Java Reference
In-Depth Information
Java syntax: new-expression
new
class-name
(
arguments
)
Example
:
new
JFrame(
"
title 1
"
)
Purpose
: Create a new folder of class
class-name
, ini-
tialize its fields using the call
class-name
(
argu-
ments
)
, and yield the name of the new folder as
the value of the new-expression.
1.3.3
Objects of class JFrame
Class
javax.swing.JFrame
is a good class to study first because you will be
able to use it and see immediately the effects of calling methods of one of its
manila folders. An instance of
JFrame
is associated with a window on your com-
puter monitor. To create an object of class
JFrame
, use this assignment:
JFrame window=
new
JFrame("first JFrame");
Figure. 1.4 shows three things. On the left is the filing cabinet with a drawer for
class
JFrame
. To the right is the object, drawn like a manila folder, that is creat-
ed by execution of the above assignment; this object belongs in
JFrame
's file
drawer. The name on the tab of the folder,
a3
, is arbitrary but different from the
names of all other folders. Finally, Fig. 1.4 contains a variable
window
, with the
name
a3
as its value. We describe the process that created the picture.
First, the
new-expression
is evaluated:
new
JFrame("first JFrame")
Evaluating this new-expression is a three-step process:
1. Create a new folder of class
JFrame
and give it a name.
2. Execute the call
JFrame("first JFrame")
, which causes the
argument
to be stored somewhere in the folder. This
String
will be the title on the
window.
3. Yield the name of the folder as the value of the new-expression.
Take a look near the top left of the folder in Fig. 1.4. It contains “
JFrame
(String)
”, which indicates that method
JFrame
is in the folder and, when this
method is called, it has to have an argument that is a
String
. This is the method
that is called in step 2 above.
Second, once the new-expression yields its value,
a3
, the assignment state-
ment stores that value in variable
window
.
The contents of folder
a3
We investigate the contents of folder
a3
in Fig. 1.4. It has method
JFrame(String)
, as discussed above, several other instance methods, and an
instance variable
title
.
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