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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
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
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?


What are they run time errors
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!


Why are they useful
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 (…)


What else can happen
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?


And of course
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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