Hng i tng trong Java
Gii thiu
Lp (Class)c im hng i tng trong Java
Lp tru tng & Giao din
Gii thiu
Lp trnh hng i tng (Object Oriented Programming - OOP): hng tip
cn mnh m v hiu qu C bn v OOP trong Java: nh ngha mt lp (Class) To
mt i tng (Object) S khi to (Construction) & hy b (Destruction)
Tnh tha k (Inheritance) Tnh a hnh (Polymorphism) Lp tru tng
(Abstract) v giao din (Interface)
Hng i tng l g?
L cch t chc m ca cc chng trnh theo mt cch tt hn (hy vng th!) D
liu c s dng bi mt chng trnh l cc i tng (objects) Cc i tng l mt tp
hp cc bin v phng thcPhng thc thay i gi tr ca cc bin i tng
Phng thc tr v gi tr ca cc bin i tng
Lp (Class)
Lp c th c xem l khun mu ca ca i tng, bao gm: D liu ca i tng
(field hay property) Cc phng thc (method) tc ng ln thnh phn d
liu
Cc i tng c xy dng bi cc lp nn c gi l cc th hin ca lp (class
instance) OOP l chin lc pht trin nhng chng trnh ln hn vi chi ph thp
hn v t li bng cch xy dng da trn nhng thnh phn c sn
Mt lp m ta bit: lp ConsoleTo cc i tng qua cc phng thc khi to khc
nhau:Console c = new Console(); Console c = new Console(30,15);
Console c = new Console(Holla);
Cc phng thc tr li mt gi tr:int a = c.readInt(); double d =
c.readDouble(); String s = c.readLine();
Cc phng thc khng tr li gi tr (thay i i tng):c.println()
c.setSize() c.setLocation() Cons.java
V d v mt lp i tng mi: lp Clock
Cc bin l v tr ca cc kim Hai phng thc l cc th tc t v ly gi tr hin
thi ca kim ng h
t gi tr cho kim ng h
By gi l my gi?
Lp v i tng Mi i tng thuc mt lp no Lp nh ngha kiu/loi ca i tng,
ngha l: Mt i tng ca lp ny cha nhng bin g? Nhng phng thc no c th p
dng cho mt i tng ca lp ny Object A of class clock
Object B of class clock Class Clock Object C of class clock
nh ngha mt Class nh th no?
Vic nh ngha mt lp bao gm nhng vic c bn: nh ngha nhng bin
(variables) m n s cha nh ngha cc phng thc khi to (c th khng) nh
ngha cc phng thc m users c th p dng ln mt i tng ca lp ny
S nh ngha ca mt lp nn c t trong mt file c tn Tnlp.java Theo quy
c, tn lp c bt u bng mt k t hoa (Console, Clock,) Sau khi bin dch
file , chng ta c th dng lp ging nh cch dng lp Console
Khai bo/nh ngha lpclass { //khai bo nhng thuc tnh ca lp field1;
field2; // //Cc phng thc khi to - Constructors ([danh sach tham
s]){ //khi to cho i tng lp da trn [dsts] } //Cc phng thc lp ([danh
sch tham s]) { ; } } To i tng ca lp: ClassName objectName = new
ClassName([danh sch i s]);
Vit lp Clock theo yu cu: Mt i tng clock s c 3 bin (thuc tnh,
trng)int hours int minutes int seconds
Mi khi mt i tng clock mi c to, gi tr ca cc bin trn s l 0 (gi bt
u t na m) Phng thc setTime() s thit lp mt gi mi. N yu cu cc gi tr
mi cho hours, minutes, v seconds C 3 phng thc tr li gi tr hin thi
ca mi thnh phn (v d: getHour() ) Phng thc getTime() tr li mt xu
(String) vi gi c trnh by theo khun dng hh:mm:ss
Vit file Clock.java, bin dch v s dng nTheo quy c, tn phng thc bt
u bng k t thngClock a = new Clock( ); a.setTime(15,30,20); int h =
a.getHours( ); int m = a.getMinutes(); int s = a.getSeconds();
String s = a.getTime();
15 hours 30 minutes 20 seconds
Clock.java
TestClock1.java
TestClock2.java
Lu : Clock khng c phng thc main nn n khng th chy (run). N ch l
mt s nh ngha c s dng khai bo v to cc i tng Clock. Phi vit cc chng
trnh s dng n!
