SCJP_Ch03 Assignments

Chapter 3 Assignments1. Stack and Heap – An OverviewInstance var and object live on heapLocal var live on stack2. Literals, Assignments and Variables2.1 Literal Value for All Primitive TypesPrimitive Integer Literal DecimalTypeOctal 0 ~ 7 int eight = 010nine = 011HexaDecimal 0 ~ f 0x7FFFFFFFlong l = 0xFFFFlFloat Pt. Literal float (32 bit) (must suffix f/F)(double by default)double (64 bit) (d/D)Boolean Literal (true/false) NOT 0/1Character Literal char test = …\u004E‟;(16 bit unsigned int) char b = 982; //intchar c = (char)70000; // castchar d = (char)-98;char c = …\”‟;float f = 3.2; // ERRORString is object, NOT primitive2.2 Assignment Operatora literal integer (e.g. 7) is implicitly an intbyte a = 27; // OK, max for byte is 127byte b = (byte)27; // OK, explicit cast=> applies to char and shortbyte b = 3; byte c = 8;byte d = b + c; // ERROR, …+‟ return intbyte d = (byte)(b + c); // OKPrimitive CastingImplicit cast => put small thing (byte) into bigger (int)=> no need to write code for the caste.g. int a = 100;long b = a;double d = 100L;Explicit cast => put large thing into smaller container => loss precisione.g. float a = 100.01f; int b = (int)a;int x = 3957.22; // ERRORint x = (int)3957.22; // OK, become 3957long l = 56L; byte b = (byte)l; // OKbyte a = (byte)128; // truncate higher order bit, become –128byte b = 3; b += 7; // OK b = (byte)(b + 7); // compile ERRORBMW extends Car Car c = new BMW( ); // OK BMW b = new Car( ); // ERRORVariable Scope 1) Static var 2) Instance var 3) Local var 4) Block varCommon ERROR1) Attempt to access inst. var from static context (typically from main) class Test { int x = 5;public static void main( … ) {x++; } }2) Attempt to access local var for nested method void go( ) { int y = 5; go2( ); }void go2( ) { y++; // ERROR }USE IS-A Test x NOT static3)Attempt to use block var after code block completedf or( … ) {Boolean test = false;}System.out.println(test); // ERROR2.3 Using Var. or Array element that is UninitInstance var => declared at class level=> they are init. to default value when instance created Type Default ValueObject Ref nullbyte, short, int, long 0float, double 0.0boolean falsechar …\u0000‟1)Primitive Instance Variable2)Object Reference, e.g. String3)Array Instance Variable => if NOT init => null=> if init => element => def. val2.4 Local Primitive and Objectlocal var MUST be initcan w/o init if you don‟t use itsometimes compiler can‟t tell for certainint x;if( xxx ) { x = 7; } // ERRORif(date == null) // ERROR, since date even not null, uninit can‟t use dot operator on null referencelocal array => element need not init, will give default value Assigning one ref. val to another- if an object is assigned to another, will pt to same objecti.e. change one will change anotherDimension a = new Dimension(5, 10);Dimension b = a;b.height = 30; // both a and b change to 30- exception is String, which is immuntable3. Passing Variables into Methods3.1 Passing Object Reference Variable=> able to modify3.2 Pass – By – Value Semantics●Java is actually pass – by – value for all var●pass – by – copy – of - var●if obje. ref => pass the bit representing ref.3.3 Passing Primitive Variable=> the value is NOT modifiedshadow an instance var => by declare same nameclass Foo {static int size = 7; WON‟T change itstatic void changeIt(int size) {size = size + 200; actually local varSystem.out.println(size); // 207}public static void main( … ) {Foo f = new