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3 Major Steps for Creating/Running a
Java Program
• You write the source code for the program
and save it as a .java file.
• You compile the .java program using the
compiler (javac). When you compile, it
checks for syntax errors. When it successfully
compiles, it creates a java bytecode file that
ends in .class
• You interpret and execute the bytecode
using the interpreter (the Java Virtual Machine,
java)
IDE
• IDE stands for Integrated Development Environment.
• It is basically a fancy text editor that gives you easy access
to the compiler and interpreter
• If you get an IDE, you still have to download the JDK
before you can use it
• Although there are better IDEs out there, some of them are
kind of complicated (like Oracle's NetBeans IDE and the
Eclipse IDE) and others cost money (like developer
versions of IDEs like JBuilder), so to make it easier we are
going to use a useful, free, and easy one called JCreator
• If you want to try Oracle's NetBeans, you can download
the "bundle" on the same page that you download the JDK
from
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public class CompSci
{
}
All Java programs start with a class.
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public class CompSci
{
public static void main(String[] args)
{
System.out.println("Comp Sci!");
}
}
OUTPUT
Comp Sci!
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public class CompSci
{ //open brace
public static void main(String[] args)
{
System.out.println("Comp Sci!");
}
} //close brace
Braces – You gotta have ‘em! Every class
and every method must have a { and a } .
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public class CompSci
{
public static void main(String[] args)
{
System.out.println("Comp Sci!");
}
}
You must put a semi-colon at the end of
all Java program statements ( ; ).
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Never put a ;
before an open { brace
;{ //illegal
}; //legal
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public class CompSci
{
public static void main(String[] args)
{
System.out.println("Comp Sci!");
}
}
Indent all code 3 spaces to make it easier to read.
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System.out
frequently used methods
Name
Use
print(x)
print x and stay on the current line
println(x)
print x and move to next line down
printf(s,x)
print x according to s specifications
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reference
command / method
System.out.print("compsci");
OUTPUT
compsci
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System.out.print("compsci");
System.out.print("compsci");
OUTPUT
compscicompsci
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System.out.println("compsci");
OUTPUT
compsci
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System.out.println("compsci");
System.out.println("compsci");
OUTPUT
compsci
compsci
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\n
\t
\r
\b
newline
tab
carriage return
backspace
System.out.println("c\tompsci");
OUTPUT
c
ompsci
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\n
\t
\r
\b
newline
tab
carriage return
backspace
System.out.println("com\tpsci");
OUTPUT
com
psci
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\n
\t
\r
\b
newline
tab
carriage return
backspace
System.out.println("comp\nsci");
OUTPUT
comp
sci
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\\
\"
\’
outs \
outs "
outs ’
System.out.println("\\compsci\"/");
OUTPUT
\compsci"/
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\\
\"
\’
outs \
outs "
outs ’
System.out.println("\\'comp\'sci\'/");
OUTPUT
\'comp'sci'/
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Escape Sequences
frequently used combinations
Name
Use
\t
tabs over five spaces
\n
moves to front of next line
\b
deletes previous character
\r
moves to front of current line
\\
nets one backslash \
\"
nets one double quote "
\’
nets one single quote ’
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//
/* */
single-line comments
block comments
//this line prints stuff on the screen
System.out.println("stuff");
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//
/* */
/*
single-line comments
block comments
this line prints stuff on the screen
*/
System.out.println("stuff");
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System.out.printf("%s","compsci\n");
OUTPUT
compsci
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Syntax errors occur when you
type something in wrong,
causing the code to not compile.
//missing semicolon - ; expected
System.out.println("stuff")
//case problem – should be System
system.out.println("stuff");
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Runtime errors occur when
something goes wrong while the
program is running.
//an out of bounds exception is thrown
String s = "runtime_error";
System.out.println( s.charAt(15) );
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Importing
• There is a whole bunch of code already written for
Java and included with it...why start from scratch with
each program?
• To use pre-existing code, you need to import the
class (ie, the file containing the code) at the top.
• To see a list of existing classes that you can use, go to
the Java API (Application Program Interface)
http://download.oracle.com/javase/7/docs/api/
• You can think of the classes in the API as building
blocks you can use to make your programs
• The JOptionPane class is for simple GUIs...we'll use
much more of it and many API classes in the future
Importing in Acition
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