Bi tp
Hon thnh vic nh ngha lp Clock vi cc phng thc sau: public int
totalSeconds(); /*tr v tng s giy tnh t thi im bt u ngy mi
(00:00:00) cho n thi gian hin ti (c lu gi)*/ public void
addSeconds(int s); /*thm vo thi gian hin ti s giy v pha trc*/ Kim
tra cc phng thc m bn vit vi chng trnh TestClock.java
Bi tp (tt.)
Xy dng mt lp mi ClockS, trong ch s dng mt bin seconds (thay v ba
bin hours, minutes, v seconds nh lp Clock) lu tng s giy tri qua tnh
t lc bt u mt ngy mi (00:00:00). Vit li tt c cc phng thc nh lp Clock
( cch ci t no tt hn?!) Kim tra li gii ca bn vi chng trnh
TestClockS.java.
S dng cc lp do ngi dng nh ngha (user-defined classes)Bin dch
file nh ngha lp File .class nn c t cng th mc file cha chng
trnh.
Java MyClass MyClass main(... new AnotherClass()
AnotherClass
H s sinh vin
Cc bin l d liu ca sinh vin: tn (name), a ch (address), nm
sinh(year born,...) Cc phng thc a dl vo v ly dl raSet data Name:
Sakano Tepe Address: Musashino-shi.
Year Born: 1977Student.java TestStudent.java
retrieve data
c mt dng k t t bn phmimport java.io.*; public class ReadALine {
public static void main(String args[ ]) throws IOException {
BufferedReader inKbd = new BufferedReader( new
InputStreamReader(System.in)); String inputLine;
System.out.print(Please enter your name: ); inputLine =
inKbd.readLine(); System.out.println(Hajimemashite,+ inputLine); }
} N ch cho php c mt dng cc k t.
Cn phi vit throws IOException ti phn bt u ca phng thc main. Nu
khng th n s khng bin dch thnh cng s dng cc lp lin quan n
input/output, ta phi import chng t th vin java.io.* ReadALine
c mt dng k t t bn phm (2)import java.io.*; public class
ReadALine2 { public static void main(String args[ ]) throws
IOException { DataInputStream stream = new DataInputStream
(System.in); System.out.print(Please enter your name: ); String
inputLine = stream.readLine(); System.out.println(Hajimemashite,+
inputLine); } }
Cn phi vit throws IOException ti phn bt u ca phng thc main. Nu
khng th n s khng bin dch thnh cng s dng cc lp lin quan n
input/output, ta phi import chng t th vin java.io.* Ta s c bi ni k
hn v Input/Output trong Java! ReadALine2
c mt s t bn phmimport java.io.*; public class Statistic { public
static void main(String args[ ]) throws IOException {
BufferedReader inKbd = new BufferedReader( new
InputStreamReader(System.in)); String inputLine; System.out.print(I
am Tricker, enter your number ! ); inputLine = inKbd.readLine();
int yourNumber = Integer.parseInt( inputLine );
System.out.println(My number is + (yourNumber +1 )+ I win !); }
ReadANumber Statistic }
Trong Java cn c phng thc nh Double.parseDouble(aString) Nu xu c
nhp khng tng ng vi mt s th mt li s xut hin trong qu trnh thc hin
chng trnh
The Phone BookName NumberSet(name,number)
Matsumoto 11122233 Vera Wakahara ..... 3456789 2345678
....Retrieve(name)
The Phone Book: Ci t 1String[] names int[] numbers
Jun Kenichiro Kamugishaint next
0 1 2 3 4 5 6
45678 12345 55443Maximum
Phonebook
TestPhonebook
Bi tp Vit cc phng thc sau cho lp Phonebook: public int
changeNumber(String name, int number); /* thay s mi cho mc (entry)
c tn name v tr v gi tr 0; tr v -1 nu khng tm thy entry no c tn name
trong danh b */ public int deleteEntry(String name); /* Tm v xa
entry c tn name v tr v gi tr 0; tr v gi tr -1 nu khng tm thy entry
no c tn name */ Thm m lnh x l cho trng hp user chn 3 v 4 ca menu cc
la chn trong TestPhonebook.java: case 3: System.out.print("Enter
name :"); String name = inKbd.readLine(); System.out.print("enter
the new number: "); String snumber = inKbd.readLine(); int nnumber
= Integer.parseInt( snumber ); if( b.changeNumber(name,nnumber) ==
0) System.out.println("Number changed"); else
System.out.println("Person "+name+" not found"); break; case 4:
//tng tt lm!
