How does this code do what it does?

[Paul Salvaggio]

This IO example is taken from Chapter 11 of Bruce Eckel's FREE online book
called 'Thinking in Java' (http://www.MindView.net). In reference to the
DirectoryFilter class in the example below, Bruce explains: "The whole reason
behind the creation of this class is to provide the accept( ) method to the
list( ) method so that list( ) can call back accept( ) to determine which
file names should be included in the list. Thus, this technique is often
referred to as a callback or sometimes a functor (that is, DirFilter is a
functor because its only job is to hold a method)."

Questions: I am confused as to how the code below actually calls the method
named 'accept'. I was under the impression that in order to run a method
from a class other than a constructor, you had to physically call it, for
example: DirFilter.accept(arg1,arg2). Where, when and how is the accept()
method called? What am I not understanding?

//: c11irList.java
// Displays directory listing.
import java.io.*;
import java.util.*;
import com.bruceeckel.util.*;

public class DirList {
public static void main(String[] args) {
try {
File path = new File(".");
String[] list;
if(args.length == 0)
list = path.list();
else
list = path.list(new DirFilter(args[0]));
Arrays.sort(list,
new AlphabeticComparator());
for(int i = 0; i < list.length; i++)
System.out.println(list[i]);
} catch(Exception e) {
e.printStackTrace();
}
}
}

class DirFilter implements FilenameFilter {
String afn;
DirFilter(String afn) { this.afn = afn; }
public boolean accept(File dir, String name) {
// Strip path information:
String f = new File(name).getName();
return f.indexOf(afn) != -1;
}
} ///:~

[Paul Clapham]

In this line:

list = path.list(new DirFilter(args[0]));

Object "path"'s method "list" is called, with a parameter that is a new
DirFilter object. As far as "list" is concerned, it receives an object that
implements FilenameFilter, which is a standard java.io interface that must
implement the "accept" method. We don't see the code here, but we can
assume that "list" is going to call "accept" -- what else could it do with
that parameter?

PC2

Paul Salvaggio <standingo@yahoo.com> wrote in message
news:39cd01d0$1@news.devx.com...
>
> This IO example is taken from Chapter 11 of Bruce Eckel's FREE online book
> called 'Thinking in Java' (http://www.MindView.net). In reference to the
> DirectoryFilter class in the example below, Bruce explains: "The whole
reason
> behind the creation of this class is to provide the accept( ) method to
the
> list( ) method so that list( ) can call back accept( ) to determine which
> file names should be included in the list. Thus, this technique is often
> referred to as a callback or sometimes a functor (that is, DirFilter is a
> functor because its only job is to hold a method)."
>
> Questions: I am confused as to how the code below actually calls the
method
> named 'accept'. I was under the impression that in order to run a method
> from a class other than a constructor, you had to physically call it, for
> example: DirFilter.accept(arg1,arg2). Where, when and how is the accept()
> method called? What am I not understanding?
>
> file://: c11irList.java
> // Displays directory listing.
> import java.io.*;
> import java.util.*;
> import com.bruceeckel.util.*;
>
> public class DirList {
> public static void main(String[] args) {
> try {
> File path = new File(".");
> String[] list;
> if(args.length == 0)
> list = path.list();
> else
> list = path.list(new DirFilter(args[0]));
> Arrays.sort(list,
> new AlphabeticComparator());
> for(int i = 0; i < list.length; i++)
> System.out.println(list[i]);
> } catch(Exception e) {
> e.printStackTrace();
> }
> }
> }
>
> class DirFilter implements FilenameFilter {
> String afn;
> DirFilter(String afn) { this.afn = afn; }
> public boolean accept(File dir, String name) {
> // Strip path information:
> String f = new File(name).getName();
> return f.indexOf(afn) != -1;
> }
> } ///:~
>

[Vikram Rajan]

hi,

I agree with Paul Clapham. The only conceivable place where accept could
be called is inside the list method.

Vikram