Java Reference
In-Depth Information
calls another constructor by using keyword this and a round bracket-delimited and
comma-separated list of arguments. For example, Image(String filename)
executes this(filename, null); to execute Image(String filename,
String imageType) .
Caution Youmustuse this tocallanotherconstructor—youcannotusetheclass's
name,asin Image() .The this() constructorcall(ifpresent)mustbethefirstcode
this( )constructorcallsinthesameconstructor.Finally,youcannotspecify this()
in a method—constructors can be called only by other constructors and during object
creation. (I will discuss methods later in this chapter.)
When present, the constructor call must be the first code that is specified within
a constructor; otherwise, the compiler reports an error. For this reason, a constructor
that calls another constructor can only perform additional work after the other con-
structor has finished. For example, Image(String filename) executes Sys-
tem.out.println("Image(String filename) called"); after the in-
voked Image(String filename, String imageType) constructor finishes.
The Image(String filename, String imageType) constructordeclares
an imageType parameterthatsignifiesthekindofimagestoredinthefile—aPortable
Network Graphics (PNG) image, for example. Presumably, the constructor uses im-
ageType tospeedupprocessingbynotexaminingthefile'scontentstolearntheimage
format.When null ispassedto imageType ,ashappenswiththe Image(String
filename) constructor, Image(String filename, String imageType)
examines file contents to learn the format. If null was also passed to filename ,
Image(String filename, String imageType) wouldn'treadthefile,but
would presumably notify the code attempting to create the Image object of an error
Afterdeclaringtheconstructors, Listing2-2 declaresa main() methodthatletsyou
create Image objectsandviewoutputmessages. main() createsthree Image objects,
calling the first constructor with no arguments, the second constructor with argument
"image.png" ,andthethirdconstructorwitharguments "image.png" and "PNG" .
Note Thenumberofargumentspassedtoaconstructorormethod,orthenumberof
operator operands is known as the constructor's, method's, or operator's arity .
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