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12: The Java I/O System

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12: The Java I/O System

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12: The Java
I/O System

Creating a good input/output (I/O) system is one of the more difficult tasks for the language designer.

This is evidenced by the number of different approaches. The challenge seems to be in covering all eventualities. Not only are there different sources and sinks of I/O that you want to communicate with (files, the console, network connections, etc.), but you need to talk to them in a wide variety of ways (sequential, random-access, buffered, binary, character, by lines, by words, etc.). Feedback

The Java library designers attacked this problem by creating lots of classes. In fact, there are so many classes for Java’s I/O system that it can be intimidating at first (ironically, the Java I/O design actually prevents an explosion of classes). There was also a significant change in the I/O library after Java 1.0, when the original byte-oriented library was supplemented with char-oriented, Unicode-based I/O classes. In JDK 1.4, the nio classes (for “new I/O,” a name we’ll still be using years from now) were added for improved performance and functionality. As a result, there are a fair number of classes to learn before you understand enough of Java’s I/O picture that you can use it properly. In addition, it’s rather important to understand the evolution history of the I/O library, even if your first reaction is “don’t bother me with history, just show me how to use it!” The problem is that without the historical perspective, you will rapidly become confused with some of the classes and when you should and shouldn’t use them. Feedback

This chapter will give you an introduction to the variety of I/O classes in the standard Java library and how to use them. Feedback

The File class

Before getting into the classes that actually read and write data to streams, we’ll look at a utility provided with the library to assist you in handling file directory issues. Feedback

The File class has a deceiving name; you might think it refers to a file, but it doesn’t. It can represent either the name of a particular file or the names of a set of files in a directory. If it’s a set of files, you can ask for that set using the list( ) method, which returns an array of String. It makes sense to return an array rather than one of the flexible container classes, because the number of elements is fixed, and if you want a different directory listing, you just create a different File object. In fact, “FilePath” would have been a better name for the class. This section shows an example of the use of this class, including the associated FilenameFilter interface. Feedback

A directory lister

Suppose you’d like to see a directory listing. The File object can be listed in two ways. If you call list( ) with no arguments, you’ll get the full list that the File object contains. However, if you want a restricted list—for example, if you want all of the files with an extension of .java—then you use a “directory filter,” which is a class that tells how to select the File objects for display. Feedback

Here’s the code for the example. Note that the result has been effortlessly sorted (alphabetically) using the java.utils.Arrays.sort( ) method and the AlphabeticComparator defined in Chapter 11:

//: c12:DirList.java
// Displays directory listing using regular expressions.
// {Args: "D.*.java"}
import java.io.*;
import java.util.*;
import java.util.regex.*;
import com.bruceeckel.util.*;

public class DirList {
  public static void main(String[] args) {
    File path = new File(".");
    String[] list;
    if(args.length == 0)
      list = path.list();
      list = path.list(new DirFilter(args[0]));
    Arrays.sort(list, new AlphabeticComparator());
    for(int i = 0; i < list.length; i++)

class DirFilter implements FilenameFilter {
  private Pattern pattern;
  public DirFilter(String regex) {
    pattern = Pattern.compile(regex);
  public boolean accept(File dir, String name) {
    // Strip path information, search for regex:
    return pattern.matcher(
      new File(name).getName()).matches();
} ///:~

The DirFilter class “implements” the interface FilenameFilter. It’s useful to see how simple the FilenameFilter interface is: Feedback

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

It says 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 structure is often referred to as a callback. More specifically, this is an example of the Strategy Pattern, because list( ) implements basic functionality, and you provide the Strategy in the form of a FilenameFilter in order to complete the algorithm necessary for list( ) to provide its service. 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. Feedback

DirFilter shows that just because an interface contains only a set of methods, you’re not restricted to writing only those methods. (You must at least provide definitions for all the methods in an interface, however.) In this case, the DirFilter constructor is also created. Feedback

The accept( ) method must accept a File object representing the directory that a particular file is found in, and a String containing the name of that file. You might choose to use or ignore either of these arguments, but you will probably at least use the file name. Remember that the list( ) method is calling accept( ) for each of the file names in the directory object to see which one should be included; this is indicated by the boolean result returned by accept( ). Feedback

To make sure the element you’re working with is only the file name and contains no path information, all you have to do is take the String object and create a File object out of it, then call getName( ), which strips away all the path information (in a platform-independent way). Then accept( ) uses a regular expression matcher object to see if the regular expression regex matches the name of the file. Using accept( ), the list( ) method returns an array. Feedback

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