newbie question about java's syntax

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donovan - 09 Dec 2005 02:54 GMT

I have a background in c++ and this syntax looks basically just like
C++. What throws me off is the first line and the last line. I'm
thinking it's a statement because it has ; at the very end, but it looks
like a function definition sqeezed in as an argument of a function call.
Can someone give me a clue?

...

ipField.addFocusListener(new FocusAdapter() {
public void focusLost(FocusEvent e) {
ipField.selectAll();
if (connectionStatus != DISCONNECTED) {
changeStatusNTS(NULL, true);
}
else {
hostIP = ipField.getText();
}
}
});

...

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Mark - 09 Dec 2005 03:18 GMT

> I have a background in c++ and this syntax looks basically just like
> C++. What throws me off is the first line and the last line. I'm
[quoted text clipped - 17 lines]
>
> ...

This is an anonymous class definition.
Basically you are defining a class that extends FocusAdapter and then
instantiating it in one hit, (without naming the class).

I does exactly the same thing as:

ipField.addFocusListener(new MyFocusAdapter());

where elsewhere you define:
class MyFocusAdapter extends FocusAdapter() {
public void focusLost(FocusEvent e) {
ipField.selectAll();
if (connectionStatus != DISCONNECTED) {
changeStatusNTS(NULL, true);
}
else {
hostIP = ipField.getText();
}
}
});

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Roedy Green - 09 Dec 2005 03:25 GMT

>I have a background in c++ and this syntax looks basically just like
>C++. What throws me off is the first line and the last line. I'm
>thinking it's a statement because it has ; at the very end, but it looks
>like a function definition sqeezed in as an argument of a function call.
>Can someone give me a clue?

this is an anonymous class. See
http://mindprod.com/jgloss/anonymousclasses.html

Signature

Canadian Mind Products, Roedy Green.
http://mindprod.com Java custom programming, consulting and coaching.

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Bjorn Abelli - 09 Dec 2005 08:24 GMT

"donovan" wrote...

>I have a background in c++ and this syntax looks basically
> just like C++. What throws me off is the first line and
> the last line. I'm thinking it's a statement because it
> has ; at the very end,

It is. If you look more closely, you'll see that it ends the following
statement:

ipField.addFocusListener( ... );

> but it looks like a function definition sqeezed in
> as an argument of a function call.

Not quite, but almost... ;-)

You're instantiating an object, which will be the argument of the function
call:

ipField.addFocusListener(new FocusAdapter() {...} );

> Can someone give me a clue?

As FocusAdapter is an abstract class, you need to implement an own subclass
of it, which in Java can be done "anonymously", i.e. without giving it a new
class name, just as you've done.

new FocusAdapter() {
public void focusLost(FocusEvent e) {
ipField.selectAll();
if (connectionStatus != DISCONNECTED) {
changeStatusNTS(NULL, true);
}
else {
hostIP = ipField.getText();
}
}
}

// Bjorn A

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donovan - 10 Dec 2005 17:01 GMT

I see. neat'o.
thanks

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newbie question about java's syntax

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