COMS%20W1004%20Introduction%20to%20Computer%20Science

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COMS W1004Introduction to Computer Science

• May 30, 2008

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Last Time

• Course intro
• What is Computer Science?
• What is an algorithm?
• Examples of algorithms
• Searching Sequential, Binary
• Sorting Selection, Insertion
• Pseudocode
• Comparing algorithms (SG chapter 3)

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Today

• Java compilation and execution
• Intro to programming concepts
• Java datatypes
• Your first Java program!
• Java programming environments
• Reading LL 1-2

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Dealing with compiler errors

• If you get a whole bunch of errors, sometime
  fixing the first error will make the others go
  away. So always start at the top (with the first
  error) and try to address that one before looking
  at the others.
• You may find that fixing an error causes more
  errors to appear. That is because the compiler
  makes two passes through the code, and looks for
  different things each time. So don't despair if
  you fix something and other errors appear.

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Class name and file name

• Calc.java7 class Calculator is public, should
  be declared in a file named Calculator.java
• public class Calculator
• This means that the name of the class does not
  match the name of the file. Remember, they MUST
  be the same, including capitalization!

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Cannot find symbol

• Calculator.java13 cannot find symbol
• symbol class Scanner
• location class Calculator
• Scanner scan new Scanner(System.in)
• Any error that starts "cannot find symbol" means
  that the compiler doesn't know what that symbol
  (which can be a variable or a class name) refers
  to.
• In the second line of the error, where it says
  "symbol class Scanner", that indicates that it
  doesn't know what the Scanner class is.
• This probably means that you forgot to include
  the line "import java.util.Scanner" at the top
  of the file.

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Cannot find symbol

• Calculator.java13 cannot find symbol
• symbol constructor Scanner()
• location class java.util.Scanner
• Scanner scan new Scanner()
• Again we see an error that says "cannot find
  symbol". This time, it is "constructor
  Scanner()". We will see what a constructor is
  later on, but in this case it probably means that
  you forgot to put "System.in" inside the
  parentheses.

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Cannot find symbol

• Calculator.java19 cannot find symbol
• symbol method nextdouble()
• location class java.util.Scanner
• double x scan.nextdouble()
• Yet another "cannot find symbol" error. Here it
  doesn't know "method nextdouble()". Can you guess
  why? Because it should be "nextDouble", with a
  capital D. Be very careful about capitalization!

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expected

• Calculator.java14 '' expected
• This error message is slightly deceiving. Clearly
  it means that you forgot a semicolon. However, it
  tells you that the error is on line 14. But that
  is not correct! (Sometimes) The forgotten
  semicolon might actually belong on the PREVIOUS
  line, which would be line 13 in this case. Don't
  ask why...

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) expected

• Calculator.java29 ')' expected
• System.out.println("The sum is " sum)
• In this case, the error is a bit misleading
  because it says that a right parenthesis is
  expected. However, as you can see, the error is
  actually that you need a plus sign between the
  two things you are concatenating.

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expected

• Calculator.java45 '' expected
• This probably means that you forgot a curly brace
  somewhere. The easiest way to avoid this is to
  vertically line up the left and right curly
  braces and indent accordingly.

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Announcements

• Homework 1 is available on the website
• Programming homework is due Weds, June 4
• Theory homework is due Monday, June 9
• Office hours start on Monday

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Programming Homework 1

• Start with the Hello, World and Calculator
  examples and build upon them
• Dont try to do too much at once
• Write a few lines, compile, repeat
• Dont worry about error handling yet

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Extra Credit!

• First, in your home directory, type cp .profile
  .profileOld
• This creates a backup of your .profile file.
• Now use pico or emacs to edit the file .profile
  adding the following two lines at the end of the
  file. Please note the single quotes and the
  spacing. export CLASSPATHCLASSPATHcdm6/retin
  a/
• alias javac'cdm6/retina/compile'