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 input/output streams used in Java and discussed in the next section. Here are some things you can do with File:
import java.io.*; // 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 provides 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, so do something ... // Then delete the lock file lock.delete(); } else { // We didn't create the file; someone else has a lock System.err.println("Can't create lock file; exiting."); System.exit(0); } // Create a temporary file to use during processing (Java 1.2 and later) File temp = File.createTempFile("app", ".tmp"); // Filename prefix and suffix // Make sure file gets deleted when we're done with it (Java 1.2 and later) temp.deleteOnExit();
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 a useful thing to do 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
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