Stream: an object that either delivers data to its destination (screen, file, etc.) or that takes data from a source (keyboard, file, etc.) –it acts as

• Stream: an object that either delivers data to its destination (screen, file, etc.) or that takes data from a source (keyboard, file, etc.)– it acts as a buffer between the data source and

destination• Input stream: a stream that provides input to a

program– System.in connects a program to the keyboard

• Output stream: a stream that accepts output from a program– System.out connects a program to the screen

Byte based FileInputStream FileOutputStream

Character basedReader

Writer

InputStream, OutputStream, Reader, Writer are abstract classes

Need to create an instance of stream of a non-abstract type Can wrap the streams to be more convenience in reading and

writingex. wrap FileWriter with processing streams

BufferedWriter PrintWriter

wrap FileReader with processing streamsBufferedFileReader

Text Human readable, Not efficient in terms of time and space

(C) 2010 Pearson Education, Inc. All rights reserved.

The Reader and Writer abstract classes are Unicode two-byte, character-based streams.

Most of the byte-based streams have corresponding character-based concrete Reader or Writer classes.

A typical codesegment for opening a textfile for output:

FileWriter out = new FileWriter("test.txt");BufferedWriter b = new BufferedWriter(out); //istead of writing a character each step, can write a number of

characte at the same time or write a line

PrintWriter p = new PrintWriter(b);

Or with anonymous (‘unnamed‘) objects:PrintWriter p = new PrintWriter( new BufferedWriter( new FileWriter("test.txt")));

CIS 068

Using FileReader is not very efficient. Better

wrap it with BufferedReader:

BufferedReader br = new BufferedReader( new FileReader(“name“));

Remark: BufferedReader contains the method readLine(), which is convenient for reading textfiles

CIS 068

Display the words separated by any of the following characters: space, new line (\n), period (.) or comma (,).

String inputLine = keyboard.nextLine();StringTokenizer wordFinder = new StringTokenizer(inputLine, " \n.,");//the second argument is a string of the 4 delimiterswhile(wordFinder.hasMoreTokens()){ System.out.println(wordFinder.nextToken());}

Entering "Question,2b.or 20." gives this output:

Question2bor20

• BufferedReader◦ read(), readLine(), … none for parsing types◦ needs StringTokenizer then wrapper class methods

like Integer.parseInt(token)

Can Use Scanner with File instead:Scanner inFile = new Scanner(new File(“in.txt”));

Scanner inFile = new Scanner(new File(“in.txt"));

int number;

while (inFile.hasInt())

{

number = inFile.nextInt();

// …

}

BufferedReader◦ readLine() returns null◦ read() returns -1

Scanner◦ use hasNext() or hasNextLine() to check first◦ nextInt(), hasNextInt(), …

BufferedReader inFile = …line = inFile.readline();while (line != null){ // … line = inFile.readline();}

-------------------

Scanner inFile = …while (inFile.hasNextLine()){ line = infile.nextLine(); // …}


Character-based input and output can be performed with classes Scanner and Formatter. Class Scanner is used extensively to input data from the

keyboard. This class can also read data from a file. Class Formatter enables formatted data to be output to any

text-based stream in a manner similar to method System.out.printf.







The constructor with two String arguments specifies an absolute or relative path and the file or directory to associate with the File object.

The constructor with File and String arguments uses an existing File object that specifies the parent directory of the file or directory specified by the String argument.

The fourth constructor uses a URI object to locate the file. A Uniform Resource Identifier (URI) is a more general form of the

Uniform Resource Locators (URLs) that are used to locate websites.

Figure 17.3 lists some common File methods. The http://java.sun.com/javase/6/docs/api/java/io/File.html




A SecurityException occurs if the user does not have permission to write data to the file.

A FileNotFoundException occurs if the file does not exist and a new file cannot be created.

static method System.exit terminates an application. An argument of 0 indicates successful program termination. A nonzero value, normally indicates that an error has occurred. The argument is useful if the program is executed from a batch

file on Windows or a shell script on UNIX/Linux/Mac OS X.



Scanner method hasNext determines whether the end-of-file key combination has been entered.

A NoSuchElementException occurs if the data being read by a Scanner method is in the wrong format or if there is no more data to input.

Formatter method format works like System.out.printf

A FormatterClosedException occurs if the Formatter is closed when you attempt to output.

Formatter method close closes the file. If method close is not called explicitly, the operating sys-tem

normally will close the file when program execution terminates.

Efficient in terms of time and space

Not readable

Class: FileInputStream/FileOutputStream

... see FileReader/FileWriter

The difference:No difference in usage, only in

input/output format

CIS 068

You can also read and write objects to files Object needs serialization If an object is to be serialized:

◦ The class must be declared as public◦ The class must implement Serializable◦ The class must have a no-argument constructor◦ All fields of the class must be serializable: either

primitive types or serializable objects

ObjectOutputStream objectOut = new ObjectOutputStream( new FileOutputStream(fileName));

objectOut.writeObject(serializableObject);

objectOut.close( );

ObjectInputStream objectIn = new ObjectInputStream( new FileInputStream(fileName));

myObject = (ObjectType)objectIn.readObject( );

objectIn.close( );

try{while (true) //readoject}catch (EOFException e)






ObjectInputStream method readObject reads an Object from a file.

Method readObject throws an EOFException if an attempt is made to read beyond the end of the file.

Method readObject throws a ClassNotFoundException if the class for the object being read cannot be located.

A stream is an expensive resource There is a limit on the number of streams

that you can have open at one time You should not have more than one stream

open on the same file You must close a stream before you can

open it again Remember to close your streams!


Interface DataOutput describes a set of methods for writing primitive types to an output stream.

Classes DataOutputStream (a subclass of FilterOutputStream) and RandomAccessFile each implement this interface to write primitive-type values as bytes.


Class JFileChooser displays a dialog that enables the user to easily select files or directories.

Documents

Stream: an object that either delivers data to its destination (screen, file, etc.) or that takes data from a source (keyboard, file, etc.) –it acts as