Reference types and their characteristics Class Definition Constructors and Object Creation Special objects: Strings and Arrays

Transcription

1 Objects nd Clsses Reference types nd their chrcteristics Clss Definition Constructors nd Object Cretion Specil objects: Strings nd Arrys OOAD 1999/2000 Cludi Niederée, Jochim W. Schmidt Softwre Systems Institute Reference Types Predefined or user-defined Clsses String Arry The vlues of reference types re not mnipulted directly. Insted hndles (references) to the ctul vlues re used. OOAD99/00-STS-Objects nd Clsses 2 1

2 Reference Type Vribles A vrible of reference type contins reference to n nonymous object in storge. An uninitilized vrible contins null-reference. Point,b; Objects re creted t run-time. new Point(0,0); Objects re nonymous. They hve n immutble identity. b 0 0 object OOAD99/00-STS-Objects nd Clsses 3 Binding Reference Type Vribles Assigning n object to vrible, the vrible gets reference to the object s its vlue. = new Point(0,0); b = new Point(3,4); Assigning vlues of other vribles mens ssigning the references! The vlue of the object does not chnge = b; My be pitfll, s the object my chnge OOAD99/00-STS-Objects nd Clsses 4 b b

3 Compring Objects Compring two reference type vribles mens compring the references they contin nd, thus, compring the identity of the referenced objects. b 3 4 == b c 3 4!= c d d == null OOAD99/00-STS-Objects nd Clsses 5 Compring the Contents of Objects The contents of objects belonging to pre-defined clsses cn be compred using the method equls. If you define clss of your own, equls bydefult only compres references, s == does. => You hve to override equls for newly defined clsses. OOAD99/00-STS-Objects nd Clsses 6 3

4 Creting New Dt Types: clss Clss keyword introduces definition of new dt type clss ATypeNme { /* clss body goes here */ ATypeNme = new ATypeNme(); Dt members clss DtOnly { int i; flot f; boolen b; Ech instnce of DtOnly gets its own copy of the dt members In clss, primitives get defult vlues. OOAD99/00-STS-Objects nd Clsses 7 Methods, Arguments nd Return Vlues Methods: how you get things done in n object My inspect nd chnge the stte of the object Trditionlly clled functions Cn only be defined inside clsses returntype methodnme(/* rgument list */) { /* Method body */ Exmple method cll = messge sending : cell1.setvlue(4); Receiver Object Method nme Prmeter(s) OOAD99/00-STS-Objects nd Clsses 8 4

5 The Argument List Types of the objects to pss in to the method Nme (identifier) to use forech one Whenever you seem to be pssing objects in Jv, you re ctully pssing hndles void reset(cell c) { c.setvlue(0); Cell cell1 = new Cell(); cell1.setvlue(4); reset(cell1); cell1.getvlue(); OOAD99/00-STS-Objects nd Clsses 9 Inside the Method Body Vrible declrtion nd ssignments Opertions on primitive dt types Object cretion Messge sending Itertion Conditionls return Sttement OOAD99/00-STS-Objects nd Clsses 10 5

6 this: Hndle to Current Object public clss Lef { privte int i = 0; Lef increment() { i++; return this; void print() { System.out.println("i = " + i); public sttic void min(string rgs[]) { Lef x = new Lef(); x.increment().increment().increment().print(); no thisin sttic methods OOAD99/00-STS-Objects nd Clsses 11 Nunce We cn deduce mening from context Wsh the shirt Wsh the cr Wsh the dog Not shirtwshthe shirt crwshthe cr dogwsh the dog OOAD99/00-STS-Objects nd Clsses 12 6

7 Method Overloding clss SuperWsher{ void wsh(shirt s) {... void wsh(cr c) {... void wsh(dog d) { One word, mny menings: overloded Unique rgument type combintions distinguish overloded methods OOAD99/00-STS-Objects nd Clsses 13 Overloding on Return Vlues Why notlso use return vlues inmethod overloding? void f() {... int f() {... Then wht would this men? f(); OOAD99/00-STS-Objects nd Clsses 14 7

8 Defult Constructor Constructors re needed to crete instnces (objects) of clss Compiler provides one for you if you write no constructor clss Bird { int i; public clss DefultConstructor { public sttic void min(string rgs[]) { Bird nc = new Bird(); // defult! OOAD99/00-STS-Objects nd Clsses 15 Constructor Definition clss Rock { Rock() { // This is the constructor System.out.println("Creting Rock"); public clss SimpleConstructor { public sttic void min(string rgs[]) { for(int i = 0; i < 10; i++) new Rock(); OOAD99/00-STS-Objects nd Clsses 16 8

9 Constructor Overloding Like methods constructors my be overloded clss Tree { int height; Tree() { System.out.println("A seedling"); height = 0; Tree(int i) { System.out.println("A new Tree, " + i + " feet tll"); height = i; OOAD99/00-STS-Objects nd Clsses 17 this in Constructors A very common kind to use this is in constructors to initilize dt members with the constructor's rguments public clss Animl { privte int numberoflegs; Animl(int numberoflegs) { this.numberoflegs = numberoflegs; OOAD99/00-STS-Objects nd Clsses 18 9

10 Arrys nd Arry Initiliztion Arrys re objects int 1[]; // this... int[] 1; // is the sme s this! Cretes hndle, not the rry. Cn t size it. To crete nrryof primitives: int [] 1 = { 1, 2, 3, 4, 5 ; Bounds re checked, length produces size of the rry 1.length If you do nything wrong either the compiler will ctch it or n exception will be thrown OOAD99/00-STS-Objects nd Clsses 19 Arrys of Objects An rry of clss objects: Animl[] = new Animl[20]; System.out.println(.length + " nimls"); for(int i = 0; i <.length; i++) [i] = new Animl((i % 2 + 1) * 2); Cn lso use brcketed list (The size is then fixed t compile-time) Integer[] = { new Integer(1), new Integer(2), new Integer(3), ; OOAD99/00-STS-Objects nd Clsses 20 10

11 Multi-dimensionl Arrys It is possible to define multi-dimensionl rrys. int[][] ; The brckets even my be distributed between type nd identifier. int[] []; Initiliztion cn be done directly int[][] = { { 1, 2, 3, { 5, 6 ; It cn lso be done by nested itertions over the rrynd its components. OOAD99/00-STS-Objects nd Clsses 21 Strings Strings re immutble objects of the clss String. String literls re zero, one or more chrcters included within double quotes. When binding to string literl is executed for the first time, new String object is creted. If ny other bindings to this literl pper, the respective vribles will hold reference to the sme object. String = "bc"; String b = "bc"; == b b "bc" OOAD99/00-STS-Objects nd Clsses 22 11

12 String Constructors String objects cn lso be creted by clling constructor. String constructors crete new objects whenever they re clled. String c = new String("bc"); b c "bc" "bc"!= c.equls(c) ; lwys use equls There re severl constructors defined for strings. OOAD99/00-STS-Objects nd Clsses 23 String Conctention Strings cn be conctented by using +. String c = "A " + "conctention"; The conctention lso cretes new String object. Vlues of other types cn be conctented to strings, too. They re implicitly converted to String. String n = "Number " + 49; OOAD99/00-STS-Objects nd Clsses 24 12