not expctey or mpctey decared statc. The Java Language Specfcaton; n obect of the outer cass creates an nstance of an nner cass: n obect of an nner cass has one outer obect; n obect of the outer cass may have zero, one or severa nstances of ts nner cass; n obect of an nner cass has access to the nstance varabes and methods of ts outer obect.

3 ndex.pdf March 20, 20 3 LnedLst<Strng> = new LnedLst<Strng>();.addLast( "" );.addlast( "" );.addlast( "" );.addlast( "" ); LnedLst<Integer> 2 = new LnedLst<Integer>(); 2.addLast( new Integer( ) ); 2.addLast( new Integer( 2 ) ); 2.addLast( new Integer( 3 ) ); Iterator<Strng>, ; Iterator<Integer> ; =.terator(); =.terator(); = 2.terator(); 2 2 f (.hasnext() ) { Strng o =.next(); 2 2 whe (.hasnext() ) { Strng o =.next(); f (.hasnext() ) { Strng o =.next();

4 ndex.pdf March 20, f (.hasnext() ) { Integer o =.next(); whe (.hasnext() ) { Integer o =.next(); pubc cass LnedLst<E> mpements Lst<E> { prvate statc cass Node<E> { Node<E> ; Each obect must have a varabe (nstance), a reference to the eement that t desgnates, : pubc cass LnedLst<E> mpements Lst<E> { prvate cass LstIterator mpements Iterator<E> { // LnedLstIterator s an nner cass; LnedLstIterator mpements Iterator; LnedLst undernedoes not mpement Iterator. prvate statc cass Node<E> { prvate Node<E> ; prvate cass LstIterator mpements Iterator<E> { prvate Node ; // <--- What are the nstance varabe of a LnedLstIterator? The new mpementaton aows for severa terators to co-exst. Its constructor ntases to nu, a vaue whch, symbocay, desgnates beng before the start of the st. pubc cass LnedLst<E> mpements Lst<E> { prvate statc cass Node<E> { prvate Node<E> ; prvate cass LstIterator mpements Iterator<E> { prvate Node<E> ; prvate LstIterator() { = nu; // <---

5 ndex.pdf March 20, 20 How to create an terator? pubc cass LnedLst<E> mpements Lst<E> { prvate statc cass Node<E> { prvate Node<E> ; prvate cass LstIterator mpements Iterator<E> { prvate Node<E> ; prvate LstIterator() { = nu; pubc Iterator<E> terator() { return new LstIterator(); // <--- terator(): s an nstance method of an obect of the cass LnedLst. pubc cass LnedLst<E> mpements Lst<E> { prvate statc cass Node<E> { prvate Node<E> ; prvate cass LstIterator mpements Iterator<E> { prvate Node<E> ; prvate LstIterator() { = nu; pubc E next() { f ( == nu ) { = ; ese { =.next; return.vaue; pubc booean hasnext() { pubc Iterator<E> terator() { return new LstIterator<E>(); Usage Lst<oube> doubes = new LnedLst<oube>(); doubes.add( new oube(. ) ); doubes.add( new oube( 3.2 ) ); doube sum = 0.0; Iterator<oube> = doubes.terator(); whe (.hasnext() ) { oube v =.next(); sum = sum + v.doubevaue(); Is t fast? We ve wored hard to create a mechansm to traverse the st. What s worth t? # nodes nsde terator (ms) (ms) 0, , , , , Exercse: trace the executon of the above statements. Many terators Lst<oube> doubes = new LnedLst<oube>(); add( E o ) and remove() Let s add two methods to the nterface Iterator, add( E o ) and remove(). for ( nt c=0; c<; c++ ) { doubes.add( new oube( c ) ); Iterator<oube> = doubes.terator(); whe (.hasnext() ) { oube Va =.next(); Iterator<oube> = doubes.terator(); whe (.hasnext() ) { oube Va =.next(); System.out.prntn( "("+Va+","+Va+")" ); (0.0,0.0), (0.0,.0),..., (0.0,4.0),..., (4.0,4.0).

