book

Head First Java, 2nd Edition

by Kathy Sierra, Bert Bates

February 2005

Beginner

720 pages

19h 12m

English

O'Reilly Media, Inc.

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Who is this book for?Who should probably back away from this book?We know what you’re thinkingAnd we know what your brain is thinkingMetacognition: thinking about thinkingHere’s what WE didHere’s what YOU can do to bend your brain into submissionWhat you need for this bookLast-minute things you need to knowTechnical EditorsOther people to : creditJust when you thought there wouldn’t be any more acknowledgements
The Way Java WorksWhat you’ll do in JavaA very brief history of JavaCode structure in JavaAnatomy of a classWriting a class with a mainWhat can you say in the main method?Looping and looping and...Simple boolean testsExample of a while loopConditional branchingCoding a Serious Business ApplicationMonday morning at Bob’sPhrase-O-MaticHow it works
Chair Wars: (or How Objects Can Change Your Life)In Larry’s cubeAt Brad’s laptop at the cafeLarry thought he’d nailed it. He could almost feel the rolled steel of the Aeron beneath his...But wait! There’s been a spec changeBack in Larry’s cubeAt Brad’s laptop at the beachLarry snuck in just moments ahead of BradBack in Larry’s cubeAt Brad’s laptop on his lawn chair at the Telluride Bluegrass FestivalSo, Brad the OO guy got the chair, right?What about the Amoeba rotate()?The suspense is killing me. Who got the chair?When you design a class, think about the objects that will be created from that class type. Think aboutThings an object knows about itself are calledThings an object can do are calledA class is not an object. (but it’s used to construct them)Making your first objectMaking and testing Movie objectsQuick! Get out of main!The two uses of main:The Guessing GameRunning the Guessing Game

