Java Reference
In-Depth Information
mentthatreturnsavalueofthistype(e.g.,
return 0;
),thecompilerwillnot
reportanerror.However,youwon'tbeabletorun
HelloWorld
becausethe
proper
main()
method would not exist.
•
(String[] args)
:Thisparameterlistconsistsofasingleparameternamed
args
oftype
String[]
.Startupcodepassesasequenceofcommand-linear-
gumentsto
args
,whichmakestheseargumentsavailabletothecodethatex-
The block of code consists of a single
System.out.println("Hello,
world!");
methodcall.Fromlefttowrite,
System
identifiesastandardclassofsys-
tem utilities,
out
identifies an object variable located in
System
whose methods let
you output values of various types optionally followed by a newline character to the
standardoutputdevice,
println
identifiesamethodthatprintsitsargumentfollowed
byanewlinecharactertostandardoutput,and
"Hello, world!"
isa
string
(ase-
quenceofcharactersdelimitedbydoublequote
"
charactersandtreatedasaunit)thatis
passedastheargumentto
println
andwrittentostandardoutput(thestarting
"
and
ending
"
double quote characters are not written; these characters delimit but are not
part of the string).
Note
AlldesktopJava/nonJavaapplicationscanberunatthecommandline.Before
graphical user interfaces with their controls for inputting and outputting values (e.g.,
textfields), these applications obtained their input and generated their output with the
help of
Standard I/O
, an input/output mechanism that originated with the Unix oper-
atingsystem,andwhichconsistsofstandardinput,standardoutput,andstandarderror
devices.
The user would input data via the standard input device (typically the keyboard, but
a file could be specified instead—Unix treats everything as files). The application's
output would appear on the standard output device (typically a computer screen, but
optionally a file or printer). Output messages denoting errors would be output to the
standarderrordevice(screen,file,orprinter)sothatthesemessagescouldbehandled
separately.
Now that you understand how
Listing 1-1
works, you'll want to create this applica-
tion. Complete the following steps to accomplish this task:
2. Execute
javac HelloWorld.java
tocompilethissourcefile.
javac
will
complain if you do not specify the “
.java
” file extension.
