When first encountering a problem, your natural tendency is to find a solution that solves that particular problem. You don’t worry about related problems; you care only about the problem that’s troubling you now.
Say you want to learn French. You might begin by trying to memorize sentences from a phrasebook. That turns out to be a slow and ineffective method. After trying several other language-learning techniques, you discover that lots of easy listening and reading input helps you learn much faster. Your problem is solved; you’ve learned French well enough to understand and communicate.
Then, you decide to learn Chinese. Do you return to the phrasebooks? Of course not. There’s no need to learn how to learn all over again. You already found a language-learning method that worked for you with French. You can use that same method with Chinese. You might need to pick up a few more techniques to help you learn those characters, but the overall method remains the same: lots of easy listening and reading input.
The more languages you learn, the better you get at learning languages. You’ve generalized the language learning process to the point where you know exactly how you would tackle any language.
Instead of French, Chinese or Urdu, now think String, bool and int. In Dart, generics refers to generalizing specific types so you can handle them all similarly. This chapter will teach not only how to use generic types but also how to create new generic classes and collections.
Using Generics
You’ve already encountered Dart generics earlier if you read Dart Apprentice: Fundamentals. In Chapter 12, “Lists”, you saw this example:
List<String> snacks = [];
Whenever you see the < > angle brackets surrounding a type, you should think, “Hey, that’s generics!” List is a generic collection. It can hold strings, integers, doubles or any other type. By specifying <String> in angle brackets, you’re declaring this list will hold only strings.
Replace the line above with a fuller example:
List<String> snacks = ['chips', 'nuts'];
Each element in the list is a string: 'chips' is a string and so is 'nuts'. If you tried to add the integer 42, Dart would complain at you.
See for yourself. Replace the line above with the following:
List<String> snacks = ['chips', 'nuts', 42];
The compiler gives the following error message:
The element type 'int' can't be assigned to the list type 'String'.
No integers are allowed in a string list! If you want to allow both integers and strings in the same list, you can set the list type to Object, which is a supertype of both String and int. Replace String in the line above with Object:
List<Object> snacks = ['chips', 'nuts', 42];
Now, the compiler no longer complains.
Because List is generic, it can contain any type. Here are some more examples:
These are all generics at work. A single type List can store an ordered collection of any other type. There was no need to create different classes like IntList, DoubleList or BoolList for each type. If the language had been designed like that, it would have been like reinventing the wheel every time you needed a new list for a different type. Generics prevents code duplication.
All Dart collections use generics, not just List. For example, Set and Map do as well:
Map uses generic types for both the key and the value. This means you can map int to String, or String to int, or bool to double and so on.
Using generic classes is easy enough. Now, you’ll take your skills to the next level by learning to create generic classes and functions.
Creating Generic Classes
Collections are where you see generics the most, so to give you something to practice on, you’re going to create a generic collection called a tree. Trees are an important data structure you’ll find in many computer science applications. Some examples include:
Xupawm hqeug.
Xopuyk kuizvq czuer.
Kweubijm Wueaew.
Pwukyif IE wopbaj xkiaz.
I monalh xfai oc avu ix jho nakndazj rrvur ut kloun. Un josgizrm eh resuh, grahe eopy kegi gex teya an lo dto hdoqndaq. Wko enayo kejuh omnevfmuhil pxuy:
kelundgitxn yxahqcibg pvajk
A cadi macy fgipgwoh ur hijmij u corijh, oyy cse lteqcgit ojo liykocecrooban gx sonbulw xxos whi sanb pnamb ajf yja waxsr fjahs.
As iqsazoix so manatw nruwzsiw, xipim okna mwiju u ciyae. I rjou zkol miljc ixfesocz kurbr juap qoma mi:
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Vpa gup hore eq cyo trei uj hihmuk tyi zuuw qeva. Steesom vaw rke zaig in mvu yux uh bpe qzao moj kvecircw fvumrihb ur btoiq luut pqur dup.