Declaring a variable“I’d like a double mocha, no, make it an int.”You really don’t want to spill that...Back away from that keyword!This table reservedControlling your Dog objectAn object reference is just another variable valueLife on the garbage-collectible heapLife and death on the heapAn array is like a tray of cupsArrays are objects tooMake an array of DogsWhat’s missing?Control your Dog (with a reference variable)What happens if the Dog is in a Dog array?A Dog example
Remember: a class describes what an object knows and what an object doesCan every object of that type have different method behavior?The size affects the barkYou can send things to a methodYou can get things back from a methodYou can send more than one thing to a methodCalling a two-parameter method, and sending it two argumentsYou can pass variables into a method, as long as the variable type matches the parameter typeJava is pass-by-value. That means pass-by-copyCool things you can do with parameters and return typesEncapsulationDo it or risk humiliation and ridiculeHide the dataEncapsulating the GoodDog classHow do objects in an array behave?Declaring and initializing instance variablesThe difference between instance and local variablesComparing variables (primitives or references)
Let’s build a Battleship-style game: “Sink a Dot Com”part of a game interactionFirst, a high-level designThe “Simple Dot Com Game” a gentler introductionDeveloping a ClassThe three things we’ll write for each class:Writing the method implementationslet’s write the real method code now, and get this puppy workingWriting test code for the SimpleDotCom classBased on this prepcode:Here’s what we should test:Test code for the SimpleDotCom classThe checkYourself() methodJust the new stuffFinal code for SimpleDotCom and SimpleDotComTesterPrepcode for the SimpleDotComGame classEverything happens in main()The game’s main() methodrandom() and getUserInput()One last class: GameHelperLet’s playA complete game interactionWhat’s this? A bug?Gasp!A different game interactionMore about for loopsRegular (non-enhanced) for loopsTrips through a loopDifference between for and whileThe enhanced for loopCasting primitives
In our last chapter, we left you with the cliff-hanger. A bugHow it’s supposed to lookA complete game interactionHow the bug looksA different game interactionSo what happened?How do we fix it ?Option one is too clunkyOption two is a little better, but still pretty clunkyWake up and smell the librarySome things you can do with ArrayListComparing ArrayList to a regular arrayComparing ArrayList to a regular arrayLet’s fix the DotCom codeNew and improved DotCom classLet’s build the REAL game: “Sink a Dot Com”part of a game interactionWhat needs to change?Who does what in the DotComBust game (and when)Prep code for the real DotComBust classThe final version of the DotCom classSuper Powerful Boolean ExpressionsUsing the Library (the Java API)How to play with the API
Chair Wars Revisited...What about the Amoeba rotate()?Understanding InheritanceAn inheritance example:Let’s design the inheritance tree for an Animal simulation programUsing inheritance to avoid duplicating code in subclassesDo all animals eat the same way?Which methods should we override?Looking for more inheritance opportunitiesWhich method is called?Designing an Inheritance TreeUsing IS-A and HAS-ABut wait! There’s more!How do you know if you’ve got your inheritance right?Who gets the Porsche, who gets the porcelain?When designing with inheritance, are you using or abusing?So what does all this inheritance really buy you?Inheritance lets you guarantee that all classes grouped under a certain supertype have all the methods that the supertype has.In other words, you define a common protocol for a set of classes related through inheritanceWith polymorphism, you can write code that doesn’t have to change when you introduce new subclass types into the programKeeping the contract: rules for overridingOverloading a method
Did we forget about something when we designed this?The compiler won’t let you instantiate an abstract classAbstract vs. ConcreteAbstract methodsYou MUST implement all abstract methodsPolymorphism in actionUh-oh, now we need to keep Cats, tooWhat about non-Animals? Why not make a class generic enough to take anything?So what’s in this ultra-super-megaclass Object?Using polymorphic references of type Object has a price...When a Dog won’t act like a DogObjects don’t barkGet in touch with your inner ObjectWhat if you need to change the contract?Let’s explore some design options for reusing some of our existing classes in a PetShop programInterface to the rescue!Making and Implementing the Pet interface
The Stack and the Heap: where things liveMethods are stackedA stack scenarioWhat about local variables that are objects?If local variables live on the stack, where do instance variables live?The miracle of object creationConstruct a DuckInitializing the state of a new DuckUsing the constructor to initialize important Duck stateMake it easy to make a DuckBe sure you have a no-arg constructorDoesn’t the compiler always make a no-arg constructor for you? No!Nanoreview: four things to remember about constructorsWait a minute... we never DID talk about superclasses and inheritance and how that all fits in with constructorsThe role of superclass constructors in an object’s lifeMaking a Hippo means making the Animal and Object parts too...How do you invoke a superclass constructor?Can the child exist before the parents?Superclass constructors with argumentsInvoking one overloaded constructor from anotherNow we know how an object is born, but how long does an object live?What about reference variables?
