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Streams Dwight Deugo Nesa Matic

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Presentation on theme: "Streams Dwight Deugo Nesa Matic"— Presentation transcript:

1 Streams Dwight Deugo Nesa Matic

2 2 © , Espirity Inc. Additional Contributors None as of September, 2004

3 3 © , Espirity Inc. Module Overview 1.Streams

4 4 © , Espirity Inc. Module Road Map 1.Streams  What are streams?  Stream types  Character streams  Byte streams  Filter streams  Object Serialization

5 5 © , Espirity Inc. What is a Stream? From an abstract point of view a stream is simply a sequence From an implementation point of view you can think of a stream as a list A stream has a start, and end, and a position Streams can let us model systems that have state without ever using assignment or mutable data

6 6 © , Espirity Inc. Stream Concept To read data serially, a Java program:  Opens a stream to a data source  file  remote socket  Reads the information serially To write data serially, a Java program:  Opens a stream to a data source  Writes the information serially Data source type doesn’t matter, concepts of reading and writing are the same Once you understand the top level classes ( java.io.Reader, java.io.Writer ), the remaining classes are much of the same

7 7 © , Espirity Inc. Stream Types Two different types of streams:  Character streams  Support reading and writing of characters, text  Contain 16-bit Unicode characters  Supported through Reader and Writer classes  Byte streams  Support reading and writing of any bytes  Contain 8-but bytes  Supported through InputStream and OutputStream classes It is possible to do conversion between character streams and byte stream  InputStreamReader and OutputStreamWriter classes can be used

8 8 © , Espirity Inc. Character Streams vs. Byte Streams Character streams (reader and writer) should be used because:  They can handle any character in the Unicode character set (while the byte streams are limited to ISO-Latin-1 8- bit bytes)  They are easier to internationalize because they are not dependent upon a specific character encoding  They use buffering techniques internally and are therefore potentially much more efficient than byte streams Byte streams should be used for handling binary data, such as image and sound files All stream classes are in the java.io package

9 9 © , Espirity Inc. Character Streams Hierarchy Object OutputStream Writer InputStream Reader > Writer > Reader FileWriterFileReaderBufferedWriterBufferedReader

10 10 © , Espirity Inc. Reader and Writer classes Parent classes for character-stream based classes Used to read and write 16-bit character streams Important methods for reading and writing to streams found in these and their descendent classes include the following: int read() int read(char buffer[]) int read(char buffer[], int offset, int length) int write( int aCharacter) int write( char buffer[]) int write( char buffer[], int offset, int length)

11 11 © , Espirity Inc. FileReader and FileWriter classes FileReader is used for reading streams of characters from a file FileWriter is used for writing streams of characters to a file File inputFile = new File("source.txt"); File outputFile = new File("final.txt"); FileReader in = new FileReader(inputFile); FileWriter out = new FileWriter(outputFile); int aCharacter; while ((aCharacter = in.read()) != 1) out.write(aCharacter); in.close(); out.close(); It’s important to close the stream

12 12 © , Espirity Inc. Writing and Reading Example Create a file Read the file FileWriter writefile = new FileWriter("source.txt"); writefile.write('A'); writefile.write("bcdefghi"); writefile.close(); Abcdefghi FileReader inFile = new FileReader("source.txt"); inFile.skip(3); // skips next 3 chars ('A' 'b' 'c' ) char[] characterArray = new char[10]; inFile.read(characterArray ); // ['d' 'e' 'f' 'g' 'h' 'i'] String myString = new inFile.read(characterArray).trim(); inFile.close(); myString

13 13 © , Espirity Inc. BufferedReader and BufferedWriter classes Used for buffering characters as being read or written  Buffer is used for storing data without conversion  Buffer size can be set  Should wrap any reader/writer whose read/write operations may be inefficient BufferedReader reader = new BufferedReader(new DataInputStream(System.in)); String input; while ((input = reader.readLine()) != null) {... //do something interesting here }

14 14 © , Espirity Inc. Byte Streams Hierarchy InputStream FileInputStreamFilterInputStream DataInputStreamBufferdInputStream ObjectInputStream Filter streams Object streams Filter streams OuputStream FileOutputStreamFilterOutputStream DataOutputStreamBufferdOutputStream ObjectOutputStream PrintStream

15 15 © , Espirity Inc. Specialized Byte Streams File streams  Used for writing data to files and reading data from files Object streams  Used for reading and writing objects  Also called object serialization Filter streams  Used for filtering data as it’s being read from streams, or written to the streams  They work with primitive data types ( int, double, boolean )  They implement DataInput and DataOutput interfaces

16 16 © , Espirity Inc. Filter Streams Filter data as it's being read from or written to a stream  Subclasses of the FilterInputStream a and FilterOutputStream Constructed on another stream (the underlying stream)  Read method reads input from the underlying stream, filters it, and passes on the filtered data to the caller  Write method writes data to the underlying stream Filtering done by the streams depends on the stream  Some streams buffer the data, some count data as it goes by, and others convert data to another form

17 17 © , Espirity Inc. Using Filter Streams… For reading primitive data types  DataInputStream class can be used FileInputStream inputFile = new FileInputStream("price.cat"); DataInputStream inputStream = new DataInputStream(inputFile); double price= inputStream.readDouble(); inputStream.close(); It’s important to know what’s in the stream

18 18 © , Espirity Inc. …Using Filter Streams For writing primitive data types  A DataOutputStream can be used FileOutputStream outputFile = new FileInputStream("price.cat"); DataOutputStream outputStream = new DataInputStream(outputFile ); outputStream.writeDouble(234.56); outputStream.flush(); outputStream.close(); Forces data to be written

19 19 © , Espirity Inc. Streams in System Class System.in - standard input  An instance of BufferedInputStream class  Used to read lines of text that user enters System.out - standard output  Instance of PrintStream class  Used to send text to the Console System.err - error output  Instance of PrintStream class  Used to send error text to the error file BufferedReader reader = new BufferedReader(new DataInputStream(System.in)); String input; while ((input = reader.readLine()) != null){ System.out.println(input);}

20 20 © , Espirity Inc. Object Serialization Supported with ObjectOutputStream class  Serialized object class must implement the Serializable interface GregorianCalendar calendar = new GregorianCalendar(); ObjectOutputStream out = new ObjectOutputStream (new FileOutputStream("calendar.dat")); out.writeObject(calendar); out.close(); public class java.util.GregorianCalendar extents java.util.Calendar{… public class java.util. GregorianCalendar extents java.lang.Object implements java.lang.Cloneable, java.io.Serializable{…

21 21 © , Espirity Inc. Serialization Protocol Classes that perform serialization and deserialization must implement special methods: Object state is saved by writing the individual fields to the ObjectOutputStream Object state is retrieved by reading the individual fields back from the ObjectInputStream private void writeObject(java.io.ObjectOutputStream out) throws IOException private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException;

22 22 © , Espirity Inc. Object Deserialization Supported with ObjectInputStream class  You must know the order in which things were written in order to cast to the correct type ObjectInputStream in = new ObjectInputStream (new FileInputStream("calendar.dat")); GregorianCalendar calendar = (GregorianCalendar)in.readObject(); in.close(); It’s important to know what’s in the stream for casting

23 23 © , Espirity Inc. Module Summary In this module you have learned:  What streams are  What are different types of streams in Java  Differences between character streams and byte streams  What filter streams are  Streams used in the System class  How to serialize objects

24 24 © , Espirity Inc. Labs Slide! Lab: Steams


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