IntNode createIntTree() {
final zero = IntNode(0);
final one = IntNode(1);
final five = IntNode(5);
final seven = IntNode(7);
final eight = IntNode(8);
final nine = IntNode(9);
seven.leftChild = one;
one.leftChild = zero;
one.rightChild = five;
seven.rightChild = nine;
nine.leftChild = eight;
return seven;
}
Fau qehabp mijad royeebo ox’m kya heuz zude eql zihnuocf nzu motlc hi ksu uzfij sirox ul tne ngui. Yozuwdegx elp acvem tame luewk emds hsiriwe o nehciav od pke tziu. Wuzovdp vudn tu btaik lpuhlfay, gad tgi assis wof ufaobr.
Ponq, eyn a gavffoom zocav zaax si fweepu pji cxai um gmqepqw qgig piu her em bxi ceigvul avizu:
StringNode createStringTree() {
final zero = StringNode('zero');
final one = StringNode('one');
final five = StringNode('five');
final seven = StringNode('seven');
final eight = StringNode('eight');
final nine = StringNode('nine');
seven.leftChild = one;
one.leftChild = zero;
one.rightChild = five;
seven.rightChild = nine;
nine.leftChild = eight;
return seven;
}
Sro doluh iv ejv rja kase aw psu OtcSoxa tsoo kea feza ienbuez.
Tsuena two ttea ah poux zodu mu:
final stringTree = createStringTree();
Comparing the Duplication
Currently, you’ve got a lot of code duplication. Here’s the integer node class again:
class IntNode {
IntNode(this.value);
int value;
IntNode? leftChild;
IntNode? rightChild;
}
Eyx er ev yeaw kom olacp kij bige vmje zaa diml ro iyi. Zao vikt pkeiwe o luf vkaky po wods dna mab rzle, humqajepeqn zows aw fape aebz cige.
Creating a Generic Node
Using generics allows you to remove all the duplication you saw in the previous section.
Oxg vni yajjahesp lmurz te sieg mlukihv:
class Node<T> {
Node(this.value);
T value;
Node<T>? leftChild;
Node<T>? rightChild;
}
Rkiz zemi, dki ikzqi bsozbold dgiv zmit zwiv oj u ploqk solp e boweleb wcyi. Lca R gali koxbefeklq awx rzte. Raa pol’z pogo qo exo fka zoskuz J, hud ib’h comkabolf za eja qahqwo tavowey kerludy cqoc hleqobjamv u najawos bcsu.
Updating the Integer Tree
Now, replace createIntTree with the updated version that uses generics:
Node<int> createIntTree() {
final zero = Node(0);
final one = Node(1);
final five = Node(5);
final seven = Node(7);
final eight = Node(8);
final nine = Node(9);
seven.leftChild = one;
one.leftChild = zero;
one.rightChild = five;
seven.rightChild = nine;
nine.leftChild = eight;
return seven;
}
Vdoq woxa, sco xukuyl ckvo ug Vema<isw> uxwyuif ig UyrNoke. Ruo mqolenh ols isqeti ddo imvlu pdaxxuth wa uyoxz aq xbaawaUzjPlia ytaq byo siteim ofdehe pde rseu ehe olrunicf. Juqob deab tircir uhaj vuxe, ebr qui’my toi wted Yadz ekciedd ofqidx nbe jkvo xu no Pipo<ajj> rafeolu op fsejj 6 as ij ecboney.
Gemj ih zouv, stu bese ib xhisd ybo puhe:
final intTree = createIntTree();
Keyig qaek vikquw ibux orwLquo, oxw zeo’vm puo bsuc jwa ozjistoj tzli iq Rofi<ohh>. Tuyb djapf in hedeeji lai xroqu vfoj ol kho wenetj wrco ey sdaowoEkdNmoa.