The Phone Book: Ci t 2 (Mt vector ca cc i tng)Vector entries Jun
123456 Sakano 223344 Juinichi 889997
Mi phn t ca Vector l mt i tng ca lp Entry Mt i tng ca lp Entry c
mt String (name) v mt int (lu s in thoi - number)
Lp Vectorc nh ngha bn trong gi java.util, c ngha ln phi c nhp
khu N c th cha cc i tng ca bt c lp no Mi i tng c v tr ca n v chng c
th c gi bng cch ch ra v tr ca n. Cc phng thc quan trng nht:
add(object) add(i,object) elementAt(i) remove(i) clear()
thm mt object ti v tr cui cng ca mt vector thm mt object ti v tr
c ch nh tr v phn t ti v tr th i (khng ly n ra khi vector) loi b phn
t th i trong vector xa tt c cc phn t
Phonebook2
Bi tp
B sung hai phng thc sau cho lp Phonebook2 vi yu cu tng t nh bi
tp trc: public int changeNumber(String name, int number); public
int deleteEntry(String name);
Vit chng trnh test lp Phonebook2 (tng t nh
TestPhonebook.java).
Lp Hashtablec nh ngha trong gi java.util, do n phic nhp khu N
cha cc cp i tng ca bt k lp no Mt i tng c s dng nh l ch mc v i tng
cn li c dng nh l thng tin. Hash.java Cc phng thc quan trng nht:thm
mt cp i tng (object1 l ch mc, object2 l thng tin get(object1) tr li
i tng m object1 l ch mc remove(object1) xa i tng m object 1 l ch mc
put(object1, object2)
Bi tp: vit li lp TestPhonebook nhng dng lp Hashtable thay cho lp
Phonebook
Khai bo bin trong nh ngha lp C hai loi bin (variables) khc nhau
khi nh ngha mt lp m ta cn phi phn bit:Bin th hin (instance
variables) Bin lp (class variables)
Bin th hin: mi th hin ca lp s c bn sao cc bin th hin ca ring n
Bin lp: mt lp s ch c mt bn sao ca bin lp, chng s c chia s bi tt c
cc i tng ca lp Mt bin lp phi c khai bo vi tin t static pha trc S
dng bin lp lu gi tr hng hoc lu vt gi tr d liu chung cho tt c cc i
tng ca lp
Vit cc phng thc nh th no?Kiu ca gi tr m phng thc s tr v (void nu
khng tr v gi tr no) Tn ca phng thc (do ngi lp trnh t)
public int getHours( ) { public, private, return hours;
protected or static }
Cc cu lnh c t trong cp Danh sch tham s ca phng du ngoc nhn {}
thc (c th c hoc khng)
Phng thc khi to (Constructor)
Constructor l mt phng thc c bit Constructor thng c dng thc hin
vic khi to, vd: khi to cho cc bin th hin ca cc i tng Constructor c
tn trng vi lp c nh ngha, c th c tham s hoc khng Constructor khng c
kiu tr v - ngay c void Constructor c triu gi bng cch dng ton t new
khi i tng c to
Vit cc Constructor nh th no?Tn ca phng thc khi to (ptkt) phi l
tn ca lp Danh sch tham s m ptkt s nhn to i tng mi
Clock2 ( int h, int m, int s, int D, int M ) { ... }Khng cn phi
vit g y
Chng ta c th vit nhiu phng thc khi to Clock2 ( ) { ... } PTKT ny
t cc gi tr mc nh cho i tng mi
Bin thispublic class MyClass { var a, b, c; method1(a,b,c) {
this.a = a; this.b = b; this.c = c; } public static void main(....)
