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www.espirity.com Streams Dwight Deugo ([email protected]) Nesa Matic ([email protected]) Additional Contributors None as of September, 2004 2 © 2003-2004, Espirity Inc. Module Overview 1. Streams 3 © 2003-2004, Espirity Inc. Module Road Map 1. Streams What are streams? Stream types Character streams Byte streams Filter streams Object Serialization 4 © 2003-2004, 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 5 © 2003-2004, Espirity Inc. Stream Concept To read data serially, a Java program: Opens a stream to a data source Reads the information serially To write data serially, a Java program: file remote socket 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 6 © 2003-2004, Espirity Inc. Stream Types Two different types of streams: Character streams Byte streams Support reading and writing of characters, text Contain 16-bit Unicode characters Supported through Reader and Writer classes 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 7 © 2003-2004, 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 8bit 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 8 © 2003-2004, Espirity Inc. Character Streams Hierarchy Object InputStream Reader OutputStream Writer <<abstract>> <<abstract>> Reader Writer FileReader BufferedReader FileWriter 9 BufferedWriter © 2003-2004, 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 int int int int int read() read(char buffer[]) read(char buffer[], int offset, int length) write(int aCharacter) write(char buffer[]) write(char buffer[], int offset, int length) 10 © 2003-2004, 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 It’s important to close the stream 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(); 11 © 2003-2004, Espirity Inc. Writing and Reading Example Create a file FileWriter writefile = new FileWriter("source.txt"); writefile.write('A'); writefile.write("bcdefghi"); writefile.close(); myString Abcdefghi Read the file 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(); 12 © 2003-2004, 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 } 13 © 2003-2004, Espirity Inc. Byte Streams Hierarchy InputStream FileInputStream FilterInputStream DataInputStream Filter streams ObjectInputStream BufferdInputStream Filter streams Object streams OuputStream FileOutputStream DataOutputStream FilterOutputStream BufferdOutputStream 14 ObjectOutputStream PrintStream © 2003-2004, Espirity Inc. Specialized Byte Streams File streams Object streams Used for writing data to files and reading data from files 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 15 © 2003-2004, 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 16 © 2003-2004, 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); It’s important to know what’s in the stream double price= inputStream.readDouble(); inputStream.close(); 17 © 2003-2004, 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); Forces data to be written outputStream.flush(); outputStream.close(); 18 © 2003-2004, Espirity Inc. Streams in System Class System.in - standard input System.out - standard output An instance of BufferedInputStream class Used to read lines of text that user enters 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);} 19 © 2003-2004, 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{… 20 © 2003-2004, Espirity Inc. Serialization Protocol Classes that perform serialization and deserialization must implement special methods: private void writeObject(java.io.ObjectOutputStream out) throws IOException private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException; 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 21 © 2003-2004, 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 22 © 2003-2004, 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 23 © 2003-2004, Espirity Inc. Labs Slide! Lab: Steams 24 © 2003-2004, Espirity Inc.