Prentice Hall Technical Java Developing Scientific And Engineering Applications Apr 2003 ISBN 0131018159

At the top of the hierarchy is the Writer class that defines methods to write one or more characters to the output stream.. The PrintWriter class can be wrapped around either a character

The character output stream class hierarchy is shown in Figure 25.4 At the top of the hierarchy is the Writer class that defines methods to write one or more characters to the output stream The BufferedWriter class provides a buffer for improved I/O performance The PrintWriter class can be wrapped around either a character or byte output stream and gives the

underlying stream access to the print() and println()

methods The OutputStreamWriter class allows you to wrap a character output stream around a byte output stream The

FileWriter class connects an output stream to a file allowing you to write data to the file

Figure 25.4 Character Output Streams

Writer Class

The Writer class is the abstract superclass for all character

output stream classes It defines methods that are available to all character output streams, including methods to close an

output stream, write the contents of a buffer to the stream, or

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public abstract void close() throws IOException

public abstract void flush() throws IOException

public void write(int c) throws IOException

public void write(char[] cbuf) throws IOException

public void write(char[] cbuf, int offset, int length) throws

IOException

public void write(String str) throws IOException

public void write(String str, int offset, int length) throws

IOException

The close() method closes the output stream and releases any resources allocated to it The flush() method causes any data stored in a memory buffer to be written to the stream The

write() method is used to write a single character, the

contents of a character array, or a String to the output stream You can also write a subset of a character array or String to the output stream

BufferedWriter Class

The BufferedWriter class provides an intermediary buffer for writing data resulting in more efficient I/O performance Instead

of writing characters or Strings immediately to a file, for

instance, the character data is first written to the buffer When the buffer reaches its capacity, its contents are written to the file The BufferedWriter class defines two public constructors

public BufferedWriter(Writer stream)

A BufferedWriter object is always wrapped around another character output stream The second constructor version allows you to specify the size of the buffer The BufferedWriter class overrides some of the Writer class methods but defines no new methods to write character data To see an example of using a

BufferedWriter, see the "Reading and Writing to a File" section later in this chapter

FileWriter Class

The FileWriter class allows you to connect a character output stream to a file FileWriter objects can be used on their own, but they are often used in conjunction with a BufferedWriter

or PrintWriter object to take advantage of the extra

functionality provided by those classes The File Writer class does not define any new methods, nor does it override any

methods inherited from the OutputStreamWriter or Writer

classes The FileWriter class does define five constructors

public FileWriter(File file) throws IOException

public FileWriter(File file, boolean append) throws IOException

public FileWriter(FileDescriptor desc)

public FileWriter(String name) throws IOException

public FileWriter(String name,boolean append) throws IOException

The file can be specified either by a File object, a

FileDescriptor object, or by providing the name and, if

necessary, path of the file If append is true, data is written

starting at the end of the file The default is false meaning that

OutputStreamWriter Class

The OutputStreamWriter class provides a way to wrap a

character output stream around a byte output stream

Character data is converted to bytes and stored in an

intermediate buffer before being written to the underlying

stream The OutputStreamWriter class overrides some of the

Writer class methods but defines no new methods to write data The class defines four public constructors

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public OutputStreamWriter(OutputStream stream)

public OutputStreamWriter(OutputStream stream, Charset cs)

public OutputStreamWriter(OutputStream stream, CharsetDecoder

dec)

public OutputStreamWriter(OutputStream stream, String

charsetName) throws UnsupportedEncodingException

The first constructor version uses the default character set

mapping scheme The other three versions allow you to specify

a character mapping to be used

PrintWriter Class

The PrintWriter class provides an underlying output stream access to the print() and println() methods These methods never throw an IOException and do not need to be enclosed in

a try block The Print Writer class is unique in that it can be wrapped around either a character or byte output stream The

public PrintWriter(OutputStream stream)

public PrintWriter(OutputStream stream, boolean autoFlush)

public PrintWriter(Writer stream)

public PrintWriter(Writer stream, boolean autoFlush)

If autoFlush is true, any call to the println() method will

flush the output buffer In addition to overriding some of the

Writer class methods, the PrintWriter class defines the

following methods

public void print(boolean b)

public void print(char c)

public void print(char[] chars)

public void print(double d)

public void print(float f)

public void print(int i)

public void print(long l)

public void print(Object obj)

public void print(String str)

The print() method prints a String or a String

representation of a primitive type value or Object to the output stream without appending a newline character to the end of the

String

public void println()

public void println(boolean b)

public void println(char[] chars)

public void println(double d)

public void println(float f)

public void println(int i)

public void println(long l)

public void println(Object obj)

public void println(String str)