6 ndex.pdf March 20, 20 6 add() y defnton,.add( vaue ) w nsert the new vaue mmedatey before the next eement that woud be returned by next(). Therefore, the next ca to next() s unaffected! doubes frst doubes frst a to.add( new oube( 3.42 ) ), addng an eement at an ntermedate poston n the st, notce that.next() returns the same vaue wth or wthout the nserton! doubes frst doubes frst doubes frst doubes frst ddng an eement when the terator s postoned to the eft of the st,.add( new oube(.68 ) ), agan.next() returns the same vaue wth or wthout nserton. ddng an eement when postoned at the end of the st,.add( new oube(.68 ) ), notce that hasnext() returns the same vaue wth or wthout nserton! pubc cass LnedLst<E> mpements Lst<E> { prvate statc cass Node<E> { prvate Node<E> frst; prvate cass LstIterator mpements Iterator<E> { prvate Node<E> ; prvate LstIterator() { = nu; pubc E next() { pubc booean hasnext() { pubc booean add(e o) { f ( o == nu ) return fase; f ( == nu ) { frst = new Node<E>( o, frst ); = frst; ese {.next = new Node<E>( o,.next ); =.next; return true; pubc vod remove() { pubc Iterator<E> terator() { return new LstIterator(); // a the usua methods of LnedLst

7 ndex.pdf March 20, sebuodtsatsrftneruc What woud the foowng test program dspay? pubc cass Test { pubc statc vod man( Strng[] args ) { LnedLst<Strng> = new LnedLst<Strng>(); Iterator<Strng> =.terator(); for ( nt c=0; c<; c++ ) {.add( "eement-" + c ); =.terator(); whe (.hasnext() ) { System.out.prntn(.next() ); eement-0 eement- eement-2 eement-3 eement-4 remove() Removes from the st the eement that was returned by next(). ca.add( vaue ) mmedatey foowed by.remove() eeps the st unchanged. doubes frst doubes frst Notce that remove() does return the eement that was removed, t s ntended that a prevous ca to next() has saved the vaue. Removng an eement at an ntermedate poston,.remove(), notce that the vaue of.next() s the same wth or wthout remova. doubes frst doubes frst doubes frst Removng the eement,.remove(), the vaue of hasnext() s the same wth or wthout remova. Removng the frst eement,.remove(), the vaue of.next() remans unchanged.

8 ndex.pdf March 20, 20 8 Iterator pubc nterface Iterator<E> { pubc abstract E next(); pubc abstract booean hasnext(); pubc abstract vod add( E o ); pubc abstract vod remove(); pubc abstract booean hasprevous(); nt sze(); vod add( E o ); E get( nt pos ); E remove( nt pos ); Iterator<E> terator(); Pubc methods of LnedLst Lmtaton of the mpementaton? on modfcatons to the st! Havng many terators on the same st poses no probems as ong as ong no con modfcatons occur. onsder: Many terators and con modfcatons [.68, 2.78, 3.42, 7.389, 9.806] What woud occur f a program cas.remove()? What shoud occur of? onsder: [.68, 2.78, 3.42, 7.389, 9.806] fter the ca.remove() what shoud be the vaue of.next()? frst frst onsder what woud occur after.remove(). s now nvad!

9 ndex.pdf March 20, 20 9 Impementaton 3: fa-fast frst frst The souton that we w adopt s caed fa-fast and conssts of mang an terator nvad as soon as a modfcaton to the st has occurred that was not made by the terator tsef;.e. the modfcaton was made by another terator or an nstance method of the cass LnedLst. Impementng ths souton requres:. ddng a counter of modfcatons to the er of the st (); 2. ddng a counter of modfcatons to the terators (expectedmodount); 0 = new LnedLst(); frst addFrst( new oube( 2.0 ) );.addfrst( new oube( ) ); frst 3.deeteFrst();. When creatng a new st, the modfcaton counter s set to 0; 2. Notce that counts the number of modfcatons and not the number of eements n the st. Foowng the ca to deetefrst(), s 3 and not. frst 2.0 frst frst frst addFrst( 2.0 );.addfrst( 3.0 );.next();.next(); frst Iterator = terator(); frst next(); reatng a new Iterator does not change! Smary, traversng the st does not change. hanges to the cass LnedLst 6 The st operatons that modfy the st, addfst, deetefrst, etc. change the vaue of. must now When creatng a new st, must be set to zero. frst frst oth terators are vad.remove(); The terator whose expectedmodount has the same vaue as the one n the er of the st remans vad; a the other ones are not.