MATH methods: as close as you’ll ever get to a global methodThe difference between regular (non-static) and static methodsWhat it means to have a class with static methodsStatic methods can’t use non-static (instance) variables!Static methods can’t use non-static methods, either!Static variable: value is the same for ALL instances of the classInitializing a static variablestatic final variables are constantsfinal isn’t just for static variables...Math methodsWrapping a primitiveBefore Java 5.0, YOU had to do the work...Autoboxing: blurring the line between primitive and objectAutoboxing works almost everywhereBut wait! There’s more! Wrappers have static utility methods too!And now in reverse... turning a primitive number into a StringNumber formattingFormatting deconstructed...The percent (%) says, “insert argument here” (and format it using these instructions)The format String uses its own little language syntaxThe format specifierThe only required specifier is for TYPEWhat happens if I have more than one argument?So much for numbers, what about dates?Working with DatesMoving backward and forward in timeGetting an object that extends CalendarWorking with Calendar objectsHighlights of the Calendar APIEven more Statics!... static imports
Let’s make a Music MachineThe finished BeatBox looks something like this:We’ll start with the basicsThe JavaSound APIFirst we need a SequencerSomething’s wrong!What happens when a method you want to call (probably in a class you didn’t write) is risky?Methods in Java use exceptions to tell the calling code, “Something Bad Happened. I failed.”The compiler needs to know that YOU know you’re calling a risky methodAn exception is an object... of type ExceptionIf it’s your code that catches the exception, then whose code throws it?Flow control in try/catch blocksFinally: for the things you want to do no matter whatDid we mention that a method can throw more than one exception?Catching multiple exceptionsExceptions are polymorphicMultiple catch blocks must be ordered from smallest to biggestYou can’t put bigger baskets above smaller basketsWhen you don’t want to handle an exception...Ducking (by declaring) only delays the inevitableGetting back to our music code...Exception RulesMaking actual soundYour very first sound player appMaking a MidiEvent (song data)MIDI message: the heart of a MidiEventChange a messageVersion 2: Using command-line args to experiment with sounds
It all starts with a windowPut widgets in the windowYour first GUI: a button on a frameBut nothing happens when I click it...Getting a user eventListeners, Sources, and EventsGetting back to graphics...Make your own drawing widgetFun things to do in paintComponent()Behind every good Graphics reference is a Graphics2D objectBecause life’s too short to paint the circle a solid color when there’s a gradient blend waiting for youWe can get an event. We can paint graphics. But can we paint graphics when we get an event?GUI layouts: putting more than one widget on a frameLet’s try it with TWO buttonsSo now we need FOUR widgetsAnd we need to get TWO eventsHow do you get action events for two different buttons, when each button needs to do something different?How do you get action events for two different buttons, when each button needs to do something different?Inner class to the rescue!An inner class instance must be tied to an outer class instanceHow to make an instance of an inner classUsing an inner class for animationListening for a non-GUI eventAn easier way to make messages / eventsExample: how to use the new static makeEvent() methodVersion Two: registering and getting ControllerEventsVersion Three: drawing graphics in time with the music
Swing componentsComponents can be nestedLayout ManagersHow does the layout manager decide?Different layout managers have different policiesThe Big Three layout managers: border, flow, and boxPlaying with Swing componentsMaking the BeatBox
Capture the BeatSaving StateWriting a serialized object to a fileData moves in streams from one place to anotherWhat really happens to an object when it’s serialized?But what exactly IS an object’s state? What needs to be saved?If you want your class to be serializable, implement SerializableDeserialization: restoring an objectWhat happens during deserialization?Saving and restoring the game charactersThe GameCharacter classWriting a String to a Text FileText File Example: e-FlashcardsQuiz Card Builder (code outline)The java.io.File classReading from a Text FileQuiz Card Player (code outline)Parsing with String split()Version ID: A Big Serialization GotchaUsing the serialVersionUIDSaving a BeatBox patternRestoring a BeatBox pattern
