He rb Sc h i ldt s Java P rog ra m m i n g Cookbook in Java

Generating Data Matrix ECC200 in Java He rb Sc h i ldt s Java P rog ra m m i n g Cookbook

Classes and Interfaces javaioFile Methods File[ ] listFiles( ) File[ ] listFiles(FileFilter ff) String getName( ) boolean isDirectory( ) boolean accept(File name)

Another common file-related task is obtaining a list of the files within a directory For example, you might want to obtain a list of all files in a directory so that they can be transmitted to a back up site or so that you can confirm that all files for an application have been properly installed Whatever the purpose, obtaining a directory listing can be conveniently accomplished using the File class

Java provides a variety of ways to obtain a directory listing The approach used by this recipe involves these steps: 1 Create a File object that represents the directory 2 Confirm that the File object exists and actually represents a valid directory (In cases in which the File object is known to represent an existent directory, this step can be skipped) 3 If you want to obtain a filtered list of files (such as those with a specified file extension), create a FileFilter object that describes the pattern that the files must match 4 To obtain a directory listing, call listFiles( ) on the File object It returns an array of File objects that represent the files in the directory There are three versions of listFiles( ) One gives you all files; the other two let you filter the files 5 To obtain the name of the file, call File s getName( ) method 6 If you are only interested in normal files and not directories, use the isDirectory( ) method to determine which files represent directories

See Obtain File Attributes for a description of File s constructors, and the exists( ) and isDirectory( ) methods It is important to understand that a directory listing can only be obtained if the File object represents a directory Therefore, it is often necessary to confirm that the object is, indeed, a valid directory before attempting to obtain the file list

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File defines three versions of listFiles( ) The first returns an array of strings that contains all files (including those that represent subdirectories) Thus, it obtains an unfiltered list of files It is shown here: File[ ] listFiles( ) To obtain a filtered file list, you can use one of the other two forms The one used by this recipe is shown here: File[ ] listFiles(FileFilter ff) It obtains a list of only those files (and directories) that meet the criteria specified by ff Given the array containing the directory listing, you can obtain the name of each file by calling getName( ), which is defined by File It is shown here: String getName( ) The string representation of the file (or directory) name is returned To restrict the list of files to only those that fit a certain criteria, implement a FileFilter FileFilter is an interface that specifies only one method: accept( ) It is shown here: boolean accept(File name) This method must return true for files that you want to be part of the directory listing and false for those that are to be excluded Depending upon how you implement accept( ), it can be used to include all files whose names fit a general pattern For example, here is a simple implementation that accepts all Java source files:

// A simple file filter for Java source files class JavaFiles implements FileFilter { public boolean accept(File f) { if(fgetName()endsWith("java")) return true; return false; } }

For more sophisticated filtering, you can use regular expressions With this approach, you can easily create filters that handle wildcards, match alternatives, ignore case differences, and so on (An overview of regular expressions is found in 2)