10 ndex.pdf March 20, 20 0 Modfcatons to LstIterator svad() When creatng a new terator, the vaue from the er of the st must be coped to the nstance varabe expectedmodount. The methods for addng and removng eements must update both, ther own expectedmodount and n the er of the st. To factate wrtng the methods of the nner cass LstIterator, we create a new method caed svad(). The method checs that expectedmodount ==. the terator s methods must be changed so that the precondton s vad? s true, otherwse they must throw the approprate excepton, onmodfcatonexcepton. Iterator and new for oop Lst<Integer> nts = rrays.aslst(, 2, 3, 4, ); nt s = 0; for ( Iterator<Integer> t = nts.terator(); t.hasnext(); ) { Integer = t.next(); s +=.ntvaue(); System.out.prntn( s ); Iterator and new for oop Java ntroduces a new syntax for the for oop that automagcay creates and uses an terator to traverse a st, and unboxes the eements returned by the teraton. Lst<Integer> nts = rrays.aslst(, 2, 3, 4, ); nt s = 0; for ( nt : nts ) { s += ; System.out.prntn( s ); Iterator and new for oop Lst<Integer> nts = rrays.aslst(, 2, 3, 4, ); nt s = 0; for ( nt : nts ) { s += ; System.out.prntn( s ); What type can be used wth the new for oop? ny type that has an terator. What s the Java way to enforce ths? ny cass that mpements the nterface Iterabe<E>. Iterabe<E> pubc nterface Iterabe<E> { pubc Iterator<E> terator(); where pubc nterface Iterator<E> { pubc booean hasnext(); pubc E next(); pubc vod remove();

11 ndex.pdf March 20, 20 Iterabe Iterabe pubc cass LnedLst<E> mpements Lst<E>, Iterabe<E> { prvate statc cass Node<T> { prvate Node<E> ; prvate cass LstIterator mpements Iterator<E> { pubc nterface Lst<E> extends Iterabe<E> { pubc cass LnedLst<E> mpements Lst<E> { prvate statc cass Node<T> { prvate Node<E> ; prvate cass LstIterator mpements Iterator<E> { rrays and new for oop For the sae of competeness, the new for oop can aso be apped to arrays. nt[] xs = new nt[] {, 2, 3 ; nt sum = 0; for ( nt x : xs ) { sum += x; ppendx : ccessng Members of the Outer Obect In the prevous exampe, the modfcaton counter of the nner cass (expectedmodount) was gven a dfferent name than that of the outer cass () to avod a name confct. The syntax LnedLst.ths. coud aso be used n the nner cass (LstIterator) to refer to the nstance varabe of the outer obect (LnedLst). pubc cass Outer { prvate nt vaue = 99; pubc cass Inner { prvate nt vaue; pubc Inner() { ths.vaue = Outer.ths.vaue + ; pubc nt getvaue() { return vaue; pubc Inner newinner() { return new Inner(); pubc nt getvaue() { return vaue; ccessng Members of the Outer Obect cass Test { pubc statc vod man( Strng[] args ) { Outer o = new Outer(); System.out.prntn( "o.getvaue() -> " + o.getvaue() ); Outer.Inner = o.newinner(); System.out.prntn( ".getvaue() -> " +.getvaue() ); System.out.prntn( "o.getvaue() -> " + o.getvaue() );

12 ndex.pdf March 20, 20 2 ccessng Members of the Outer Obect dvanced (optona) exampe // > ava Test // o.getvaue() -> 99 //.getvaue() -> 00 // o.getvaue() -> 99 pubc cass { prvate nt vaue = 99; pubc cass { prvate nt vaue; pubc () { ths.vaue =.ths.vaue + ; pubc cass { prvate nt vaue; pubc () { ths.vaue =.ths.vaue + ; pubc nt getvaue() { System.out.prntn( ".ths.vaue = " +.ths.vaue ); System.out.prntn( ".ths.vaue = " +.ths.vaue ); System.out.prntn( "ths.vaue = " + ths.vaue ); return vaue; cass { pubc statc vod man( Strng[] args ) {.. abc = new ().new ().new (); System.out.prntn( "abc.getvaue() -> " + abc.getvaue() ); // > ava //.ths.vaue = 99 //.ths.vaue = 00 // ths.vaue = 0 // abc.getvaue() -> 0