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Exceptions

Exceptions. What are they? Why are they useful? What else can happen at runtime…?. Wouldn’t it be wonderful? Or would it? From website http://www.dribbleglass.com/subpages/strange/exceptions.htm. What are they?.

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Exceptions

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  1. Exceptions • What are they? • Why are they useful? • What else can happen at runtime…? Wouldn’t it be wonderful? Or would it? From website http://www.dribbleglass.com/subpages/strange/exceptions.htm

  2. What are they? • The program seems to be running along just fine, minding its own business and all, when suddenly… Exception in thread "AWT-EventQueue-0" java.lang.ArithmeticException: / by zero at EyeBall.look(EyeBall.java:47) at Eye.look(Eye.java:78) at Eye.mouseMoved(Eye.java:56) at java.awt.AWTEventMulticaster.mouseMoved(AWTEventMulticaster.java:271) at java.awt.Component.processMouseMotionEvent(Component.java:5533) at java.awt.Component.processEvent(Component.java:5257) at java.awt.Container.processEvent(Container.java:1966) at java.awt.Window.processEvent(Window.java:1153) at java.awt.Component.dispatchEventImpl(Component.java:3955) at java.awt.Container.dispatchEventImpl(Container.java:2024) at java.awt.Window.dispatchEventImpl(Window.java:1766) at java.awt.Component.dispatchEvent(Component.java:3803) at java.awt.EventQueue.dispatchEvent(EventQueue.java:463) at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:234) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:163) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149) at java.awt.EventDispatchThread.run(EventDispatchThread.java:110) Good grief! What’s all this mean?

  3. What are they? – Run-time errors! • Java detects a problem in executing the code. • Java “throws an exception.” • If you don’t “catch” the exception, the program stops and… • Displays “where it was” at many levels – the “dump” you see like when JavaEyes divided by zero.  Start at the top, looking at the dump’s info!

  4. Why are they useful? • Exceptions tell us about serious problems running the program, which Java can’t deal with. • We then have a chance to fix those… Would we be better off if Java hadn’t told us? Generic exception code: try { … } catch (…)

  5. What else can happen…? • Exceptions are not the only run-time error… • There are plenty of “runtime” errors you have to spot yourself. • For example… I wanted three eyeballs, not two! That button was supposed to say “Exit”, not “Quit”! Why isn’t the red square moving?

  6. And of course… • Hey – DoublePedant is producing the wrong output! Rule 1 of software development is:Everything the program is supposed to do must be tested or… It doesn’t work!!!

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