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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 classand 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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© A+ Computer Science - www.apluscompsci.com
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System.outfrequently 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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System.out.print("compsci");
reference command / method
OUTPUTcompsci
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System.out.print("compsci");System.out.print("compsci");
OUTPUTcompscicompsci
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System.out.println("compsci");
OUTPUTcompsci
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System.out.println("compsci");System.out.println("compsci");
OUTPUTcompscicompsci
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System.out.println("c\tompsci");
\n newline\t tab\r carriage return\b backspace
OUTPUTc ompsci
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System.out.println("com\tpsci");
OUTPUTcom psci
\n newline\t tab\r carriage return\b backspace
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System.out.println("comp\nsci");
OUTPUTcompsci
\n newline\t tab\r carriage return\b backspace
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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 Sequencesfrequently 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 screenSystem.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");
OUTPUTcompsci
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© A+ Computer Science - www.apluscompsci.com
Syntax errors occur when you type something in wrong, causing the code to not compile.
//missing semicolon - ; expectedSystem.out.println("stuff")
//case problem – should be Systemsystem.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 thrownString 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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