Real-time Beat Box ChatConnecting, Sending, and ReceivingMake a network Socket connectionA TCP port is just a number. A 16-bit number that identifies a specific program on the serverTo read data from a Socket, use a BufferedReaderTo write data to a Socket, use a PrintWriterThe DailyAdviceClientDailyAdviceClient codeWriting a simple serverDailyAdviceServer codeWriting a Chat ClientJava has multiple threads but only one Thread classWhat does it mean to have more than one call stack?Every Thread needs a job to do. A method to put on the new thread stackTo make a job for your thread, implement the Runnable interfaceThe Thread SchedulerPutting a thread to sleepUsing sleep to make our program more predictableMaking and starting two threadsWhat will happen?Um, yes. There IS a dark sideThreads can lead to concurrency ‘issues’The Ryan and Monica problem, in codeThe Ryan and Monica exampleWe need the makeWithdrawal ( ) method to run as one atomic thingUsing an object’s lockThe dreaded “Lost Update” problemLet’s run this code...Make the increment() method atomic. Synchronize it!The deadly side of synchronizationNew and improved SimpleChatClientThe really really simple Chat Server
Tracking song popularity on your jukeboxHere’s what you have so far, without the sort:But the ArrayList class does NOT have a sort() method!ArrayList is not the only collectionYou could use a TreeSet... Or you could use the Collections.sort() methodAdding Collections.sort() to the Jukebox codeBut now you need Song objects, not just simple StringsChanging the Jukebox code to use Songs instead of StringsIt won’t compile!The sort() method declarationGenerics means more type-safetyLearning genericsUsing generic CLASSESUsing type parameters with ArrayListUsing generic METHODSHere’s where it gets weird...Revisiting the sort( ) methodIn generics, “extends” means “extends or implements”Finally we know what’s wrong...The Song class needs to implement ComparableThe new, improved, comparable Song classWe can sort the list, but...Using a custom ComparatorUpdating the Jukebox to use a ComparatorUh-oh. The sorting all works, but now we have duplicates...We need a Set instead of a ListThe Collection API (part of it)Using a HashSet instead of ArrayListWhat makes two objects equal?How a HashSet checks for duplicates: hashCode() and equals()The Song class with overridden hashCode() and equals()And if we want the set to stay sorted, we’ve got TreeSetWhat you MUST know about TreeSet...TreeSet elements MUST be comparableWe’ve seen Lists and Sets, now we’ll use a MapFinally, back to genericsUsing polymorphic arguments and genericsBut will it work with ArrayList<Dog> ?What could happen if it were allowed...Wildcards to the rescueAlternate syntax for doing the same thing
Deploying your applicationImagine this scenario...Separate source code and class filesPut your Java in a JARRunning (executing) the JARPut your classes in packages!Packages prevent class name conflictsPreventing package name conflictsCompiling and running with packagesThe -d flag is even cooler than we saidMaking an executable JAR with packagesSo where did the manifest file go?Java Web StartThe .jnlp file
Method calls are always between two objects on the same heapWhat if you want to invoke a method on an object running on another machine?Object A, running on Little, wants to call a method on Object B, running on BigBut you can’t do thatThe role of the ‘helpers’Java RMI gives you the client and service helper objects!How does the client get the stub object?How does the client get the stub class?Be sure each machine has the class files it needsYeah, but who really uses RMI?What about Servlets?A very simple ServletHTML page with a link to this servletJust for fun, let’s make the Phrase-O-Matic work as a servletPhrase-O-Matic code, servlet-friendlyEnterprise JavaBeans: RMI on steroidsFor our final trick... a little JiniAdaptive discovery in actionSelf-healing network in actionFinal Project: the Universal Service browser
Final BeatBox client programFinal BeatBox server program
#10 Bit ManipulationWhy do you care?#9 ImmutabilityWhy do you care that Strings are Immutable?Why do you care that Wrappers are Immutable?#8 Assertions#7 Block Scope#6 Linked Invocations#5 Anonymous and Static Nested Classes#4 Access Levels and Access Modifiers (Who Sees What)#3 String and StringBuffer/StringBuilder Methods#2 Multidimensional ArraysAnd the number one topic that didn’t quite make it in...#1 Enumerations (also called Enumerated Types or Enums)

Content preview from Head First Java, 2nd Edition

Chapter 4. Methods Use Instance Variables: How Objects Behave

State affects behavior, behavior affects state. We know that objects have state and behavior, represented by instance variables and methods. But until now, we haven’t looked at how state and behavior are related. We already know that each instance of a class (each object of a particular type) can have its own unique values for its instance variables. Dog A can have a name “Fido” and a weight of 70 pounds. Dog B is “Killer” and weighs 9 pounds. And if the Dog class has a method makeNoise(), well, don’t you think a 70-pound dog barks a bit deeper than the little 9-pounder? (Assuming that annoying yippy sound can be considered a bark.) Fortunately, that’s the whole point of an object—it has behavior that acts on its state. In other words, methods use instance variable values. Like, “if dog is less than 14 pounds, make yippy sound, else...” or “increase weight by 5”. Let’s go change some state.

Remember: a class describes what an object knows and what an object does

A class is the blueprint for an object. When you write a class, you’re describing how the JVM should make an object of that type. You already know that every object of that type can have different instance variable values. But what about the methods?