SCJP_Ch01 Declaration and Access Control.doc

SCJP Study Notes: Chapter 1 Declarations and Access ControlChapter 1 Declarations and Access Control1. Identifiers and JavaBean1.1 Legal IdentifiersMust start with a letter, $ or _ , CANT start with numberLegal: _a, $c, _2_w, _$Illegal: :b, -d, e#, .f, 7gJava Keywordabstractbooleanbreakbytecasecatchcharclassconstcontinuedefaultdodoubleelseextendsfinalfinallyfloatforgotoifimplementsimportinstanceofintinterfacelongnativenewpackageprivateprotectedpublicreturnshortstaticstrictfpsuperswitchsynchronizedthisthrowthrowstransienttryvoidvolatilewhileassertenum1.2 JavaBeans StandardsJavaBeans = help Java developer create component= class that have properties1. JavaBean Property Naming Rule- if not Boolean, getter method prefix must be get, e.g. getSize( )- if boolean, getStopped( ) or isStopped( )- setter method prefix must be set, e.g. setSize( )- setter method must marked public with void return type, with arg. Represent property value- getter method must marked public with no arg, return type match arg type of setter method2. JavaBean Listener Naming Rule- register listener prefix with add, eg. AddActionListener( )- unreg = remove- Listener method name must end with “Listener”E.g. valid JavaBean Signature:public void setMyValue(int v)public int getMyValue( )public Boolean isMyStatus( )public void addMyListener(MyListener m)public void removeMyListener(MyListener m)E.g. Invalid JavaBean Signature:void setCustomer(String s)/ must be publicpublic void modfiyMyValue(int v)/ cant use modifypublic void addXListener(MyListenser m)/ Mismatch2. Declare Classes2.1 Source File Declaration Rules- there can be ONLY ONE public class per src. file- package, import, class- a file can have more than 1 non-public class2.2 Class Declaration and ModifiersAccess modifier: public, protected, private, default (package access)Non access modifier: strictfp, final, abstractPublic = all package can access, still need to import itFinal = cant be subclassedAbstract class = cant be instantiatedabstract class Car private double price;public abstract void goFast( );/ end with ; instead of 3. Declare InterfaceInterface = a contract for what a class can do w/o saying how itll dowhat you declareinterface Bounceablevoid bounce( );what compiler seeinterface Bounceablepublic abstract void bounce( );implicitALL interface method mustbe implemented and mark publicclass Tire implements Bounceablepublic void bounce( ) Interface = 100% abstract class = can only have abstract methodAll interface method are implicitly public and abstractAll var. in interface must be public, static and final, i.e. constInterface method must NOT be staticInterface cant extend anything except another InterfaceInterface cant implement another interfaceLegal interface declaration:public abstract interface Rollable public interface Rollable Illegal interface method:final void bounce( );static void bounce( );private void bounce( );protected void bounce( );Declaring Interface ConstantALWAYS: public static final = can be omitted= CANT change the value no matter how you declare4. Declare Class Member4.1 Access Modifierclass = default or public (just two)member = public, protected, default, privatePublic Member- access from diff. package, need to import it first3 ways of access a method- in same class- using reference of the class (.operator)- in inherited method (no need .operator)For subclass, if declared public, can be in diff. packagePrivate Member= subclass cant inherit / overriding itProtected and Default Member (Most confusing and complicated)Almost identical EXCEPT protected member can be accessed by subclass in diff. package= the diff. is ONLY in subclassDefault access = Package restrictionProtected = Package + Kid = ONLY through inheritancepackage Cert;public class Parent protected int x = 9;package Other;import Cert.Parent;CASE 1class Child extends Parent public void testit( ) CASE 2System.out.println(x); / OKParent p = new Parent( );System.out.println(p.x); / ERRORNeighbor = same package of child instantiate a child objectCASE 3 = cannot access xLocal VariableAccess modifier NOT applied to local var., except finalPublicProtectedDefaultPrivateFrom same classYYYYOther class in same packageYYYNFrom subclass in