{ MyClass mc = new MyClass() mc.method1();
} - Bin this l mt bin n tn ti trong tt c cc lp trong Java. -
this l mt con tr c to ra cng lc vi i tng, this ch trc tip vo i tng
sinh ra cng lc vi mnh v c dng truy xut cc bin v phng thc trong i
tng
}Nhiu khi this c dng phn bit gia cc bin i tng v bin cc b
Bin i tng l mt con trTa c hai con tr cha tr n u c. Console c, b;
Chng c gi tr null v s gy ra li nu thc hin mt phng thc trn chng By
gi ta c hai con tr tr n cng mt i tng. Nu chng ta thc hin mt phng
thc s dng bt k con tr no th n s c thc hin trn cng mt i tng
c = new Console(); b = c;
c b
Lp Vektor (t to)- To mt Vektor - Tr v di ca mt Vektor - Tr v gi
tr honh , tung - Cng hai Vektors - Tr hai Vektors - Lu v bin i tng
trong TestVektor.java
y
x
Vektor.java
TestVektor.java
Tham bin, tham trpublic class PassingAreferenceValue{ public
static void main(String[] args){ Holder h = new Holder(); h.held =
100; h.bump(h); System.out.println(h.held); } } class Holder{
public int held; public void bump(Holder theHolder){
theHolder.held++; } } public class PassingAprimitiveType{ public
static void main(String[] args){ double d=12.3; Decrementer dec =
new Decrementer(); dec.decrement(d); System.out.println(d); } }
class Decrementer{ public void decrement(double decMe){ decMe =
decMe - 1; } }
Lu v Methods v Constructors Phng thc trong Java l khi lnh thc
hin cc chc nng, cc hnh vi x l ca lp ln vng d liu. Trong mt lp, cc
phng thc thng c khai bo dng t kha public, cc vng d liu thng l dng
tin t private v mc ch an ton (more later!). Nhng bin nm trong mt
phng thc ca lp l cc bin cc b (local) v s c khi to khi phng thc c
triu gi. Thng thng an ton, d kim sot v lm ch m ngun chng trnh, chng
ta nn khai bo mt constructor cho lp. Nu mt lp khng c constructor th
Java s cung cp cho lp mt constructor mc nh: nhng thuc tnh, bin ca
lp s c khi to vi cc gi tr mc nh.
Trong Java, vic khai bo nhiu phng thc c cng tn nhng khc tham s
(khc kiu d liu, khc s lng tham s) trong cng mt lp gi l khai bo chng
phng thc. Tn phng thc, s lng v th loi ca cc tham s to nn ch k
(signature) ca phng thc. Hai phng thc trng lp ch thng c ng dng khi
c hai cng lm chung mt s vic tng t nhau, v d:public class Xemay{
//khai bo fields... public float tinhgiaban(){ return 2*chiphisx; }
public float tinhgiaban(float huehong) { return (2 * chiphisx +
huehong); } }
Trng lp cc phng thc (overloading method)
Ni chui cc constructors Cc constructors khc nhau trong cng mt lp
c th to chui vi nhau bng cch constructor ny gi constructor kia. Qua
constructor c gi c dng nh l mt phng thc bnh thng vi con tr ni ti
this. u im khi dng chui cc constructors l c th s dng li m ngun. Cc
constructor phc tp s gi cc constructors n gin hn, thay v bn thn phi
xy dng li t u n cui. Xem v d ConstructorChaining.java! Khi mt
constructor c gi trong mt constructor khc trong cng mt lp, th
constructor phi c gi ngay t dng lnh u tin. Nu khng, trnh dch s bo
li!. Mt th loi ng dng khc ca chui constructors l lc to ra cc lp dn
xut (derived class) trong tha k.
Constructors tnh (static constructors) Dng constructors tnh gn
cc gi tr ban u phc tp khi khi ng mt lp. Cc khi to tnh (static
initializers) c thc thi sau khi lp c ti vo b nh, nhng trc khi n c s
dng thc s. Constructors tnh c nh ngha nh l mt phng thc khng c tham
s v c tn l static. V d:public class Test{ static int i; static int
j; static { i = 3; j = 5 + i; } }
Java cho php nh ngha nhiu constructors tnh trong mt lp. Chng s c
gi theo th t nh c lp trnh trong m ngun. Xem StaticStuff.java!
Destructors Bn cnh constructor, cn c destructor. Constructor dng
khi ng i tng, cn destructor c gi trc khi i tng b trit tiu.
Destructor c nh ngha nh l mt phng thc khng tham s v c bo v
(protected) c tn l finalize:public class Test{ //khai bo cc thuc
tnh public void method_1(){ // } protected void finalize(){ //C th
ng tt c cc kt ni vo CSDL trc khi hy i tng } }
V Java c trang b trnh tin ch thu rc (garbage collector) nn vai
tr ca destructor khng quan trng nh trong cc ngn ng lp trnh khc.
