Re: Newbie needs help understanding code snippet
NoSpam@aol.com wrote:
... Now I'm starting again at the beginning of the part on swing [sic]. I
don't understans a statement in an example, probably because I forgot
some
syntax I studied earlier but can't find now. Here is the code:
javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});
createAndShowGUI is defined in the example. I do not understand the
argement to invokeLater. Could someone explain each part of the
argument?
First, did you read
<http://java.sun.com/javase/6/docs/api/javax/swing/SwingUtilities.html#invokeLater(java.lang.Runnable)>
What to the statements and {} within an agrument mean?
This is an example of declaring an anonymous inner class.
It's not so much that it's within an argument but that it is the target of the
'new' operator.
Daniel Pitts wrote:
The snippet you probably need to understand is this:
new Runnable() {
public void run() {
createAndShowGUI();
}
}
Runnable is an interface. The snippet defines a brand new class that
implements the Runnable interface, and then creates a new instance of
that class. This is called an anonymous class, because it doesn't have
any name associated with it (well, behind the scenes it does, but you
don't worry about that generally)
So, you can do something like this:
public static void main(String[] args) {
Runnable runnable = new Runnable() {
public void run() {
System.out.println("This is run!");
}
}
// Now, runnable is a Runnable object with an implementation for run()
runnable.run();
System.out.println("This is main!");
// invoke again.
runnable.run();
}
So now, the SwingUtilities.invokeLater() call is taking a Runnable object.
See the Java Language Specification (JLS), section 15.9.5:
<http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.9.5>
The Java Tutorial
<http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html>
references a blog article at
<http://blogs.sun.com/darcy/entry/nested_inner_member_and_top>
and discusses anonymous classes briefly at
<http://java.sun.com/docs/books/tutorial/java/javaOO/innerclasses.html>
and more fully at
<http://java.sun.com/docs/books/tutorial/uiswing/events/generalrules.html#innerClasses>
For your future reference, java.sun.com is an excellent resource for such
knowledge. A minute's search there
<http://search.sun.com/main/index.jsp?qt=new+anonymous+&cid=113298&col=main-developer-learning>
turned up
<http://blogs.sun.com/darcy/entry/nested_inner_member_and_top>
You can create an anonymous inner class like the following:
ActionListener listener = new ActionListener() {
public void actionPerformed(ActionEvent ae) {
System.out.println("Anonymous class example");
}
};
Daniel Pitts wrote:
BTW, SwingUtilities.invokeLater() should be replaced with
EventQueue.invokeLater() when possible. They do the same thing,
SwingUtilities is just older (and calls upon EventQueue itself)
<http://java.sun.com/javase/6/docs/api/java/awt/EventQueue.html#invokeLater(java.lang.Runnable)>
Just to drive the point home, the API docs are the single richest source of
information on API calls.
Summary:
____________________
Tutorial
JLS
API docs
Search java.sun.com
____________________
If those aren't enough, IBM DeveloperWorks, Google.
--
Lew