Command-line Java
optional: encoding messages

Compiling and Running Java from the command line

We can still write, compile, and run Java programs without using an IDE like BlueJ or Eclipse.

To write your program, use any editor. Try jEdit (Just just type type that into the search box of the Start menu, or choose Start > Programs > College of Science and Technology > Editors > jEdit.)
You can also use any other editor ¹, e.g. pfe (programmer's file editor, which happens to be what the lab computers are configured to start if you click on a .java file in a browser).
A class named Foo must be contained inside a file named Foo.java.
Running and compiling will be done by the command-line, so the first step is to bring up a command shell. In Windows, Start and type cmd into the search box. (On Mac, run the program Terminal, found in /Applications/Utilities).
From the command shell, useful commands are:
- H: to change your current-directory to your H: drive.
- dir (directory-listing), to see the list of files in your current directory.
  (This is the text analog of looking at the icons in a folder.)
- cd someDir (change directory) to change your current-directory into the folder someDir.
  (This is the text analog of double-clicking on a folder.)
To compile your program from the command-shell:
javac Foo.java
If there are errors, the compiler will print them and stop. If everything works fine, nothing apparent will happen, but the file Foo.class has been created. (Use dir to look for it.)

Note that Foo.class contains a version of your program translated into byte code, a language that can be understood by the computer directly ².
To run your program from the command-shell:
java Foo
This causes the Java runtime system to find the class Foo (in the file Foo.class), and run its main method, whose signature has to be public static void main(String[] args).
(Now you see why main must be public and static.)

Your task

cd to a directory on your H: drive where you keep your ITEC120 files.
Write a program with just a main that prints out main to world, main to world, come in world?. Compile and run it.
Modify your main so that it prompts the user to enter their favorite number and their favorite beverage, and (for now) just prints out that information.
```
Enter your favorite number: -3
Enter your favorite beverage: ginger-ale
I read the int -3 and the string ginger-ale.
```

Instead of printout that information, print the user's favorite verse of the beverage song.

Enter your favorite number: -3
Enter your favorite beverage: ginger-ale
In that case, you'll love this verse:
-3 bottles of ginger-ale on the wall,
-3 bottles of ginger-ale;
  take one down,
  pass it around,
-4 bottles of ginger-ale on the wall.

Of course you'll modify copy in your lab10 solution to add main appropriately. (You can also use posted solution [link opens better in Firefox than IE].)

Finally: Modify main so that it does the above repeatedly, until they enter control-C (sometimes written ^C) to signify the end-of-input, instead of a number+name.
Windows ^C vs Unix ^D:
On unix (and mac), use control-D (^D) to signify end-of-input rather than control-C (for Windows).

unverified: On Windows-8, you also need to use control-D?

Use java.util.Scanner#hasNext.
Scanner timing: Note that like all Scanner methods, hasNext can feel slightly counterintuitive since it answers about the next input past or future; if there is no past input queued up, then the function-call will block until some input finally does arrive. This can be mis-interpreted as an infinite loop, if the user doesn't realize the computer is waiting for them to type something.
```
Enter your favorite number (or ^C to quit): -3
Enter your favorite beverage: ginger-ale
In that case, you'll love this verse:
-3 bottles of ginger-ale on the wall,
…
-4 bottles of ginger-ale on the wall.

Enter your favorite number: 12345
Enter your favorite beverage: cheerwine
In that case, your favorite song must be:
12345 bottles of cheerwine on the wall,
…
12344 bottles of cheerwine on the wall.

Enter your favorite number (or ^C to quit): ^C
Bye!
```

Practice: Encryption (optional)

Write a program which reads in a message, and prints out an encrypted version of the message (for any of the three low-brow encryption schemes mentioned below, or one of your own).
In all cases though, use helper functions as appropriate. (Can you think of useful helper functions you'll write, in advance?)

Vowel-removal: Remove all vowels and spaces between words.
```
Please enter your message (^C when done): Four score and seven years ago
Frscrndsvnyrsg
Our forefath--hmmm, what about our foremothers, were they doing nothing?
rfrfth--hmm,whtbtrfrmthrs,wrthydngnthng?
^C
bye!
```
Note that although your output is just one long line, it might get printed across several lines because our display is intertwining our output with the user's input³ (which contains newlines).

rot13: rotate every letter by 13 places (wrapping around). For your first version, assume that your input is all lower-case letters (although handling upper-case letters correctly and leaving punctuation unchanged is a fine improvement to make).

Please enter your message (^C when done): Four score and seven years ago
Sbhe fpber naq frira lrnef ntb
Our forefath--hmmm, what about our foremothers, were they doing nothing?
Bhe sbersngu--uzzz, jung nobhg bhe sberzbguref, jrer gurl qbvat abguvat?
Sbhe fpber!
Four score!
^C
Bye!

pig-latin: Convert the message to pig latin.

Please enter your message (^C when done): Four score and seven years ago
ourFay orescay anday evensay earsay agoay
Our forefath--hmmm, what about our foremothers, were they doing nothing?
Ouray orefath--hmmm,ay atwhay aboutay ouray oremothers,fay ereway eythay oingday othing?nay
^C
Bye!

You are of course welcome to handle punctuation better.

Thoughts:

Which of the above encryptions are fully, 100% invertable? That is, can any message always be decrypted? (If not: give two different inputs which encrypt to the same result.)
Add a newline to the output after printing more than 80 characters (or better yet, before printing 80 characters).

If you really want: Let the user select which of the three encryption schemes they want.

Welcome to Encryption Central.
Pick your scheme:
(1) vowel-removal
(2) rot13 
(3) pig-latin
(0) quit
What is your choice? 2
Enter your message (^C to end): Hello there
Uryyb gurer
^C

Welcome to Encryption Central.
Pick your scheme:
(1) vowel-removal
(2) rot13 
(3) pig-latin
(0) quit
What is your choice? 2
Enter your message (^C to end): Uryyb gurer
Hello there
^C

Welcome to Encryption Central.
Pick your scheme:
(1) vowel-removal
(2) rot13 
(3) pig-latin
(0) quit
What is your choice? 0
Bye!

(Hint: after ^C is typed, you have to create a new Scanner to read further input.)

¹ You could even use Microsoft Word, if you save-as plain text. Word isn't tailored for Java though, so it won't give you any help with indentation or editing. You'd want to switch to a monospaced font like Courier or Consolas. ↩

² Actually, byte code is interpreted by the Java Virtual Machine (JVM); but the JVM is a program that your computer does run directly. ↩

³ Technically, the issue is that the terminal automatically echoes all typed input. ↩

This page licensed CC-BY 4.0 Ian Barland
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(incl. typos, broken links)
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Command-line Java optional: encoding messages

Compiling and Running Java from the command line

Your task

Practice: Encryption (optional)

Command-line Java
optional: encoding messages