Question

I have this code here. (this code was taken from Thinking in Java 4th Edition)

//{Args: "D.*\.java}
import  java.util.regex.*;
import java.io.*;
import java.util.*;

public class DirList {
    public static void main (String[] args){
        File path = new File(".");
        String[] list;
        if(args.length == 0)
            list = path.list();
        else
            list = path.list(new DirFilter(args[0]));

        Arrays.sort(list,String.CASE_INSENSITIVE_ORDER);
        for(String dirItem : list ){
            System.out.println(dirItem);
        }
    }
}

class DirFilter implements FilenameFilter{
    private Pattern pattern;
    public DirFilter(String regex){
        pattern = Pattern.compile(regex);
    }

    public boolean accept(File Dir,String name){
        return pattern.matcher(name).matches();
    }
}

And then as I read its explanation I have encountered this

DirFilter’s sole reason for existence 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 structure is often referred to as a callback.

Soo what is specifically a callback?

Answer 1

A callback is a method that you provide to a library that you are using so that the library can call your method to perform work (or rather call back to your code, thus the name).

More generally, any two layers of code can interact through a callback function.