Object-Oriented Programming with Java Lecture Notes

19 March 2009 • I-O &c

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Outline

• File management.

• Random-access files.

• New I-O.

• I-O Interfaces.

File Management

• An instance of the File class represents a file in the local file system.

  • For some definition of “local”.

• The represented file may or may not exist.

  • The exits() method tells you that.

  • The createNewFile() method creates the file if it doesn't exist.

File-Management Methods

• File has methods to query the file status.

  • Name, parent, length, last-modified time.

• It has methods to manipulate files.

  • Create, delete, and rename, for example.

Files vs. Directories

• The Unix-oid point of view blurs the distinction between files and directories.

  • Java does the same, but other systems don't.

• Use the isDirectory() method to test for directory-ness.

• Use the mkdir() method (more Unixisms) to turn a File into an existing directory (permission permitting).

Random Access

• Random access allows easy access to all parts of the byte stream.

  • Random access is not a property of all streams.

• I-O occurs relative to a file pointer, a reference to a particular byte in the stream.

• Methods query and set the file pointer position.

Random Access Files

• The RandomAccessFile represents random-access streams.

  • It implements the DataInput and DataOutput interfaces.

    • Providing readBoolean(), writeDouble() and so on.

• RandomAccessFile constructors only accept File instances or names of files, guarenteeing the random-access capability.

Random-Access Methods

• The seek(long n) method sets the file pointer to n.

  • raf.seek(raf.length) moves to the end of the file.

• getFilePointer() returns the file pointer.

• All the DataInput and DataOutput methods (e.g., writeDouble() and readBoolean()).

New I-O

• Java's original I-O abstractions weren't suitable for life behind the server.

  • File locking or asynchronous I-O capabilities, for example.

• There were other, more general problems too.

  • Internal-external character representation mismatches.

• The New I-O (NIO) classes were designed to deal with these problems.

  • Introduced in Java 1.4.

Feature Interfaces

• Interfaces can be used to specify the (syntatic) behavior of a single class.

• An interface can also be used to specify the (syntatic) behavior of a particular feature.

  • A single feature is usually not enough to make an interesting class.

  • Comparable is an example of a single-feature interface.

    • It implies ordering among class instances.

  • A Single-feature interface is sometimes called a trait.

I-O Interfaces

• Java 5 introduced four new interfaces:

  1. Closeable: The class instance can be closed, releasing allocated resources.

     void close() throws IOException
     

  2. Flushable: The class instance can flushed, pushing data out of the pipe into the sink.

     void flush()
     

  3. Readable: The class instance is able to move characters from the source into a CharBuffer.

     int read(CharBuffer cb)
     

  4. Appendable: The class instance adds new characters to the end of existing characters, rather than overwriting them.

     Appendable append(char c)
     Appendable append(CharSequence cs)
     

I-O Interface Implementers

• Closeable: Input- and OutputStream, Reader, and Writer.

• Flushable: OutputStream and Writer.

• Reader: Reader.

• Appendable: Writer.