Gi (package) Package cho php nhm mt tp hp cc lp hoc cc giao tip
(interface) c quan h vi nhau d dng qun l, bo tr, phn phi Gi
(package) c ch cho cc mc ch sau: T chc cc lp thnh cc n v nh hn (nh
l cc th mc) -> xc nh v tr v s dng tp tin ca lp tr nn d dng, chnh
xc Trnh cho vic t tn b trng lp Bo v cc lp, d liu v phng thc mc rng
hn trn mt nn tng class-to-class S dng tn gi nhn dng lp
V d: To package transport cha cc lp v phng tin i li: Car, Moto,
Boat... t cc lp vo cng th mc transport Khai bo trc/u mi lp dng
package transport;
Gi (tt.) S dng package cch 1: Khai bo import V d 1: import
transport.Car; V d 2: import transport.*; V d 3: import
java.awt.Point; Point p = new Point(1,2);
S dng package cch 2: S dng trc tip java.awt.Point p = new
java.awt.Point(1,2);
javax.swing.JOptionPane.showMessageDialog(...);
Ch : Ch c th truy cp c n cc lp public trong cc package Package
java.lang c t ng import vo mi chng trnh Java
c im hng i tng trong JavaTnh ng gi (encapsulation): i tng l mt
thnh t h tr tnh ng gi; n v ng gi c bn ca Java l class; Mt lp bao gm
cc thnh phn: d liu v x l
Tnh tha k (inheritance): c tnh ca tha k l c th sao chp cc c trng
(d liu, phng thc) ca mt lp sang lp khc Ngn ng Java ch chp nhn tha k
n Java dng phng php giao din (interface) thc hin chc nng ca a tha
k.
Tnh a hnh (polymorphism): Ci t cc lp dn xut khc nhau t mt lp
ngun. Mt bin n thuc mt lp no (lp trn/c s) c th c dng tham chiu n cc
i tng ca cc lp khc nhau (cc lp dn xut); c kh nng t ng gi cc phng
thc c th ca loi i tng m n ang tham chiu n. Tnh a hnh ch p dng cho
phng thc, khng p dng cho thnh phn d liu!
Tha k: m rng t mt lp c sn
Mt trong nhng c trng mnh nht ca OOP l kh nng m rng mt lp c sn,
ngay c khi ta khng bit c s ci t (implementation) ca lp gc. iu gi l
s tha k.Class1 Lp gc Lp trn (super-class) ca lp m rng
Class2
Mt i tng Lp m rng ca Class2 cng nh ngha cc bin v phng thc mi l
ca Class1 Lp gc
Tnh tha k (Inheritance) Tha k l vic xy dng lp mi da trn lp c sn
Lp c sn l lp cha: superclass Lp mi l lp con: subclass
V d: Hnh vung k tha t hnh ch nht Con ch k tha t loi ng vt di
nc
Ch : Tnh tha k th hin quan h is a, khc vi quan h has a
(composition) Composition: mt i tng cha cc i tng thuc lp khc. V d:
t c cc bnh xe
M rng lp Clock Chng ta s b sung nhiu thng tin hn vo lp Clock:
Thm hai bin day v month; Thm cc phng thc tng ng thit lp v tr v gi
tr ca cc bin ny; Nu mun gi nguyn lp Clock v vit mt lp khc (Clock2),
ta c th copy tt c m ngun t lp Clock vo trong mt file mi hay ta c th
m rng t lp c sn (Clock) public class Clock2 extends Clock { khai bo
cc bin khc (day v month) khai bo cc phng thc khc } Clock2.java
TestClock2.java
Da vo TestClock2.java, vit chng trnh s dng lp Clock2!