Foo( );System.out.println(size); // 7changeIt(size);System.out.println(size); // 7}}class Bar { int barNum = 28; }class Foo {Bar myBar = new Bar( );void changeIt(Bar myBar) {myBar.barNum = 99;System.out.println(myBar.barNum); // 99myBar = new Bar( );myBar.barNum = 420;System.out.println(myBar.barNum); // 420}public static void main(String [ ] args) {Foo f = new Foo( );System.out.println(f.myBar.barNum); // 28f.changeIt(f.myBar);System.out.println(f.myBar.barNum); // 99}}4. Array Declaration, Construction and Initialization4.1 Declaring an Arrayint [ ] key;int key [ ];String [ ][ ][ ] key;String [ ] key [ ];int [5] key; // ERROR, size should not be specified4.2 Constructing an Array int [ ] key; OR int [ ] key = new int[3]; key = new int[3];int [ ] key = new int[ ]; // size missingMulti – dimensional Array => Array of Array int [ ][ ] key = new int[3][ ]; key[0] = new int[2]; key[0][0] = 6; key[0][1] = 7;key[1] = new int[3];key[1][0] = 9; key[1][1] = 8; key[1][2] = 5;4.3 Initializing an ArrayInit. element in loopint [ ] mykey = new key[6];for(int x = 0; x < mykey.length; x++) {mykey[x] = new key( ); }for(key k : mykey) k = new key( );Short Cut Syntax int [ ] key = {6, 7, 8};key[ ] mykey = {new key(“test”), new key(“test2”)}; int [ ][ ] key = {{5, 2, 4, 3}, {9, 2}, {3, 4}};Anonymous Array Creation int [ ] key;key = new int [ ] {4, 7, 2}don‟t even need to assign array to anything => just in time array => parm. passing myfunc(int [ ] myArray)myfunc(new int [ ] {1, 2, 3});Array of Primitiveint [ ] key = new int[5]; byte b = 4; char c = …c‟; short s = 7;key[0] = b;key[1] = c; key[2] = s;DON‟T confuse with length( ) in String element promoted implicitlyArray of Object=> can put the subclass (IS-A test)class Car { }class BMW extends Car { }Car [ ] myCar = { new Car( ), new BMW( ) }If the array is an interface type, element can refer to instance of class that implement the declared interfaceinterface Sporty { }class BMW extends Car implements Sporty { }class Benz { }Sporty [ ] sportCar = new Sporty[3];SportCar[0] = new BMW( ); // OKSportCar[1] = new Benz( ); // ERROR, it doesn‟t implementArray Ref. Assignmentint [ ] splats;int [ ] dats = new int[4];char [ ] letters = new char[5];splats = dats; // OKsplats = letters; // ERRORHonda extends CarCar [ ] cars;Honda [ ] cutCars = new Honda[5];cars = cutCars; // OKBeer [ ] beers = new Beer[99];cars = beers; // ERRORcuteCars = cars; // ERROR, can‟t reverseBox implements FoldableFoldable [ ] folding;Box [ ] box = new Box[3];folding = box; // OKint [ ] blots;int [ ][ ] key = new int[3][ ];blots = key; // ERRORint [ ] blocks = new int[6];blots = blocks; // OKint [ ][ ] book = new int[3][ ];int [ ] numbers = new int[6];int aNum = 7;book[0] = aNum; // ERRORbook[0] = numbers; // OK4.4 Initialization Blocko Init. Block run when the class is first loaded (static init. block) or when instance is created (instance init. block)o Instance init. block run after the call to super( )o the order of init. block mattersclass Init {Init(int x) { System.out.println(“1-arg const”); }Init( ) { System.out.println(“no-arg const”); }static { System.out.println(“1st static init”); }{ System.out.println(“1st instance init”); }static { System.out.println(“2nd static init”; }}new Init( );new Init(7);1st static init2nd static init1st instance initno-arg const1st instance init1-arg const5. Using Wrapper Class and Boxing5.1 OverviewPurpose: 1) to provide mechanism to “wrap” primitive value in object