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The javautilzip package provides the ability to read and write files in the popular ZIP and GZIP file formats Both ZIP and GZIP input and output streams are available Other classes implement the ZLIB algorithms for compression and decompression
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The javautiljar package provides the ability to read and write Java Archive (JAR) files You will see in 25 that JAR files are used to contain software components known as Java Beans and any associated files
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17: Input/Output: Exploring javaio
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This chapter explores javaio, which provides support for I/O operations In 12, we introduced Java's I/O system Here, we will examine the Java I/O system in greater detail As all programmers learn early on, most programs cannot accomplish their goals without accessing external data Data is retrieved from an input source The results of a program are sent to an output destination In Java, these sources or destinations are defined very broadly For example, a network connection, memory buffer, or disk file can be manipulated by the Java I/O classes Although physically different, these devices are all handled by the same abstraction: the stream A stream, as explained in 12, is a logical entity that either produces or consumes information A stream is linked to a physical device by the Java I/O system All streams behave in the same manner, even if the actual physical devices they are linked to differ Note For an overview of Java's stream-based I/O, see 12
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The Java I/O Classes and Interfaces
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The I/O classes defined by javaio are listed here: BufferedInputStream FileWriter PipedInputStream PipedOutputStream PipedReader PipedWriter PrintStream PrintWriter PushbackInputStream PushbackReader RandomAccessFile Reader SequenceInputStream SerializablePermission StreamTokenizer StringReader StringWriter
BufferedOutputStream FilterInputStream BufferedReader BufferedWriter FilterOutputStream FilterReader
ByteArrayInputStream FilterWriter ByteArrayOutputStream InputStream CharArrayReader CharArrayWriter DataInputStream DataOutputStream File FileDescriptor FileInputStream FileOutputStream FilePermission InputStreamReader LineNumberReader ObjectInputStream ObjectInputStreamGetField ObjectOutputStream ObjectOutputStreamPutField ObjectStreamClass ObjectStreamField OutputStream
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FileReader
OutputStreamWriter
Writer
The ObjectInputStreamGetField and ObjectOutputStreamPutField inner classes were added by Java 2 The javaio package also contains two classes that were deprecated by Java 2 and are not shown in the preceding table: LineNumberInputStream and StringBufferInputStream These classes should not be used for new code The following interfaces are defined by javaio: DataInput DataOutput Externalizable FileFilter FilenameFilter ObjectInput ObjectInputValidation ObjectOutput ObjectStreamConstants Serializable
The FileFilter interface was added by Java 2 As you can see, there are many classes and interfaces in the javaio package These include byte and character streams, and object serialization (the storage and retrieval of objects) This chapter examines several of the most commonly used I/O components, beginning with one of the most unique: File
File
Although most of the classes defined by javaio operate on streams, the File class does not It deals directly with files and the file system That is, the File class does not specify how information is retrieved from or stored in files; it describes the properties of a file itself A File object is used to obtain or manipulate the information associated with a disk file, such as the permissions, time, date, and directory path, and to navigate subdirectory hierarchies Files are a primary source and destination for data within many programs Although there are severe restrictions on their use within applets for security reasons, files are still a central resource for storing persistent and shared information A directory in Java is treated simply as a File with one additional property-a list of filenames that can be examined by the list( ) method The following constructors can be used to create File objects: File(String directoryPath) File(String directoryPath, String filename) File(File dirObj, String filename) Here, directoryPath is the path name of the file, filename is the name of the file, and dirObj is a File object that specifies a directory The following example creates three files: f1, f2, and f3 The first File object is constructed with a directory path as the only argument The second includes two arguments-the path and the filename The third includes the file path assigned to f1 and a filename; f3 refers to the same file as f2 File f1 = new File("/"); File f2 = new File("/","autoexecbat"); File f3 = new File(f1,"autoexecbat");
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Note Java does the right thing with path separators between UNIX and Windows/DOS conventions If you use a forward slash (/) on a Windows version of Java, the path will still resolve correctly Remember, if you are using the Windows/DOS convention of a backslash character (\\), you will need to use its escape sequence (\\\\) within a string The Java convention is to use the UNIX- and URL-style forward slash for path separators File defines many methods that obtain the standard properties of a File object For example, getName( ) returns the name of the file, getParent( ) returns the name of the parent directory, and exists( ) returns true if the file exists, false if it does not The File class, however, is not symmetrical By this, we mean that there are many methods that allow you to examine the properties of a simple file object, but no corresponding function exists to change those attributes The following example demonstrates several of the File methods: // Demonstrate File import javaioFile; class FileDemo { static void p(String s) { Systemoutprintln(s); } public static void main(String args[]) { File f1 = new File("/java/COPYRIGHT"); p("File Name: " + f1getName()); p("Path: " + f1getPath()); p("Abs Path: " + f1getAbsolutePath()); p("Parent: " + f1getParent()); p(f1exists() "exists" : "does not exist"); p(f1canWrite() "is writeable" : "is not writeable"); p(f1canRead() "is readable" : "is not readable"); p("is " + (f1isDirectory() "" : "not" + " a directory")); p(f1isFile() "is normal file" : "might be a named pipe"); p(f1isAbsolute() "is absolute" : "is not absolute"); p("File last modified: " + f1lastModified()); p("File size: " + f1length() + " Bytes"); }
When you run this program, you will see something similar to the following: File Name: COPYRIGHT Path: /java/COPYRIGHT Abs Path: /java/COPYRIGHT Parent: /java exists is writeable is readable is not a directory is normal file is absolute File last modified: 812465204000 File size: 695 Bytes Most of the File methods are self-explanatory isFile( ) and isAbsolute( ) are not isFile( ) returns true if called on a file and false if called on a directory Also, isFile( ) returns false for some special files, such as device drivers and named pipes, so this method can be used to make sure the file will behave as a file The isAbsolute( ) method returns true if the file has an absolute path and false if its path is relative
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File also includes two useful utility methods The first is renameTo( ), shown here: boolean renameTo(File newName) Here, the filename specified by newName becomes the new name of the invoking File object It will return true upon success and false if the file cannot be renamed (if you either attempt to rename a file so that it moves from one directory to another or use an existing filename, for example) The second utility method is delete( ), which deletes the disk file represented by the path of the invoking File object It is shown here:
boolean delete( ) You can also use delete( ) to delete a directory if the directory is empty delete( ) returns true if it deletes the file and false if the file cannot be removed Java 2 adds some new methods to File that you might find helpful in certain situations Some of the most interesting are shown here: Method void deleteOnExit( ) Description Removes the file associated with the invoking object when the Java Virtual Machine terminates Returns true if the invoking file is hidden Returns false otherwise Sets the time stamp on the invoking file to that specified by millisec, which is the number of milliseconds from January 1, 1970, Coordinated Universal Time (UTC) Sets the invoking file to read-only
boolean isHidden( )
boolean setLastModified(long millisec) boolean setReadOnly( )
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