The println() method prints a String or a String

representation of a primitive type value or Object to the output stream A newline character is appended to the end of the

String The no-argument version simply writes a newline to the output stream

Other Writer Subclasses

The Writer class has some other subclasses that you probably won't use much in your scientific and engineering programming work These include the CharArrayWriter, FilterWriter,

PipedWriter, and StringWriter classes We won't discuss these classes in this chapter If you want more details, consult the Sun Java doc pages

A third way to implement I/O functionality is to read data from

or write data to a file File I/O is achieved by connecting an I/O stream to a file File I/O can be done with either byte or

character streams To read or write byte data, you can use the

FileInputStream or FileOutputStream classes To read or write character data, you can use the FileReader or

FileWriter classes Another I/O stream is usually wrapped around the file I/O stream to make it easier to read and write data from the file

One advantage of using file I/O is it can handle a large number

of input parameters There is no maximum length to an input file You can also place descriptive comments inside the input file Another advantage is that you have a permanent record of the input and output You can reuse an input file or modify a large input file by making only a few changes

Disadvantages include having to write code to parse the input file The structure of the input file has to match what the

parsing algorithm expects There are more things that can go wrong with file I/O than the other methods; an input file may not be found, for example

The FileDemo class demonstrates how input data can be read from a file and output data can be written to a file It is

especially true about file I/O that there are many ways to

accomplish the reading and writing of data The FileDemo class uses character streams but you could (although it wouldn't be advisable) rewrite the program using byte streams instead

A FileReader object is created that connects to an input file named "USatm76.inp." A BufferedReader is wrapped around the FileReader to allow the readLine() method to read a line

of data at a time The contents of the "USatm76.inp" file used

units (SI or English) = SI

altitude (m or ft) = 20000.0

The first line of the input file contains the system of units to be used The BufferedReader reads this line as a String, which is then split into substrings using the split() method with the =

character as the delimiter The last substring is the one we

want, so we assign it to a String variable named units The

BufferedReader then reads the second line of the input file Once again the resulting String is split into substrings The last substring, containing the altitude value we want, is converted into a value of type double and assigned to a variable named

altitude The two inputs are then sent to the USatm76

constructor

The resulting atmospheric conditions are written to a file using

FileWriter and BufferedWriter objects The FileWriter is connected to a file named "USatm76.out." The BufferedWriter

is wrapped around the FileWriter to improve I/O performance The BufferedWriter object uses its write() method to write a sequence of String objects to the file The FileDemo class

source code is shown here

import java.io.*;

public class FileDemo

{

public static void main(String args[]) {

BufferedReader reader;

BufferedWriter writer;

String line, units;

String strings[];

double altitude;

// BufferedReader and FileReader objects are

try {

reader = new BufferedReader(

new FileReader("USatm76.inp"));

// Read the first line of the input file and

// split it into substrings The last substring // is the one we want to keep after any leading // and trailing white space is trimmed

line = reader.readLine();

strings = line.split("=");

units = strings[strings.length-1].trim();

// Read the second line of the input file

// Convert the last substring of the line into a // double value containing the altitude

line = reader.readLine();

strings = line.split("=");

altitude =

Double.parseDouble(strings[strings.length-1]);

// Create a USatm76 object with the recently

// acquired input values Print out the

// atmospheric data

USatm76 atm = new USatm76(units, altitude);

// Use a BufferedWriter and FileWriter object to // write the output to a file

writer = new BufferedWriter(

new FileWriter("USatm76.out"));

String label[] = atm.getLabels();

// The write() method will not automatically // add a newline character

writer.write("\ngeometric altitude = " +

atm.getAltitude() + label[0]); writer.write("\ngeopotential altitude = " +

atm.getGeoPotentialAltitude() + label[1]); writer.write("\ntemperature = " +

atm.getTemperature() + label[2]); writer.write("\npressure = " +

atm.getPressure() + label[3]);

writer.write("\nmolar mass = " +

atm.getMolarMass() + label[4]); writer.write("\ndensity = " +

atm.getDensity() + label[5]);

// Close the streams

reader.close();

writer.flush();

writer.close();

} catch (IOException ioe) {

System.out.println("IO Exception occurred");

System.exit(1);

}

}

}

Output (contents of "USatm76.out" file)

geometric altitude = 20000.0 m

geopotential altitude = 19937.27227876952 m

temperature = 216.65 K

pressure = 5529.256361823237 N/m^2

molar mass = 0.0289645 kg/mole

density = 0.08890932913275061 kg/m^3