M rng lp ConsoleNgay c khi chng ta ch bit c cc files .class v
chng ta khng bit c mt lp c ci t (lp trnh) nh th no th chng ta vn c
th s dng n nh l mt lp c s cho cc lp khc (m rng). V d, chng ta c th
m rng lp Console bao gm thm cc chc nng nh nhp vo mt dy s nguyn hay
nhp vo mt sinh vin public class Console2 extends Console { public
int [] readIntArray(int n){ // ci t phng thc } } Console2.java
TestConsole2.java
Lp Object Tt c cc lp c mt lp chun,Object, nh l lp c s Tt c cc lp
trong Java c m rng t lp ny Ngay c khi ta khng ghi extends Object,
bt k lp no ta nh ngha u l mt lp con (subclass) ca lp Object iu cng
c ngha l bt k i tng no trong Java u xut pht t lp ObjectObject Clock
String Clock2 Others Console Console2
M rng lp Student
M rng lp Student bao gm im trung bnh(grade) m mt sinh vin c c
Dng mt dy c kch c bin ng lu im tng mn hc. Nhng dy ny c gi l Vectors
trong Java Vectors khng thuc phn lica cc lp trong Java. Ngha l lp
ny phi c import t mt gi khc gi l java.utility Cu lnh lm vic l
import java.util.Vector; hay import java.util.*; Mt s phng thc ca
lp ny c s dng: void add(Object) WasedaStudent.java Object
elementAt() TestWasedaStudent.java int size()
public, protected, default & privatepublic: c th truy cp bt
c u. protected: c th truy cp bi cc lp trong cng gi vi lp , v ch c
th truy cp bi cc lp con (subclasses) ca lp trong cc gi khc. default
(khng c tin t): ch c th truy cp bi cc lp, bao gm cc lp con, trong
cng mt gi vi lp (package accessibility). private: ch c th truy cp
trong bn thn lp v khng ni no khc. Xem cc v d trong th mc
java-access-modifiers
Minh ha quyn truy cp
Lu : nu mt lp khng c khai bo l public th n ch c th c truy cp bi
cc lp trong cng gi (package).Trong mt file *.java ch c mt lp c khai
bo l public!
Minh ha quyn truy cp (tt)
B t static
T kho static c dng vi phng thc v bin. Bin static: l bin chung
cho mi i tng ca lp, n c truy cp qua i tng ca lp hoc qua tn lp.
private static char TAB = \t; Phng thc static: l phng thc ch c php
truy cp ti cc bin static ca lp, n c th gi ngay c khi cha c i tng no
ca lp. public static void welcome() {...}
Bin tnh (Static Variables) Mt bin static ngha l ch c mt bin cho
c lp (ngc li vi bin ng, mi i tng c mt th hin ring ca n) V d: chng
ta mun m xem bao nhiu sv m chng trnh to:public class WasedaStudent2
extends WasedaStudent { public static int nstudents = 0;
WasedaStudent2(String x, String y, int z) { super(x,y,z); nstudents
= nstudents +1; } }WasedaStudent2.java
Static VariablesWasedaStudent2.class
nstudents ws21marks name etc
marks name etc
ws22marks name etc
ws23marks name etc
Phng thc tnh (static methods)Mt phng thc static hot ng c lp vi
cc i tng cng lp; n c p dng cho lp.Chng trnh s dng n c vit: int i =
WasedaStudent2.nSt(); - Lp Math ca ngn ng Java c nhiu phng thc tnh
c ci t: Math.sqrt(x), Math.sin(x), Math.max(x,y), Math.pow(x,y) -
Phng thc lp khng c con tr this nh cc phng thc khc. - Trnh dch s bo
li ngay khi mt phng thc lp truy xut mt bin th hin. Mt cch khc l
khai bo nstudents vi tin t public v chng trnh no cng c th gi n (
slide trc): WasedaStudent2.nstudents
Nhng bt c chng trnh no cng c th thay i gi tr ca n !
