Agenda Lec05Exceptions API

Reference- and Type-AnonymitySwing

Review

Exceptions:

Checked versus Unchecked Exceptions

http://www.ibm.com/developerworks/java/library/j-

jtp05254/

Probable Source

Example of Error: System-out-of-memory ErrorExamples of RuntimeException: ClassCastException, NumberFormatException.Example of checked Exceptions: EOFExcception,

Within your control Probable Source Checked User should Recover

Error NO JVM NO NO

RuntimeException (unchecked)

YES Programmer NO NO

All other exceptions(checked)

NO User, External resources, Thread

YES YES

Your program running in JVM

Hosting system

Green = uncheckedBlue = checked

Situations which cause Exceptions (or Errors) to be thrown

• An internal Error occurs in the VM (OutOfMemoryError)

• 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 Why did the engineers of the Java language decide to create Unchecked Exceptions?decide to create Unchecked Exceptions?

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 green-zone are recoverable from the chaos of the exterior environment.

Checked versus Unchecked

The debate over checked and unchecked

See: http://www.ibm.com/developerworks/java/library/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

checkedMost unchecked “programers fault” exception 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

TimeTestOuterActionListener

yes EventListenerOuter In separate java file

TimeTestInner ActionListener

yes EventListenerInner In same java file

TimeTestLocalActionListener

yes anonymous Same method

TimeTestAnonActionListener

no anonymous inline

See inner example

Cross-Cutting Concerns

• Exceptions• Assertions• Logging

BlackJack 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 Paulsonhttps://www.youtube.com/watch?v=GCi_PIz5ekU

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 real-world 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

Reference anonymous - gravity https://www.youtube.com/watch?v=vKW-Gd_S_xc

Concrete classes: rules of fight clubhttps://www.youtube.com/watch?v=vJMC_S-DB2I

Type anonymous : Fight Club - Robert Paulsonhttps://www.youtube.com/watch?v=GCi_PIz5ekU

Dubugger: x-men quicksilver https://www.youtube.com/watch?v=WGDXO9mlprM

Some fun videos

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-core-1.1.jar and add as library to project

Get the following plugin: generateTestCasesCtrl-Shift-A “download” Downloads...Browse repositoriesGet GenerateTestCases plugin

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

How to use the Form Designer in IntelliJ