The File class

Bruce Eckel's Thinking in Java Contents | Prev | Next

A directory lister

Here’s the code for the example: (See page 97 if you have trouble executing this program.)

//: DirList.java
// Displays directory listing
package c10;
import java.io.*;
 
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]));
      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;
  }
} ///:~ 

The DirFilter class “implements” the interface FilenameFilter. (Interfaces were covered in Chapter 7.) It’s useful to see how simple the FilenameFilter interface is:

public interface FilenameFilter {
  boolean accept(File dir, String name);
}

It says that all that this type of object does is provide a method called accept( ). 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). Because list( ) takes a FilenameFilter object as its argument, it means that you can pass an object of any class that implements FilenameFilter to choose (even at run-time) how the list( ) method will behave. The purpose of a callback is to provide flexibility in the behavior of code.

Anonymous inner classes
//: DirList2.java
// Uses Java 1.1 anonymous inner classes
import java.io.*;
 
public class DirList2 {
  public static FilenameFilter 
  filter(final String afn) {
    // Creation of anonymous inner class:
    return new FilenameFilter() {
      String fn = afn;
      public boolean accept(File dir, String n) {
        // Strip path information:
        String f = new File(n).getName();
        return f.indexOf(fn) != -1;
      }
    }; // End of anonymous inner class
  }
  public static void main(String[] args) {
    try {
      File path = new File(".");
      String[] list;
      if(args.length == 0)
        list = path.list();
      else 
        list = path.list(filter(args[0]));
      for(int i = 0; i < list.length; i++)
        System.out.println(list[i]);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~ 

//: DirList3.java
// Building the anonymous inner class "in-place"
import java.io.*;
 
public class DirList3 {
  public static void main(final String[] args) {
    try {
      File path = new File(".");
      String[] list;
      if(args.length == 0)
        list = path.list();
      else 
        list = path.list(
          new FilenameFilter() {
            public boolean 
            accept(File dir, String n) {
              String f = new File(n).getName();
              return f.indexOf(args[0]) != -1;
            }
          });
      for(int i = 0; i < list.length; i++)
        System.out.println(list[i]);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~ 

The argument to main( ) is now final, since the anonymous inner class uses args[0] directly.

This shows you how anonymous inner classes allow the creation of quick-and-dirty classes to solve problems. Since everything in Java revolves around classes, this can be a useful coding technique. One benefit is that it keeps the code that solves a particular problem isolated together in one spot. On the other hand, it is not always as easy to read, so you must use it judiciously.

A sorted directory listing
//: SortedDirList.java
// Displays sorted directory listing
import java.io.*;
import c08.*;
 
public class SortedDirList {
  private File path;
  private String[] list;
  public SortedDirList(final String afn) {
    path = new File(".");
    if(afn == null)
      list = path.list();
    else
      list = path.list(
          new FilenameFilter() {
            public boolean 
            accept(File dir, String n) {
              String f = new File(n).getName();
              return f.indexOf(afn) != -1;
            }
          });
    sort();
  }
  void print() {
    for(int i = 0; i < list.length; i++)
      System.out.println(list[i]);
  }
  private void sort() {
    StrSortVector sv = new StrSortVector();
    for(int i = 0; i < list.length; i++)
      sv.addElement(list[i]);
    // The first time an element is pulled from
    // the StrSortVector the list is sorted:
    for(int i = 0; i < list.length; i++)
      list[i] = sv.elementAt(i);
  }
  // Test it:
  public static void main(String[] args) {
    SortedDirList sd;
    if(args.length == 0)
      sd = new SortedDirList(null);
    else
      sd = new SortedDirList(args[0]);
    sd.print();
  }
} ///:~ 

A few other improvements have been made. Instead of creating path and list as local variables to main( ), they are members of the class so their values can be accessible for the lifetime of the object. In fact, main( ) is now just a way to test the class. You can see that the constructor of the class automatically sorts the list once that list has been created.

The sort is case-insensitive so you don’t end up with a list of all the words starting with capital letters, followed by the rest of the words starting with all the lowercase letters. However, you’ll notice that within a group of file names that begin with the same letter the capitalized words are listed first, which is still not quite the desired behavior for the sort. This problem will be fixed in Java 1.2.

Checking for and creating directories

//: MakeDirectories.java
// Demonstrates the use of the File class to
// create directories and manipulate files.
import java.io.*;
 
public class MakeDirectories {
  private final static String usage =
    "Usage:MakeDirectories path1 ...\n" +
    "Creates each path\n" +
    "Usage:MakeDirectories -d path1 ...\n" +
    "Deletes each path\n" +
    "Usage:MakeDirectories -r path1 path2\n" +
    "Renames from path1 to path2\n";
  private static void usage() {
    System.err.println(usage);
    System.exit(1);
  }
  private static void fileData(File f) {
    System.out.println(
      "Absolute path: " + f.getAbsolutePath() +
      "\n Can read: " + f.canRead() +
      "\n Can write: " + f.canWrite() +
      "\n getName: " + f.getName() +
      "\n getParent: " + f.getParent() +
      "\n getPath: " + f.getPath() +
      "\n length: " + f.length() +
      "\n lastModified: " + f.lastModified());
    if(f.isFile())
      System.out.println("it's a file");
    else if(f.isDirectory())
      System.out.println("it's a directory");
  }
  public static void main(String[] args) {
    if(args.length < 1) usage();
    if(args[0].equals("-r")) {
      if(args.length != 3) usage();
      File 
        old = new File(args[1]),
        rname = new File(args[2]);
      old.renameTo(rname);
      fileData(old);
      fileData(rname);
      return; // Exit main
    }
    int count = 0;
    boolean del = false;
    if(args[0].equals("-d")) {
      count++;
      del = true;
    }
    for( ; count < args.length; count++) {
      File f = new File(args[count]);
      if(f.exists()) {
        System.out.println(f + " exists");
        if(del) {
          System.out.println("deleting..." + f);
          f.delete();
        }
      } 
      else { // Doesn't exist
        if(!del) {
          f.mkdirs();
          System.out.println("created " + f);
        }
      }
      fileData(f);
    }  
  }
} ///:~ 

In fileData( ) you can see the various file investigation methods put to use to display information about the file or directory path.



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