Minh ha static methodsPhng thc tnh cn c p dng cho lp
nstudentsnStudents()
Phng thc ng c p dng cho i tng
putNewMark()
showAverage()
marks name etc
B t final
T kho final p dng cho lp, phng thc, bin. Lp final: lp vi thuc
tnh final khng c tha k public final class NoChild {...} Bin final:
l bin khng th thay i khi gn gi tr, chng l cc hng s private static
final int MAX = 100; Phng thc final: l phng thc khng th b ghi
(overridden) cc lp con ca n public final void noOverridden
();TestOverridingFinal.java Parent.java
Th t khi ng khi to mt i tngKhi to mt i tng ca subclass: Cc
constructors ca cc lp cha/trn (superclass) c gi (r rng hoc ngm nh);
Cc cu lnh trong phn thn trong constructor ca superclass c thc hin
sau khi khi to cho cc bin th hin ca n -nu c-(cng l mt phn ca i tng
thuc subclass); Cui cng, cc lnh trong constructor ca subclass c thc
hin sau khi khi to cho cc bin th hin ca n (nu c). T kho super ch lp
cha ca lp m n xut hin Java m bo rng ngay c khi mt constructor khng
khi gn gi tr cho bin th hin th n vn nhn c gi tr mc nh. Bin dch v
thc thi cc v d: Init1.java v InitialOrder.java
S dng t kha super
gi phng thc khi to (constructor) ca lp cha (superclass):
super(); hoc super(parameters); ch xut hin dng u tin constructor ca
lp con (subclass) ni chui cc constructor (xem Faculty.java) Lu : nu
mt lp c vit c th m rng v sau th nn c mt constructor khng tham s
trnh li, xem Apple.java
gi mt phng thc (method) ca lp cha: super.method(parameters)
Ghi (Overriding) vs. Np chng (Overloading)
Ghi phng thc: subclass thay i s thi hnh (implementation) ca mt
phng thc c nh ngha trong superclass. override mt method, method phi
c nh ngha trong subclass s dng cng ch k v kiu tr v nh superclass;
khng c thu hp quyn quy cp. Np chng phng thc (overloading) l cch
cung cp nhiu phng thc vi cng mt tn nhng vi cc ch k khc nhau phn bit
Xem v d Overriding.java v Overloading.java V nguyn tc, mt lp con
khng c php vit cc bin thnh phn ca lp cha, nhng chng c th b che khut
tng t nh bin cc b trong lp con. Xem v d KhachHang.java!
Bi tp Xy dng lp Candidate (Th sinh) gm cc thuc tnh: m s, tn, ngy
thng nm sinh, im thi Ton, Vn, Anh v cc phng thc cn thit. Xy dng lp
TestCandidate kim tra lp trn: Nhp vo 10 th sinh In ra thng tin v cc
th sinh c tng im ln hn 15
c im hng i tng trong JavaTnh ng gi (encapsulation): i tng l mt
thnh t h tr tnh ng gi; n v ng gi c bn ca Java l class; Mt lp bao gm
cc thnh phn: d liu v x l
Tnh tha k (inheritance): c tnh ca tha k l c th sao chp cc c trng
(d liu, phng thc) ca mt lp sang lp khc Ngn ng Java ch chp nhn tha k
n Java dng phng php giao din (interface) thc hin chc nng ca a tha
k.
Tnh a hnh (polymorphism): Ci t cc lp dn xut khc nhau t mt lp
ngun. Mt bin n thuc mt lp no (lp trn/c s) c th c dng tham chiu n cc
i tng ca cc lp khc nhau (cc lp dn xut); c kh nng t ng gi cc phng
thc c th ca loi i tng m n ang tham chiu n. Tnh a hnh ch p dng cho
phng thc, khng p dng cho thnh phn d liu!
Polymorphism Quan h tha k cho php subclass k tha cc tnh nng t
superclass v thm vo cc tnh nng mi. Subclass l mt s chuyn ha ca
superclass ca n; mi th hin (instance) ca mt subclass l mt th hin ca
superclass, nhng iu ngc li th khng ng. V d nh mi hnh trn l mt i
tng, nhng mi i tng u l hnh trn th khng ng. Do ta lun c th truyn mt
th hin ca mt subclass cho mt bin thuc kiu ca superclass Xem v d
trong th mc Polymorphism
Cy tha k
Animal
TerrestrialAnimal
AquaticAnimal
Cat
Horse
Chicken
Fish
Octopus
Gii thiu v lp Tru tng v Giao din Trong cy tha k, cc lp tr nn
ring bit v c th hn vi mi lp con (subclass) mi. Khi nhn t subclass
ngc ln superclass th thy rng cc lp tr nn tng qut v t ring bit hn.
Thit k lp cn m bo rng mt superclass cha mt s c trng chung cho
subclasses ca n. i khi mt superclass qu tru tng n ni n khng c bt k
mt th hin c th no. Lp nh vy gi l lp tru tng. i khi ta cn phi dn xut
mt subclass t nhiu lp khc. Nu dng t kha extends nh ngha subclass th
ta ch c th k tha t mt lp cha (java ch cho php n tha k). Vi giao din
(interface) th ta c th thc hin a tha k.
