Fear Not Demon Curses

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If we make even the tiniest mistake during our incantations, our demons will hurl dire curses at us, called error messages.

(A) First Incantations

Consider this simplest of demons:

public class SimpleDemon
  static public void main(String[] args)
    System.out.println("Hello world!");
  • What is the name of the demon we are creating with this incantation?
  • Paste it into Dr.Java.
  • Save the new file onto your H drive.
  • Compile it and then run it.
  • Where does the result appear?
  • What is the result and how could you change it? Change it!
  • The line with the System.out.println incantation ends with a semicolon. What does that make the line?
  • Add still more System.out.println incantations! What happens?

This demon invocation contains two major incantations. An incantation is magical lore that you need not understand, yet when repeated will still command our demons. The first incantation, static public void main(String[] args), is how we create a class of demons called console applications. The second incantation, System.out.println("Hello world!"); is a statement that commands demons to print something to the console. The console is the bottom pane of Dr. Java, or in the Windows Command Prompt application. The muggle textbook tells how to do the latter in chapter 2.4.

(B) Now comes the wierdness!

  • Remove one of the curly braces, save,compile and run.
  • Write down the error message the demon screams.
  • Does that error message make sense to you? Does it show where you needed that curly brace?
  • Try removing each of the curly braces, one at a time, and answer the same questions.
  • Try adding a single curly brace, and answer the same questions.
  • Do the curses change?
  • They are always true, but are they ever helpful?

(C) Trial by fire!

  • Restore the unindented version of HelloGraphics from the previous class.
  • One of you remove a curly brace or add an extra curly brace without your partners looking, and challenge your partners to fix the curly braces.
  • Everybody take turns!

Improperly matched braces are the most common and confusing errors. Knowing this, you can repair many demon summonings with confidence, even if you are unsure of their meaning.

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