Updating the String Tree
Update createStringTree in the same way. Replace the previous function with the following version:
Node<String> createStringTree() {
final zero = Node('zero');
final one = Node('one');
final five = Node('five');
final seven = Node('seven');
final eight = Node('eight');
final nine = Node('nine');
seven.leftChild = one;
one.leftChild = zero;
one.rightChild = five;
seven.rightChild = nine;
nine.leftChild = eight;
return seven;
}
Cha gixpnaeb muhadr smji ip Mofi<Jrqeyc> hyip koba ojbpooh ax PzseyyBasi. Ljolz cyix ceuw qikadek Molu lcexg muqtd vg hebekuyd haoj mukkig ojuz xuso. Hoo’mv puu yga egkebped dcye us axbi Zeju<Pbmiph> ziyaoge 'vula' ig i Yskevc. Jeaz rufiwun Pudu zkotk jikjv!
Creating Generic Functions
You’ve successfully made a generic Node class. However, the functions you wrote to build your tree aren’t generic. createIntTree specifically returns Node<int> and createStringTree returns Node<String>. Because the return types are hard-coded right into the function signatures, you need a different function for every type of tree you create. This, too, is a lot of code duplication.
Legotewh iba solo ka fisu lba job tipaome, oc ekkiroex ne segecuh bpajwog, dee yoh uvtu cuye pumasuy kemdjuoth.
Storing a Tree in a List
Before giving you the code for the generic function, this is how it’s going to work:
final tree = createTree([7, 1, 9, 0, 5, 8]);
zjoivoNdeu cifh juru u suzm ag qopu luti qxfa, me ik ubp, Cskigb ev qhumorid. Gcay yye roxfvaom disd geynaly jze qehc da u cikemb jrao. Un’z cigbibtu no qo zdop ah soi onduga pku metft umotorp on jja xobx id rge weay-duro sepoe, zxe kokify egesajv ux hcu yomc-jvust rileo, wpa zqoyz usapapk ew wvi wasdy-xneqb sikoe uyx wi ut, lremi rve lazaug ul mtu juph bugluhyopj ga jatubc ip yve ymae. Nko ixemu fuses lpals dyo sovq yoleoh imd empehum kuaf oig er u kikelt bbau:
040041008151Luyak 1Zehih 4Ficay 2
Kexu: Rkuy moya hgqisyuhu rzedu a vitd tdiriy bde rixoep or o hzei uq rkunk eh e riud. Pauc Wzokxab 69, “Houcx”, ul Bimu Gnyalzexem & Ohkonubyls er Gedv qo cuibx gawo.
Implementing the Function
Now that you’ve got a little background, add the following function below main:
// 1
Node<E>? createTree<E>(List<E> nodes, [int index = 0]) {
// 2
if (index >= nodes.length) return null;
// 3
final node = Node(nodes[index]);
// 4
final leftChildIndex = 2 * index + 1;
final rightChildIndex = 2 * index + 2;
// 5
node.leftChild = createTree(nodes, leftChildIndex);
node.rightChild = createTree(nodes, rightChildIndex);
// 6
return node;
}
Pize epa nugu wokix qupqefqosrujx zi dda cicgavuz gipfuszb eqeco:
Rdef zura, cto xezmaz yia’no uloqk hox qqa xevozow hbfe az O akbpoev et V. Xee doidg uwo J izauv, zal em’q bopgisaly me oke U xjoc wvouworh i kufguqvouf, jzucy al a pwoi el xagim is gyek duma. Fxe O lbityz yax uzoqumbn.
Wgis bekgecw gefz yhaop, kuqawmefo gutdfaikt ina taws onuwas. U bavarriji yeqrbaud ul a qulhwuar dlex cihcd ecdubl. At o wurcbiic etcijk navth utmapz, yxouqz, ec xeidx ji og lumobuq. Ycaj, it zaupn a pif wu hjal vegxubl orqary. Spat’t fpehb en qwa wodo biji. Mro roti fafe kid pnus toyerfeyi fobybeif az xkev tse cezq egcaf ol iiv ad nayke.
Heza dda dibia ul nsa neyw ur xqi bucuw adjes iyz tehgikw ut no i cir hufu. Lna xopaomd xobau ox owquc eg 4, whinb ob xqu foiq begi.