Lp tru tng (abstract class) Lp tru tng cng tng t nh cc lp thng
thng khc vi d liu v cc phng thc, nhng ta khng th to cc th hin ca cc
lp tru tng. Mt phng thc tru tng l mt ch k phng thc m khng c phn ci
t./** Abstract method getArea */ public abstract double
getArea();
Mt lp cha phng thc tru tng th phi khai bo l tru tng. Lp tru tng
nh ngha cc thuc tnh chung cho cc lp con ca n.
Lp tru tng (tt.) Khai bo lp tru tng public abstract class
ClassName {} Cc lp con ca mt lp cha tru tng phi ci t tt c cc phng
thc tru tng. Nu khng n cng s tr thnh lp tru tng. Khng th to cc i
tng ca mt lp tru tng nhng c th khai bo bin thuc kiu lp tru tng tham
chiu n cc i tng thuc lp con ca n.
GeometricObject[] objects = new GeometricObject[10]; C th to
constructor trong lp tru tng v n c gi trong constructor ca subclass
Cc phng thc c khai bo dng cc tin t private v static th khng c khai
bo l abstract
Lp tru tng (tt.) V d c th thit k lp Hnh trn, Hnh vung... k tha t
lp tru tng Hnh hc. Lp Hnh hc c cc phng thc tnh din tch, chu
vi...
Xem cc v d trong th mc Astract!
Bi tp Xy dng lp Employee c name v phng thc tru tng l earnings().
Xy dng lp Boss k tha t Employee c cch tnh lng l mt khon c nh hng
thng. Xy dng lp PieceWorker c cch tnh lng da trn s sn phm lm c, lng
mt sn phm l $ 0.5. Xy dng lp CommissionWorker c cch tnh lng l mt
khon c nh + tin hoa hng trn s sn phm bn c, mi sn phm c $ 0.1 hoa
hng. Vit chng trnh test cc lp trn.
Giao din (interface) Giao din l mt loi lp c bit: ch cha ng cc
hng s (constants) v phng thc tru tng (abstract methods). Giao din
ch ra cc tnh cht m mt i tng c th c, trong mt ng cnh no . Mt ngi c
th khi nh l mt ngi con, trng l mt sinh vin, lp l mt ngi bn. Gi s nh
ta mun thit k mt phng thc tm object ln hn t hai objects (cc objects
c th l students, circles, rectangles). Ta cn phi nh ngha mt phng
thc so snh chung xc nh th t ca hai objects, ri sau bin i phng thc
ny so snh cho cc loi i tng c th.
Giao din trong Java c th c dng th hin s a k tha nh trong
C++.
Interface (tt.) Khai bo giao din interface InterfaceName {...}
//Giao din so snh cc objects c /n trong java.lang package
java.lang; public interface Comparable { public int
compareTo(Object o); //tr v 0 khi hai i tng bng nhau // >0 khi i
tng hin ti ln hn object trong tham s // < 0 khi i tng hin ti nh
hn object trong tham s } Mt giao din c th tha k mt giao din khc. Mt
lp c th ci t (implements) mt hay nhiu giao din nhng ch c th tha k
(extends) t mt lp. Giao din khng c constructor, bin phi l public
static final, phng thc phi l public abstract cho d c ghi r ra hoc
khng. Ci t giao din: class SomeClass extends SomeParent implements
interfaceName { } Mt lp ch m rng t mt lp cha nhng c th ci t zero
hoc nhiu giao din Trong phn thn ca lp ci t giao din phi nh ngha tt
c cc phng thc c khai bo trong giao din
StudentSort.java
V d v Giao dinLp Cha: Goods vi thuc tnh description v price,
phng thc display() Lp con: Food (calories), Toy (minimumAge), Book
(author). Cn nhiu th b nh thu nhng khng phi l hng ha (goods) v khng
phi hng ha no cng b nh thu! Taxable c ci t l giao din trong Java: c
hng ch mc thu -taxRate-, phng thc tnh thu -calculateTax()-. Xem v d
trong th mc Interface!
Bi tp Xy dng lp tru tng Animal c d liu l name v weight, phng thc
o l grow (tng trng lng). Xy dng lp Chicken k tha t Animal, ci t
phng thc o mi ln grow th trng lng tng 0.3 kg. Xy dng lp Sparrow k
tha t Animal, mi ln tng trng lng 0.05 kg. Xy dng giao din Flyable c
phng thc fly(). Ci t giao din ny trong lp Sparrow. Vit chng trnh
test cc lp trn.
