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Agenda Lec05 Exceptions API Reference- and Type-Anonymity Swing Review Review: Aperture and underlying object Object Number Double S C A reference is an aperture. A reference is an aperture. Object obj = new Double(5.87); reference Object Underlying object Double A reference is an aperture. Number num = new Double(89.17); reference Underlying object Number Double A reference is an aperture. Double dub = new Double(72.17); reference Double Underlying object Double A reference is an aperture. Comparable com = new Double(72.17); reference C Underlying object Double The only way to create an object is to use the new keyword (exception to this rule is String) Once instantiated on the heap, that object's type can NEVER change. You may widen or narrow the aperture only. casting/CastObjectsDriver Exceptions: Checked versus Unchecked Exceptions http://www.ibm.com/developerwo rks/java/library/j-jtp05254/ Probable Source Error Within your control Probable Source Checked User should Recover NO JVM NO NO Programmer NO NO RuntimeException YES (unchecked) All other exceptions (checked) NO User, External YES resources, Thread YES Example of Error: System-out-of-memory Error Examples of RuntimeException: ClassCastException, NumberFormatException. Example of checked Exceptions: EOFExcception, Your program running in JVM Hosting system Green = unchecked Blue = checked Situations which cause Exceptions (or Errors) to be thrown • An internal Error occurs in the VM (OutOfMemoryError) this can happen if the gc can not catch up with your program. • An unchecked Exception is generated and thrown (ArrayIndexOutOfBounds) • A checked Exception is thrown (EOFException) • You purposefully throw an Exception. Exception sequence • Once an exception is generated, the execution sequence immediately terminates until the exception is caught (or propagates up the call stack to the VM). In practice that means that any code following your statement which threatens to throw an exception will NOT be executed if an exception occurs. No return value! • An exception is thrown up the call stack See debug dir examples Some Java naming wierdness in the Exception Handling API • Despite having the 'able' suffix, Throwable is a class, not an interface. • Errors are considered part of the "Exception Handling" API • All Exceptions and Errors occur at Runtime, so RuntimeException is a useless name. Some things to remember about Exceptions • Anything that inherets from RuntimeException is unchecked. • Why did the engineers of the Java language decide to create Unchecked Exceptions? Checked versus Unchecked Unchecked: You are the master of your JVM green-zone and therefore you are responsible for ensuring there are no exceptions in your code which are ultimately your responsibility. If the Java language required all methods which threaten to throw an exception, including those in the green-zone, then your code would be riddled with try/catch blocks rendering it effectively unreadable. Imagine if every time you wanted to call an instance method from an implicit parameter (reference), you were required to check a NullPointerException. Checked versus Unchecked Checked: Any time you attempt to leave the relative safety of the green-zone of the JVM, you enter an unpredictable space where things can and do go wrong. This includes, unpredictable user input; servers, networks, and databases that are unavailable or down; I/O in the users file system. The world of concurrency can be an unsafe one as well, so Java has many checked methods in the concurrency API. Java checks these exceptions to impose discipline on the programmer, so that all calls made beyond the JVM greenzone are recoverable from the chaos of the exterior environment. The debate over checked and unchecked See: http://www.ibm.com/developerworks/java/libra ry/j-jtp05254/ 1/ are you dealing with user, system, or remote data? 2/ should your user recover? If yes, it'll probably threaten to throw a checked exception If you create your own Exception class, it should probably be checked Most unchecked exceptions are the “programers fault” and these situations have already been covered by the SDK. Source Code Online Source Code Online • Please avail yourselves of the many excellent sources for source code, both full applications and snippets. • http://stackoverflow.com/ (best for snippets) • GitHub.com • BitBucket.org • http://www.planet-source-code.com/ (best for full apps) • http://www.freewarejava.com/ • http://www.javagalaxy.com/ Guidelines for using found source code • If you know the URL where you got it, cite like so: – //http://stackoverflow.com/questions/26305/how -can-i-play-sound-in-java • Don't copy an entire application; that is plagiarism. • Use found source code within reason. • Minimum 60% of your app must be your own code. Reflection Reflection • Very useful when first learning an OO language. • Reference anonymous - gravity https://www.youtube.com/watch?v=vKW-Gd_S_xc Reflection • Reflection allows you to inspect the type (class) of the implicit parameter at runtime. • We will use reflection to gain a deeper understanding of polymorphism and the java event model. Every class has a class object which you can access like so: java.util.Date.class, or like so: Class.forName(strFullyQualifiedClass); See reflection example Reflection Name of Driver implemnts Implicit param EventListener type Defined yes EventListenerOuter In separate java file yes EventListenerInner In same java file yes anonymous Same method no anonymous inline ActionListener TimeTestOuter ActionListener TimeTestInner ActionListener TimeTestLocal ActionListener TimeTestAnon See inner example Cross-Cutting Concerns BlackJack • Exceptions • Assertions • Logging Shoe Dealer Inner and Anonymous Classes • Though you are captive in the OO paradigm, you can use inner classes and anonymous classes to get around this constraint and write procedural-like code. • Often times, no one but the enclosing class cares about an object. In this case, you may consider using anonymous inner classes. • Often times, these anon inner classes are both reference- and type-anonymous. Anonymous inner classes Type anonymous : Fight Club - Robert Paulson https://www.youtube.com/watch?v=GCi_PIz5ek U Rules of Fight club https://www.youtube.com/watch?v=MPITE_Inec Fight Club Rules of Java 1/ Only concrete classes can be instantiated. Fight Club Rules of Java 1/ Only concrete classes can be instantiated. 