Java Input/Output CSE301 University of Sunderland Harry Erwin, PhD Half Lecture

Background Java’s I/O model is defined in terms of streams—ordered sequences of data with a source or destination. Initially, these were (8-bit) byte streams like cin, cout, and cerr for C++. The corresponding byte streams for Java are: –System.in (an InputStream) –System.out (a PrintStream) –System.err (an unbuffered PrintStream) Later, Unicode character streams were added. These are called Readers and Writers. These classes and interfaces are found in java.io.* This is only a short introduction to a long subject! Read the text! Test questions on this material!

Stacking Streams The important concept in working with Java I/O is stacking streams. That is, you apply adaptors to a stream to specialise it for a specific task. For example, a File can be used via a FileStream. A.gz file can be used via a GZIPInputStream.

Some Byte Stream Operations java.io.InputStream (base class for byte input) –int read()—reads a byte –int read(byte[])—reads an array of bytes –void close()—closes the stream java.io.OutputStream (base class for byte output) –void write(int)—writes a byte passed as int –void write(byte[])—writes an array –void flush()—flushes the stream –void close()—closes the stream

Some Character Stream Operations java.io.Reader –int read()—reads a Unicode character –int read(char[])—reads a character array –boolean ready()—returns true if there is data to read –void close() java.io.Writer –void write(int)—writes a character –void write(char[]) –void write(String)—writes a String –void flush() –void close()

Formatted Input/Output DataInput and DataOutput are interfaces used to transfer data other than bytes or characters (e.g., binary data). Implemented by: –DataInputStream—readType() –DataOutputStream—writeType() –RandomAccessFile—implements both and allows positioning in the file. where Type includes all the primitive types. readLine(), writeBytes(String), writeChars(String), and writeUTF(String) are used for String data.

Data Conversion The wrapper classes for the primitive types (e.g, Double for double) provide static methods for data conversion from a String to that type. All classes also define a toString() method to convert the class data to a String. This is used in println() and when Strings are constructed using the + and += operators. If you want this method to be useful, you should override the default method inherited from Object. Test questions!

Various InputStreams FileInputStream—built around a file. PipedInputStream—PipedInputStreams are connected to PipedOutputStreams and are used between threads. ByteArrayInputStream—Has an internal byte array provided by the creator of the stream and where the data are stored. StringBufferInputStream—Like a ByteArrayInputStream. The internal buffer is a String. SequenceInputStream—Reads from multiple streams, finishing one before going on to the next.

More InputStreams FilterInputStream—Generally built around some other InputStream, using it as a source of data and doing further conversions. This ‘stacking’ can involve multiple filters. Examples include: –BufferedInputStream—Buffers the input and supports mark and reset operations. –DataInputStream—Formatted input operations. –LineNumberInputStream—Reads one line at a time, keeping count –PushBackInputStream—Allows the last byte to be ‘unread’ if desired. Useful for user interfaces.

Various OutputStreams PrintStream—provides print(arg) and println(arg) operations to output data in a readable form. Both operate by applying the toString() method to arg and then printing the resulting String. println() outputs an end of line as well. File-, Piped-, ByteArray-, Filter-, Buffered-, and DataOutputStream work generally as you would be expect.

Unicode Character Streams BufferedReader, BufferedWriter, LineNumberReader CharArrayReader, CharArrayWriter FilterReader, PushBackReader, FilterWriter InputStreamReader, OutputStreamWriter, FileReader, FileWriter PipedReader, PipedWriter StringReader, StringWriter PrintWriter

Files and Directories An instance of class File represents the pathname (a String) to some file or directory. File creation and deletion are handled through the File class. There are some problems having to do with pathnames for files in Java. No bugs; just a collection of surprises.

File Input/Output A FileInputStream or FileOutputStream is built around a pathname or a File. Otherwise, it is basically a Stream of the appropriate type. You can build a FilterInputStream or a FilterOutputStream around these streams.

Data Compression Specialized filter streams exist to operate on data. E.g., java.util.zip.GZIPOutputStream if created around another OutputStream will apply gzip compression to the data. You can also use other classes in the java.util package to generate a checksum for the stream. The class java.util.zip.ZipFile allows you to access a ZIP archive and read the entries through a stream.

Interfacing to Arrays The ByteArrayXXX, StringReader/ StringWriter and CharArrayXXX streams allow you to read and write data stored in byte arrays and character strings.

Serialization This is the capacity to serialize objects, sending them over a stream and deserializing them at the other end to reconstruct the object. This can also be used to save objects offline. java.io.ObjectOutputStream and java.io.ObjectInputStream participate in this. Methods used: writeObject() and readObject(). Used in: –Remote Method Invocation (RMI) –Handling persistent objects –Distributed agent networks

Networking In the java.net.* package. Supports TCP and UDP protocols. URL class is used to represent a Uniform Resource Locator Also supports sockets/ports with the Socket class. I have watched one of my students write a working IRC client in Java in less than 15 minutes. Secure Socket Layer is available in Java 1.4.

Logging New in Java 1.4 In the java.util.logging.* package. Automates the creation of a log with multiple severity thresholds. Can be used to create an audit trail for security.

Non-Blocking I/O New for Java 1.4 In java.nio.* Traditionally, Java I/O was blocking, with the thread having to wait for its data. This is non-blocking I/O. Supports high-performance I/O.

XML New for Java 1.4 Supports XML documents and document formats. SAXP, JAXP, and XDOC. Used in JavaServer Pages. If you care about XML, you care a lot.