same packageYYYNFrom subclass outside packageYYNNFrom non-subclass class outside packageYNNNthrough inheritance4.2 Non - Access ModifierFinal Method- final prevent a method from being overridden in subclasscant be modifiedFinal Argumentpublic int getRecord(final int number Abstract Method- if one method declared abstract, the class must be abstract- the first concrete subclass of an abstract class must implement all abstract method- if abstract extends abstract = no need to implement- abstract cant be combined with final or static or privateSynchronized Method- the method can be accessed by only one thread at a time- can only be applied to method, NOT var, NOT classpublic synchronized Record getRecord(int id) Method with Variable Argument List (var-args)- must specified type of arg., can be object type- can have other parm, but var-args must be last parm, can only have one var-argLegal:within the functionx0, x1void func(int x) void func(char c, int x) void func(MyObj myobj) Illegal:void func(int x ) void func(int x, char y) void func(String s, byte b) 4.3 Constructor DeclarationConstructor cant have return typeclass test protected test ( ) / constructorprotected void test ( ) / method, NOT constructor- if dont create explicitly, compiler will build one for you- constructor can have normal access modifier, except final, abstract- must take same name as the class- cant mark static, final or abstractclass test2 Legal constructortest2 ( ) private test2 (byte b) test2 (int x) Illegal constructorvoid test2 ( ) test ( ) test2 (short s);static test2 ( );4.4 Variable DeclarationPrimitive:Typechar = 16 bit unicodeBoolean = true/falseBitsbyte8short16int32long64float32double64Reference var: e.g. Object O, String s1Instance var: can be marked final, transient, abstract, synchronized, strictfp, native, static, public, private, protectedLocal var: must be initialized, ONLY finalfinal var: final referenceTransient var: JVM will ignore the var when serializing the obj.Volatile varStatic varArray = are objectint key;int key ;String test;String mgr ;int 5 test;/ ERROR4.5 Declaring Enumsenum CoffeeSize BIG, HUGE, OVERWHELMING ; CoffeeSize cs = CoffeeSize.BIG;Enums can be declared as their own separate class, or as a class member, however they must not be declared within a method! 1) Declaring an enum outside a class: enum CoffeeSize BIG, HUGE, SUPER / cannot be private or protected class Coffee CoffeeSize size; public class CoffeeTest1 public static void main(String args) Coffee drink = new Coffee(); drink.size = CoffeeSize.BIG; / enum outside class * The enum declared like this MUST be public or default2) Declaring an enum inside a class: class Coffee2 enum CoffeeSize BIG, HUGE, OVERWHELMING CoffeeSize size;public class CoffeeTest2 public static void main(String args) Coffee2 drink = new Coffee2(); drink.size = Coffee2.CoffeeSize.BIG; / enclosing class / name required ILLEGALpublic class CoffeeTest1 public static void main(String args) enum CoffeeSize BIG, HUGE, OVERWHELMING / WRONG! Cannot declare enums in methods Coffee drink = new Coffee(); drink.size = CoffeeSize.BIG; Optional Semicolonenum CoffeeSize BIG, HUGE, OVERWHELMING ; o remember enums are not String or into each of the enumerated types (BIG, HUGE) are actually instances of CoffeeSizeo think of an enums as a kind of classclass CoffeeSize public static final CoffeeSize BIG = new CoffeeSize(BIG, 0); public static final CoffeeSize HUGE = new CoffeeSize(HUGE, 1); public CoffeeSize(String enumName, int index) public static void main(String args) System.out.println(CoffeeSize.BIG); Declaring Constructors, Methods and Variables in an enumYou could make some kind of a lookup tableenum CoffeeSize / 8, 10 and 16 are passed as values to the constructorBIG(8), HUGE(10), OVERWHELMING(16); CoffeeSize(int ounces) this.ounces = ounces; / assign the value to an instance variable private int ounces; / an instance variable each enum value has public int getOunces() return ounces; class Coffee CoffeeSize size; / each instance of Coffee has-a CoffeeSize enumpublic static void main(String args) Coffee drink1 = new Coffee(); drink1.size = CoffeeSize.BIG;Coffee drink2 = new Coffee();drink2.size = CoffeeSize.OVERWHELMING;System.out.println(drink1.size.getOunces();