Understanding Java I/O: Streams, Readers, Writers, and Object Serialization

1. What We Will Learn • Using directory listings • Using Streams • Decorators • Readers and Writers • Tokenizers • New I/O * • Compression * • Object Serialization

2. Files and Directories • File class in java.io package • Represents name of a file or a set of files • You cannot read or write data using File • Used for • Manipulating filenames • Working with directories • Checking file attributes

3. Input and Output • Looking into input and output in an abstract way • Input: • Reading data from a source • Source: anything that we can read data items from it • Output: • Writing data to a sink • Sink: anything that we can write data items to it

4. Streams • Abstract class representing a data source/sink • InputStream: • Represents a source • Provides read() methods • OutputStream: • Represent a sink • Provides write() methods

5. Kinds of Streams • Streams are categorized according to type of source or sink they represent • Files • Arrays of bytes • Strings • Pipes • Internet connections

6. InputStream Example publicvoidecho(InputStreamin) throwsIOException { intb; while ((b = in.read()) != -1) System.out.print((char)b); } • Using echo: • echo(System.in) • echo(new FileInputStream("test.txt")) • It works for any kind of input stream

7. Input Stream Classes • ByteArrayInputStream • Arrays of bytes • FileInputStream • Files • StringBufferInputStream • Strings • ObjectInputStream • Serialized objects

8. Adding Features • Adding various features to streams such as: • Compression • Line-numbering • Buffering • Push-back • Combining these features with every stream causes inheritance tree explosion! • CompressedFileInputStream • BufferedFileInputStream • CompressedBufferedFileInputStream, ...

9. Filter Streams • Wrap around other streams • Adding features to them • Without changing the interface • Such as: • BufferedInputStream • LineNumberInputStream • PushbackInputStream • DataInputStream • ZipInputStream

10. Filter Example • We want to read • from a file • which is compressed • with buffering mechanism • with ability to pushback data into it • We use one filter for each

11. Pushback I S Zip I S close Buffered I S close read File I S read close close read read Filter Example

12. Using Filters PushbackInputStreamin = newPushbackInputStream( newZipInputStream( newBufferedInputStream( newFileInputStream("in.dat")))); echo(in); in.pushback(89); in.close(); • After construction, in can be used as an ordinary input stream • Additional methods for pushback is available

13. Decorator Pattern • All filters are subclasses of FilterInputStream • Which is a InputStream itself • Takes an InputStream upon construction • And filters data going through it • The interface is not changed • Since the filter is also an Input Stream • This way to define classes to add features is called the Decorator pattern

14. Readers and Writers • A separate hierarchy but pretty similar to streams • Added from Java 1.1 to handle Unicode IO • Reader is similar to InputStream • Writer is similar to OutputStream • Similar usage of filters

15. Readers Example publicvoidechoLn(Readerin) throwsIOException { LineNumberReaderst = newLineNumberReader(in); Stringline; while ((line = st.readLine()) != null) System.out.println(st.getLineNumber() + ":" + line); } • Using echoLn: • echoLn(new FileReader("in.txt"));

16. Streams to Readers Readerr = newInputStreamReader(System.in); echoLn(r); • To read from a stream as a reader, use InputStreamReader • OutputStreamWriter is used for output • Reverse conversion is invalid

17. Formatted Input • Is not easy! • One way is to read lines and then parse items • Using tokenizers is another way • StringTokenizer: breaking strings into tokens • StreamTokenizer: reads from a stream or reader token by token

18. StreamTokenizer • Can be used as a scanner • Set of configurable items: • whitespaces, punctuations, comment characters • Can recognize • ordinary tokens • numbers • quoted strings • comments

19. New I/O • Added in JDK 1.4 • Has one goal: speed • You benefit from its speed • When writing code using nio • When using “old” I/O packages (because they have been reimplemented using nio) • You should read TIJ and learn nio yourselves!!!!!!

20. Compression • Java I/O has classes that support compression • You should read TIJ and learn compression yourselves!!!!!!

21. Object Serialization • Turning an object into a sequence of bytes • The object should implement the Serializable interface • Serializable is a tagging interface and has no methods • Is platform independent

22. Object Serialization • Lightweight persistence • For more sophisticated persistence mechanism consider JDO or a tool like Hibernate

23. Object Serialization Worm w = new Worm(); // a serializable obj ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("worm.out")); out.writeObject("Worm storage\n"); out.writeObject(w); out.close(); // Also flushes output

24. Object Serialization • A clever aspect of Java serialization is that when you save an object, it find all the objects referenced in that object and save those. • This process is performed recursively and a “web of objects” is saved.

25. Object Serialization ObjectInputStream in = new ObjectInputStream( new FileInputStream(new File("..", "X.file"))); Object mystery = in.readObject();

26. Object Serialization • The transient keyword • You can control serialization by implementing Externalizableinstead of serializable