can invoke ine.g. collection2) to provide assortment of utility func for primitive e.g. radix5.2 Creating Wrapper ObjectWrapper object are immuntableInteger i1 = new Integer(42);Integer i2 = new Integer(“42”);Float f1 = new Float(3.14f);Float f1 = new Float(“3.14f”);Character c1 = new Character(…c‟);Boolean b = new Boolean(“false”);if(b) // WONT‟T compile before JAVA 1.4// JAVA 5 => OK => unboxInteger i2 = Integer.valueOf(“101011”, 2);// radix5.3 Using Wrapper Conversion Utilities1) xxxValue( ) => convert wrapper numeric to primitive Integer i2 = new Integer(42);byte b = i2.byteValue( );short s = i2.shortValue( );Float f2 = new Float(3.14f);Short s = f2.shortValue( ); // 3 => truncated, NOT round2) parseXXX( ) => convert string to primitivevalueOf( ) => return newly created wrapped obj. of that type double d4 = Double.parseDouble(“3.14”);// 3.14Double d5 = Double.valueOf(“3.14”);// return new Double long L2 = Long.parseLong(“101010”, 2);// 42Long L3 = Long.valueOf(“101010”, 2);3) toString( ) => give some meaningful representation of object Double d = new Double(“3.14”);d.toString( );Long.toString(254.16); => “fe”4) toXxxString( ) (Binary, Hexadecimal, Octal)String s3 = Integer.toHexString(254);Common Wrapper Conversion Method5.4 AutoboxingBoxing, unboxingBefore Java5: Integer y = new Integer(567);int x = y.intValue( ); // unwrapx++;y = new Integer(x); // re – wrapNEW: y++;Integer y = 567;Integer x = y;System.out.println(y == x); // truey++; since new object System.out.println(y == x); // false==, equal( )equals( ) => test meaningfully equalInteger i1 = 1000;Integer i2 = 1000;i1 == i2 => truei1.equals(i2) => truei1 != i2 => trueif primitive value is same => == return trueclass Boxing {static Integer x;public static void main( … ) {doStuff(x);}static void doStuff(int z) {int z2 = 5;z2 + z <= NullPointerException}}6. Overloadingvoid go(int x)void go(long x)void go(double x)byte b = 5;short s = 5;long l = 5;float f = 5.0f;=> int int long doubleOverloading with Boxing and Var-args1) void go(Integer x)void go(long x)int i = 5;go(i); // long2) void go(int x, int y)void go(byte … x)byte b = 5;go(b, b); // int, int3) void go(Byte x, Byte y)void go(byte … x)byte b = 5;go(b, b); // Byte, Bytevar – args => looserWidening Ref. Valuevoid go(Car c)BMW extends Cargo(bmw) // OK, since BMW IS-A CarNOT valid to widen wrapper class => IS-A testOverloading when Combining Widening and Boxingvoid go(Long x)byte b = 5;go(b); // ERROR => too much for compilervoid go(Object o) {Byte b2 = (Byte)o;}byte b = 5;go(b); // OKOverloading in Combination with Var-Argsvoid wide_vararg(long … x)void box_vararg(Integer … x)int i = 5;wide_vararg(5, 5); // longbox_vararg(5, 5); // IntegerSUMMARYo Primitive widening use …smallest‟ method arg. possibleo used individually, boxing & var-arg are compatible with overloading o CAN‟T widen from one wrapper type to another (IS-A test)o CAN‟T widen and then box (int can‟t become Long)o can box and then widen (int => object via Integer)o can combine var-args with either widening or boxing7. Garbage Collectiono GC is under the control of JVMo Java program can ask JVM to run GC, but not guaranteeo an object is eligible for GC when no more reachable ref to ite.g. nulling the ref myobj = null;SCJP Study Notes: Chapter 3 Assignments e.g. re – assign a ref varStringBuffer s1 = …StringBuffer s2 = …s1 = s2; // s1 is eligible for GCo if obj. return from method NOT eligibleIsland of IsolationPage 11 of 11。