CS 112 Introduction to Programming - Yale University

CS 112 Introduction to Programming
(Spring 2012)
Lecture #3: Programming Environment
Zhong Shao
Department of Computer Science
Yale University
Office: 314 Watson
http://flint.cs.yale.edu/cs112
Acknowledgements: some slides used in this class are taken directly or adapted from those accompanying the two textbooks: Introduction to Programming in Java: An
Interdisciplinary Approach by Robert Sedgewick and Kevin Wayne and Building Java Programs: A Back to Basics Approach by Stuart Reges and Marty Stepp

Programming in Any Language
q Programming consists of three tasks
❍ Create and edit source code file
• The source code is usually human understandable
❍ Compile the source code into machine code
• Machine code ranges from vaguely understandable to
nondescript 0's and 1’s
❍ Execute/run/test machine code
• Your computer (specifically the CPU) executes the
machine code, 1 instruction at a time.

Java Programming
q Programming in Java consists of the same tasks, but they are
slightly specialized
o
Create and edit “Java source code” (.java files)
• Eg. Hello.java
q Compile into “Java bytecode” (.class files)
o
Eg. javac Hello.java to produce Hello.class
q Execute/run/test bytecode with “Java interpreter”
o
Eg. java Hello
source code
compile
byte code
run
output

A Simple Java Program
public class Hello {
public static void main(String[] args) {
System.out.println("Hello, world!");
System.out.println();
System.out.println("This program produces");
System.out.println("four lines of output");
}
}
Program output (in red for emphasis):
$ java Hello
Hello, world!
This program produces
four lines of output
$
4

Java Is Simple
q All you have to do is to write a text file
using the correct syntax, and then:
1) Execute “javac” with the filename
l
This gives you a .class file
2) Execute “java” with the .class file
q You're probably wondering, “How do I write
a text file using the correct syntax”
❍ There are two main choices
5

Writing Java Programs
q The basic way is to use a text editor
❍ Example editors: Notepad, emacs, pico, vim,
kwrite, kate, gedit, etc.
• Note: MS Word is NOT a text editor
❍ The key is that your .java file cannot include
any markup or stylistic formatting; just text.
❍ You enter your Java code following Java
Language syntax, then you compile it.
6

Writing Java Programs (Step 1): Create/Edit
Create/Edit the program by typing it into a text
editor, and save it as HelloWorld.java.
/*******************************************
* Prints "Hello, World"
* Everyone's first Java program.
*******************************************/
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World");
}
}
HelloWorld.java

Writing Java Programs (Step 2): Compile
Create the program by typing it into a text editor, and
save it as HelloWorld.java.
Compile it by typing at the command-line:
javac HelloWorld.java.
command-line
% javac HelloWorld.java
This creates a Java bytecode file named: HelloWorld.class.

Writing Java Programs (Step 3): Execute
Create the program by typing it into a text editor, and
save it as HelloWorld.java.
Compile it by typing at the command-line:
javac HelloWorld.java.
Execute it by typing at the command-line:
java HelloWorld.
command-line
% javac HelloWorld.java
% java HelloWorld
Hello, World

Writing Java Programs using an IDE
q Another way is to use an Integrated Development
Environment (IDE)
❍ Example IDEs: DrJava, Eclipse, NetBeans, BlueJ, etc.
❍ An IDE usually presents the user with a space for text (like an
editor) but layers additional features on top of the text for
the user's benefit.
• Note: The underlying file contains pure text, just like a text editor.
❍ These features can be very useful and save time.
• Example features are code completion, instant compilation, and
syntax highlighting.
❍ IDEs need to know where to find the “java” and “javac”
programs.
q For simplicity and consistency, we will use DrJava.
10

Setting up DrJava on your Laptop
q Please follow the directions on the main
textbook site --- see Part 1 of Assignment 0.

Pantheon Environment
The "Pantheon" is a collection of
computers at Yale named after
Greek gods
Generally, you use Secure Shell to
login to eli.yale.edu; eli.yale.edu will
automatically direct you to
one of the machines
http://pantheon.yale.edu/help/ssh/
They run a variant of Unix
Linux
General help on pantheon is available at:
http://pantheon.yale.edu/help/
12

Login, Edit, Compile, Run on a Pantheon Computer
- The Pantheon is the first method of Java Programming (ie. text
editor, no IDE)
- Login to a pantheon machine using ssh
❍ Instructions at http://www.yale.edu/its/stc/faq/Pantheon
/SSHTerminal.html
- Create a directory for your cs112 files for the semester
$ mkdir cs112
- Change to the directory
$ cd cs112
- Edit your Java program using any text editor, e.g., pico
$ pico HelloWorld.java
type in your program, press control-X, type y, press return
13

Login, Edit, Compile, Run on a Pantheon Machine
(Cont.)
- Compile a Java program
$ javac HelloWorld.java
- Take a look to see that HelloWorld.class is generated
$ ls
HelloWorld.java HelloWorld.class
- Run Java interpreter
$ java HelloWorld
14

q White space
Syntax: White Space
❍ includes spaces, new line characters, tabs
❍ white space is used to separate words and
symbols in a program
❍ extra white space is ignored
q White space allows a Java program to be
formatted in many ways, and should be
formatted to enhance readability
o the usage of white space forms part of
programming style
15

Syntax: Comments
q Two types of comments in Java
• single-line comments use //…
// this comment runs to the end of the line
• multi-lines comments use /* … */
/* this comment runs to the terminating
symbol, even across line breaks */
q Comments are ignored by the compiler:
❍ used only for human readers (i.e., inline
documentation)
16

Using comments
q Where to place comments:
❍ at the top of each file (a "comment header")
❍ at the start of every method (see later)
❍ to explain complex pieces of code
q Comments are useful for:
❍ Understanding larger, more complex programs.
❍ Multiple programmers working together, who
must understand each other's code.

Practice Slides

Questions
q What is the output of the following println
statements
System.out.println("\ta\tb\tc");
System.out.println("\\\\");
System.out.println("'");
System.out.println("\"\"\"");
System.out.println("C:\nin\the downward
spiral");
q Write a println statement to produce this
output:
/ \ // \\ /// \\\

Answers
q Output of each println statement:
\\
'
"""
C:
in
a b c
he downward spiral
q println statement to produce the line of output:
System.out.println("/ \\ // \\\\ /// \\\\\\");

Questions
q What println statements will generate this output
This program prints a
quote from the Gettysburg Address.
"Four score and seven years ago,
our 'fore fathers' brought forth on
this continent a new nation."
q What println statements will generate this output
A "quoted" String is
'much' better if you learn
the rules of "escape sequences."
Also, "" represents an empty String.
Don't forget: use \" instead of " !
'' is not the same as "

Answers
q println statements to generate the output:
System.out.println("This program prints a");
System.out.println("quote from the Gettysburg Address.");
System.out.println();
System.out.println("\"Four score and seven years ago,");
System.out.println("our 'fore fathers' brought forth on");
System.out.println("this continent a new nation.\"");
q println statements to generate the output:
System.out.println("A \"quoted\" String is");
System.out.println("'much' better if you learn");
System.out.println("the rules of \"escape sequences.\"");
System.out.println();
System.out.println("Also, \"\" represents an empty
String.");
System.out.println("Don't forget: use \\\" instead of
\" !");
System.out.println("'' is not the same as \"");