Reader in Java

Generation QR Code ISO/IEC18004 in Java Reader

Reader is an abstract class that defines Java's model of streaming character input All of the methods in this class will throw an IOException on error conditions Table 17-3 provides a synopsis of the methods in Reader Table 17-3 The Methods Defined by Reader

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abstract void close( )

Closes the input source Further read attempts will generate an IOException Places a mark at the current point in the input stream that will remain valid until numChars characters are read Returns true if mark( )/reset( ) are supported on this stream Returns an integer representation of the next available character from the invoking input stream 1 is returned when the end of the file is encountered Attempts to read up to bufferlength characters into buffer and returns the actual number of characters that were successfully read 1 is returned when the end of the file is encountered Attempts to read up to numChars characters into buffer starting at buffer[offset], returning the number of characters successfully read 1 is returned when the end of the file is encountered Returns true if the next input request will not wait Otherwise, it returns false Resets the input pointer to the previously set mark Skips over numChars characters of input, returning the number of characters actually skipped

void mark(int numChars)

boolean markSupported( )

int read( )

int read(char buffer[ ])

abstract int read(char buffer[ ], int offset, int numChars) boolean ready( )

void reset( ) long skip(long numChars)

Writer is an abstract class that defines streaming character output All of the methods in this class return a void value and throw an IOException in the case of errors Table 17-4 shows a synopsis of the methods in Writer

The FileReader class creates a Reader that you can use to read the contents of a file Its two most commonly used constructors are shown here: FileReader(String filePath) FileReader(File fileObj) Table 17-4 The Methods Defined by Writer

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abstract void close( )

Closes the output stream Further write attempts will generate an IOException Finalizes the output state so that any buffers are cleared That is, it flushes the output buffers Writes a single character to the invoking output stream Note that the parameter is an int, which allows you to call write with expressions without having to cast them back to char Writes a complete array of characters to the invoking output stream Writes a subrange of numChars characters from the array buffer, beginning at buffer[offset] to the invoking output stream

abstract void flush( )

void write(int ch)

void write(char buffer[ ])

abstract void write(char buffer[ ], int offset, int numChars) void write(String str) void write(String str, int offset, int numChars)

Writes str to the invoking output stream Writes a subrange of numChars characters from the array str, beginning at the specified offset

Either can throw a FileNotFoundException Here, filePath is the full path name of a file, and fileObj is a File object that describes the file The following example shows how to read lines from a file and print these to the standard output stream It reads its own source file, which must be in the current directory // Demonstrate FileReader import javaio*; class FileReaderDemo { public static void main(String args[]) throws Exception { FileReader fr = new FileReader("FileReaderDemojava"); BufferedReader br = new BufferedReader(fr); String s; while((s = brreadLine()) != null) { Systemoutprintln(s); } } frclose();

FileWriter creates a Writer that you can use to write to a file Its most commonly used constructors are shown here: FileWriter(String filePath)

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FileWriter(String filePath, boolean append) FileWriter(File fileObj) They can throw an IOException or a SecurityException Here, filePath is the full path name of a file, and fileObj is a File object that describes the file If append is true, then output is appended to the end of the file Creation of a FileWriter is not dependent on the file already existing FileWriter will create the file before opening it for output when you create the object In the case where you attempt to open a read-only file, an IOException will be thrown The following example is a character stream version of an example shown earlier when FileOutputStream was discussed This version creates a sample buffer of characters by first making a String and then using the getChars( ) method to extract the character array equivalent It then creates three files The first, file1txt, will contain every other character from the sample The second, file2txt, will contain the entire set of characters Finally, the third, file3txt, will contain only the last quarter // Demonstrate FileWriter import javaio*; class FileWriterDemo { public static void main(String args[]) throws Exception { String source = "Now is the time for all good men\\n" + " to come to the aid of their country\\n" + " and pay their due taxes"; char buffer[] = new char[sourcelength()]; sourcegetChars(0, sourcelength(), buffer, 0); FileWriter f0 = new FileWriter("file1txt"); for (int i=0; i < bufferlength; i += 2) { f0write(buffer[i]); } f0close(); FileWriter f1 = new FileWriter("file2txt"); f1write(buffer); f1close(); FileWriter f2 = new FileWriter("file3txt"); f2write(buffer,bufferlengthbufferlength/4,bufferlength/4); f2close(); } }