Shadowing this
When an inner class refers to
this,
it refers to the current instance of the inner class.
To refer to the instance of the outer class from the
inner class,
this must be qualified
by the name of the outer class.
Example of referring to outer this from inner class
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
|
// ShadowThis.java -- Illustrate shadowing of this.
// Fred Swartz - 2003 - Jun
class ShadowThis {
public static void main(String[] args) {
ShadowThis st = new ShadowThis();
st.testOuter();
}//end main
private void testOuter() {
System.out.println(this);
InnerShadowThis ist = new InnerShadowThis();
ist.testInner();
}//end test
class InnerShadowThis {
void testInner() {
System.out.println(this);
System.out.println(ShadowThis.this);
}
}//end class InnerShadowThis
}//end class ShadowThis
|
Produces the following output.
ShadowThis@601bb1
ShadowThis$InnerShadowThis@ea2dfe
ShadowThis@601bb1
Typical usage
The above example fails to show why referencing an outer
this might be used. A common
situation is to implement an action listener as an
inner class of a JPanel. If the listener wants to
display a dialog (eg, a JFileChooser), it may tell
which component to center the dialog over.
The component is exactly the outer
this.
For example, this is from an inner class listener that
wants to center a file dialog over the current TestGUI
panel.
int retval = fileChooser.showOpenDialog(TestGUI.this);
Some IDEs put no code in an inner class listener except a call to
an outer method to process the action. The same code could be used,
but the need to refer to the outer
this
is no longer necessary.