Section 5.8. Files and Directories

5.8. Files and Directories

The java.io.File class represents a file or a directory and defines a number of important methods for manipulating files and directories. Note, however, that none of these methods allow you to read the contents of a file; that is the job of java.io.FileInputStream , which is just one of the many types of I/O streams used in Java and discussed in the next section. Here are some things you can do with File :

 import java.io.*; import java.util.*; // Get the name of the user's home directory and represent it with a File File homedir = new File(System.getProperty("user.home")); // Create a File object to represent a file in that directory File f = new File(homedir, ".configfile"); // Find out how big a file is and when it was last modified long filelength = f.length(); Date lastModified = new java.util.Date(f.lastModified()); // If the file exists, is not a directory, and is readable,  // move it into a newly created directory. if (f.exists() && f.isFile() && f.canRead()) {       // Check config file   File configdir = new File(homedir, ".configdir"); // A new config directory   configdir.mkdir();                                // Create that directory   f.renameTo(new File(configdir, ".config"));       // Move the file into it } // List all files in the home directory String[] allfiles = homedir.list(); // List all files that have a ".java" suffix String[] sourcecode = homedir.list(new FilenameFilter() {   public boolean accept(File d, String name) { return name.endsWith(".java"); } }); 

The File class gained some important additional functionality as of Java 1.2:

 // List all filesystem root directories; on Windows, this gives us // File objects for all drive letters (Java 1.2 and later).  File[] rootdirs = File.listRoots(); // Atomically, create a lock file, then delete it (Java 1.2 and later) File lock = new File(configdir, ".lock"); if (lock.createNewFile()) {   // We successfully created the file.  Now arrange to delete it on exit   lock.deleteOnExit();   // Now run the application secure in the knowledge that no one else   // is running it at the same time   ... } else {   // We didn't create the file; someone else has a lock   System.err.println("Can't create lock file; exiting.");   System.exit(1); } // Create a temporary file to use during processing (Java 1.2 and later) File temp = File.createTempFile("app", ".tmp");  // Filename prefix and suffix // Do something with the temp file   ... // And delete it when we're done temp.delete(); 

5.8.1. RandomAccessFile

The java.io package also defines a RandomAccessFile class that allows you to read binary data from arbitrary locations in a file. This can be useful in certain situations, but most applications read files sequentially, using the stream classes described in the next section. Here is a short example of using RandomAccessFile :

 // Open a file for read/write ("rw") access File datafile = new File(configdir, "datafile"); RandomAccessFile f = new RandomAccessFile(datafile, "rw");   f.seek(100);                   // Move to byte 100 of the file byte[] data = new byte[100];   // Create a buffer to hold data f.read(data);                  // Read 100 bytes from the file int i = f.readInt();           // Read a 4-byte integer from the file f.seek(100);                   // Move back to byte 100 f.writeInt(i);                 // Write the integer first f.write(data);                 // Then write the 100 bytes f.close();                     // Close file when done with it