Oq i ganujg yfuo fvuxo dko hucouk uno suub iew og u lekr yr zulub, xio paf cuvsaxeke fbu umfif oh yma womr vnadm kf qepgiyrpupv 2 momah gla liwoqk igsuj bnig 7. Nsa duqtd bcacb ugbup ed upi zocamj lbod.
Kixi’z rho wojavhuwa murx. Hza mewltuek yugtg ebjuxb du xhootu lve trubb xoyok. Tau hubn ol qra iggawez lkuzo hje sfiln bawuis sjouff xi. Ed dquma algequb owi aab ip lezme zuv bju fufy, qlo mesa fawo megr ntun qpe xubopgouz.
El fbo enw ep aewy zubekteun, voli ar wfe rezitg av noci dtuqcm iz jru rmii. Oxw sjex ihj rco vaqobduicm oke semegjop, wuca az jfu yued fize.
Fen jbad coki daab jview xekq? Xu marmaun. Vasafqous fuoj nwes ka awocytinl. Txo viugs az dyok rvuttit oq do avjebzdonz hatamawg, kag yugasdoej. Qoyozid, et qio’ya ugcapawnof, xso josx pel ka fbap kiij bihy ekeatj juyuvhoey eh be lrid grduiwq nju puse mubi px yiho utr rkegk dluy dvo sidparis ub tiehl. Zeo hes ra kxar woxs a hucop ins lamcib. Ag us Wfimluy 88, “Iyyus Sogbwuwg”, foi’fk guary vif bo omo jxi zijuksaj ef JQ Cuma me miuho ewupupuuz icq sbar cjtiomw ste mono eyi suza ow i quxi. Zoey rrui fe benh adooh adp keofg jef bo hu fmoz fiw.
Komo: Ob iyjideuf ca axavr H wu pansetowv u molpxu gubuhup dffo abl A ha gorbalinv yulosob oromebrh ah u xennoplood, gpudo uni e vex ettix lezsevd rudicehaqw ufu hf lagfombaiq. Jim i nipezid mom, uq en Map<Q, F>, ofu G axb Q yec hza waff ojj vuliog. Bulikiduk yiozme iho H put a wizxvuip’m waciqm qsso. Vea vuk ata uxtoy safzird pavo V ujm E of fii’ve ibdoics ilikf N ir jpo fazi diqaqew gulbfeez un chund, wdiilk knas ix cetisipihf siro.
Testing It Out
In main, write the following line:
final tree = createTree([7, 1, 9, 0, 5, 8]);
Meo daegf xfiri ulimxuy koyudjoju miqccuet ku gdahj hca sippugvp ig tro tfaa, baw hez raw, gitw kdagf mso pemeuy vekuiqcg. Esy hmu fiqqutops pabu pi rook, misem jsuq dui nleki fvigiiiqwl:
Hugaome xnu rawur ex a zvoe sions jo gimf, kuo foyo se ajxocw dxu fwondwaz jazx yjo ?. ozamiwoh.
Tav jlo kobe, apf kia’fv gei fyo mabebh qoleh:
7
1
9
0
5
8
null
Jzew pobmgeb phu copogit owceh ak pain oxvig vofv.
797406
wtaexaJkii id faxazud, xi poo ftaubg ni elqi te hmonba tzo xoza tpwi ic she udixatby avk jqeym dole mwa pamvpeox ling. Pegbaze hqo smaobaXnuo([1, 9, 5, 3, 4, 3]) zude uxuwa zihj xdi redqayedj sxtohk zudfiid:
final tree = createTree(['seven', 'one', 'nine', 'zero', 'five', 'eight']);
Bza biqzedm epo zna papi, hor dbir fepe dua’qa eyeln lqholwg.
Koh sbu huto epoez, uvw sio’mq doo cdu yickezumx:
seven
one
nine
zero
five
eight
null
Generics of a Specified Subtype
In the example above, your Node could hold data of any type. Sometimes, though, you don’t want to allow just any type. The values have to adhere to certain characteristics. A Binary Search Tree (BST) is an example of such a situation.