2/ Only concrete classes can be instantiated. Fight Club Rules of Java 1/ Only concrete classes can be instantiated. 2/ Only concrete classes can be instantiated. 3/ The type of any Object must be of type Concrete_class. (corollary to 1 and 2) 4/ References may be concrete, abstract, or interface. Fight Club Rules of Java 5/ You may create type-anonymous abstractclasses and interfaces by overriding ALL their contract methods when you declare them. 6/ The “type” of a type-anonymous class is $x aka Robert Paulson. Java Event Model • Used to program behavior in GUIs • Used extensively in Android. If a tree falls in a forest and no one is around to hear it, does it make a sound? Event-Source (Button) No Event-Listener listening Event (onClick) No Catcher No Event Listener Event-Source (Button) Event-Listener listening Event (onClick) ActionList ener Any Object Catcher ready to catch Wrong Event Event source not registered Event-Source (Button) Event-Listener listening Event (Action) ActionListener OnMouseListener Any Object Event (Mouse) Catcher ready to catch Anonymous inner classes Type anonymous : Fight Club - Robert Paulson https://www.youtube.com/watch?v=GCi_PIz5ek U $X Only concrete classes Rules of Fight club https://www.youtube.com/watch?v=MPITE_Inec Model a system • Object-oriented programming is about modeling a system. • Write the problem out as requirements -- this will essentially be a math world problem (your favorite kind to solve!) • Your computer objects map directly to realworld objects (nouns). • Your methods map directly to real-world actions (verbs). Write a very simple application for a contact manager. The the sake of simplicity, each contact will have a name and a phone number only. The user should be able to create new contacts and diplay all contacts. Build a GUI Write a very simple application for Latin dictionary. The the sake of simplicity, each entry has a Latin word and an English equivalent, which could be several words. The user should be able to add a new definition, delete a definition. Build a GUI Describe the system: The game of BlackJack; a single player plays against the house for money. His bet is consistently $100.00 and he starts with 1,000.00. There is a shoe of six 52-card decks which is reshuffled when the shoe is half used. If the player wins the hand, his gets his bet back plus the amount of the bet. If he loses, he loses the money, and if he gets blackjack, he get's his bet * 1.5. The player plays against a dealer who must follow the following strict rules; aces are worth 11points only, and the dealer must hit on 16 or below. The player however, is allowed to hit or hold on any hand-value. Furthermore, aces are worth either 1 or 11, whichever is more advantageous. An initial two hands are dealt on seperate sides of a table consisting of two cards apiece. The dealer's hand displays only one card up. The player has the option to hit, hold, split, double-down, buy insurance, etc. For this initial version of the game, we'll consider only hit, hold, and deal. blue are nouns (objects or fields) red are verbs (methods) Interfaces • A class implements an interface rather than extends it. Any class that implements the interface must override all the interface methods with it's own methods. • Interface names often end with "able" to imply that they add to the capabilty of the class. • An interface is a contract; it defines the methods Some fun videos Reference anonymous - gravity https://www.youtube.com/watch?v=vKW-Gd_S_xc Concrete classes: rules of fight club https://www.youtube.com/watch?v=vJMC_S-DB2I Type anonymous : Fight Club - Robert Paulson https://www.youtube.com/watch?v=GCi_PIz5ekU Dubugger: x-men quicksilver https://www.youtube.com/watch?v=WGDXO9mlprM Debugger (lecture 06) Breakpoints, Bookmarks, TODO • • • • • • • • Alt-2 (see breakpoints and bookmarks) Cntl-F11 (add bookmark) //TODO your comment here Left-click left-margin to add breakpoint Right-click red-circle to edit View breakpoints Conditional breakpoints All exceptions !(this instanceof java.lang.ClassNotFoundException) Stepping through code • • • • F8 step-over (used most frequently) F7 step-into Shift-F8 step-out If you want to continue execution until the next breakpoint is reached or end of execution is reached. Press PLAY Watching and evaluating • Variables: usually provides enough data • Notice that changing data is blue when you step • You can right-click any primitive or object and “add to watches” • Right-click to “evaluate expression” JUNIT Need to add junit-4.xx.jar and hamcrest-core1.1.jar and add as library to project Get the following plugin: generateTestCases Ctrl-Shift-A “download” Downloads... Browse repositories Get GenerateTestCases plugin How to use the Form Designer in IntelliJ best video on Form Designer in Intellij (in German, genießen) https://www.youtube.com/watch?v=0I_1HYMUQrg (ok video, no sound, English subtitles) https://www.youtube.com/watch?v=G4jMzEGMKfg Public and Private (so far) Modifier | Class | Package | Subclass | World ————————————+———————+—————————+—————————— +——————— public | ✔ | ✔ | ✔ | ✔ ————————————+———————+—————————+—————————— +——————— protected | ✔ | ✔ | ✔ | ✘ ————————————+———————+—————————+—————————— +——————— no modifier | ✔ | ✔ | ✘ | ✘ ————————————+———————+—————————+—————————— +——————— private | ✔ | ✘ | ✘ | ✘