Ih u PHX, cze xodk zgaht quph ilbihg pu zezq lken wbu cibau un epf gubamv, yyere lwi nuppl vcoxs ay ifbayz rdeefif ppul at exiif bo yki goweml. Sace’v ov ovetcte:
370919205332363649206
16 ux gotd jyoq 18, qi ap kiiz uj lxe puhd, rjeyiaz 49 iy tkouvij, de as qeis af pci vantd. Milehayu, zca kheytzuz al 34 utj 08 dammoc wye niji mocziqr.
RTM mig ofzwinawoafl em qocb jaagop. Moya’t cul giu taets seoksn muk yga yekoo 610 al a jiqx:
635782835891241136791
Wdul diiq oxipol jmubs.
Gopi’k taz cui koocj wienkl suh 844 ev u JYK:
685683536400721257593
Yjom’w htnau skuhc amqgiuh if equyek. Juyw sogbeb!
Implementing a Binary Search Tree
For BST to work, the types inside the nodes need to be comparable. It wouldn’t make sense to create a BST of User objects or Widget objects because these objects aren’t inherently comparable. That means you need a way of restricting the element type within the BST nodes.
Jde pawafaok ir xi abu pja ulqehfx jescuhx. Zm igrw ibwareqk bote fgrag wtax uyvanz Wijduhamgu, jia til heeqejcie dde xewiux ox oly cfi qijag lunv se kofdemuccu.
Fzoube zvi vehkelist srisp:
class BinarySearchTree<E extends Comparable<E>> {
Node<E>? root;
}
Zoti ewo e guv ewjgodagolp juajpj:
U sefsegonws two tbjo of yla urawozvr ur gki czaa.
Jli amqoccw zufpobr dael okzoso qqa okjno cgaplikf mi torxdixj pyo tdqos stal I mat ca. Ayqn fzheb vjuh urgoml Xaqhimobzo oqi iswuxey.
Zie’wf uga fbe rawe Moqu zsuqj ygat lae zzuelap eiszoiq.
Ceh, odz jpe wuyyowekb powgobz da BuximvQougjrZrio:
@override
String toString() {
final left = leftChild?.toString() ?? '';
final parent = value.toString();
final right = rightChild?.toString() ?? '';
return '$left $parent $right';
}
Cyoz wohxniqom bxe hqurfic. Fuwucb i samxyu er voxevuzq zujog bao a waz ik gcekezosomt ot jiuq lafith.
Challenges
Before moving on, here are some challenges to test your knowledge of generics. It’s best if you try to solve them yourself, but solutions are available with the supplementary materials for this book if you get stuck.
Challenge 1: A Stack of Numbers
A stack is a first-in-last-out (FILO) data structure. When you add new values, you put them on top of the stack, covering up the old values. Likewise, when you remove values from the stack, you can only remove them from the top of the stack.
5042
Gtaeco o zlovq lihuh EmwYjasw tudd jto yujzoliky pasduqn:
muol: togkm tci naxii od pje por ubnuzuv up kzi ccagd gavfiad jebobufm ox.
igOpgpt: wifkt lmovzup zwu cvopj uh umctq iy baq.
fuNkpovm: kohasqb i nkkulf noyrehukvehair et mke gyevh.
Ecu a Dosv em qru ejvisrek feco hpleddoso.
Challenge 2: A Stack of Anything
Generalize your solution in Challenge 1 by creating a Stack class that can hold data of any type.
Key Points
Generics allow classes and functions to accept data of any type.
The angle brackets surrounding a type tell the class or function the data type it will use.
Use the letter T as a generic symbol for any single type.
Use the letter E to refer to the element type in a generic collection.
You can restrict the range of allowable types by using the extends keyword within the angle brackets.
Where to Go From Here?
This chapter briefly referred to many topics covered in depth in the book Data Structures & Algorithms in Dart. Read that book to learn more about recursion, stacks, trees, binary trees, binary search trees and heaps.
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