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The RandomAccessFile Class

The stream classes just described operate on files in a strictly sequential fashion. However, Java also allows

you to access the contents of a file in non-sequential order. To do this, you will use RandomAccessFile,

which encapsulates a random-access file. RandomAccessFile is not derived from InputStream or

OutputStream. Instead, it implements the interfaces DataInput and DataOutput (which are described

shortly). RandomAccessFile supports random access because it lets you change the location in the file at

which the next read or write operation will occur. This is done by calling its seek( ) method.

Table 3.4 The Methods Defined by Reader

Method Description

abstract void close( ) throws

IOExceptionCloses the input source.

void mark(int numChars)throws IOException

Places a mark at the current point in the input stream that willremain valid until numChars characters are read. Not all streamssupport mark( ).

boolean markSupported( ) Returns true if mark( )/reset( ) are supported on this stream.

int read( ) throwsIOException

Returns an integer representation of the next available characterfrom the input stream. 1 is returned when the end of the file isencountered.

int read(char buffer[ ])

throws IOException

Attempts to read up to buffer.length characters into buffer andreturns the actual number of characters that were successfully read.

1 is returned when the end of the file is encountered.

abstract int read(char

buffer[ ],int offset,int numChars)throws IOException

Attempts to read up to numChars characters into buffer starting atbuffer[offset], returning the number of characters successfully read.1 is returned when the end of the file is encountered.

boolean ready( ) throwsIOException

Returns true if input is pending. Otherwise, it returns false.

void reset( ) throwsIOException

Resets the input pointer to the previously set mark. Not all streamssupport reset( ).

long skip(long numChars)throws IOException

Skips over numChars characters of input, returning the number ofcharacters actually skipped.

The File Class

In addition to the classes that support file I/O, Java provides the File class, which encapsulates information

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about a file. This class is extremely useful when manipulating a file itself (rather

than its contents) or the file system of the computer. For example, using File you can determine if a file is

hidden, set a files date, set a file to read-only, list the contents of a directory, or create a new directory,

among many other things. Thus, File puts the file system under your control. This makes File one of the

most important classes in Javas I/O system.

Table 3.5 The Methods Defined By Writer

Method Description

Writer append(char ch)throws IOException

Appends ch to the end of the invoking output stream. Returns areference to the stream.

Writer

append(CharSequencechars)

throws IOException

Appends chars to the end of the invoking output stream. Returns areference to the stream.

Writer

append(CharSequencechars,int begin, int end)throws IOException

Appends a subrange of chars, specified by begin and end, to the end

of the output stream. Returns a reference to the stream.

abstract void close( ) throwsIOException

Closes the output stream.

abstract void flush( ) throwsIOException

Finalizes the output state so that any buffers are cleared. That is, itflushes the output buffers.

void write(int ch) throwsIOException

Writes the character in the low-order 16 bits of ch to the outputstream.

void write(char buffer[ ])throws IOException

Writes a complete array of characters to the output stream.

abstract void write(charbuffer[ ],

int offset,int numChars)throws IOException

Writes a subrange of numChars characters from the array buffer,beginning at buffer[offset] to the output stream.

void write(String str) throwsIOException

Writes str to the output stream.

void write(String str, int

offset,

int numChars)

Writes a subrange of numChars characters from the string str,beginning at the specified offset.

The I/O Interfaces

Javas I/O system includes the following interfaces (which are packaged in java.io):

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Closeable DataInput DataOutput

Externalizable FileFilter FilenameFilter

Flushable ObjectInput ObjectInputValidation

ObjectOutput ObjectStreamConstants Serializable

Those used either directly or indirectly by the recipes in this chapter are DataInput, DataOutput, Closeable,

Flushable, FileFilter, FilenameFilter, ObjectInput, and ObjectOutput.

The DataInput and DataOutput interfaces define a variety of read and write methods, such as readInt( ) and

writeDouble( ), that can read and write Javas primitive data types. They also specify read( ) and write( )

methods that parallel those specified by InputStream and OutputStream. All operations are byte-oriented.

RandomAccessFile implements the DataInput and the DataOutput interfaces. Thus, random-access file

operations in Java are byte-oriented.

Table 3.6 The Character Stream Classes

Method Description

BufferedReader Buffered input character stream.

BufferedWriter Buffered output character stream.

CharArrayReader Input stream that reads from a character array.

CharArrayWriter Output stream that writes to a character array.

FileReader Input stream that reads from a file.

FileWriter Output stream that writes to a file.

FilterReader Filtered reader.

FilterWriter Filtered writer.

InputStreamReader Input stream that translates bytes to characters.

LineNumberReader Input stream that counts lines.

OutputStreamWriter Output stream that translates characters to bytes.

PipedReader Input pipe.

PipedWriter Output pipe.

PrintWriter Output stream that contains print( ) and println( ).

PushbackReaderInput stream that allows characters to be returned to the inputstream.

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Reader Abstract class that describes character stream input.

StringReader Input stream that reads from a string.

StringWriter Output stream that writes to a string.

Writer Abstract class that describes character stream output.

The Closeable and Flushable interfaces are implemented by several of the I/O classes. They provide a

uniform way of specifying that a stream can be closed or flushed. The Closeable interface defines only one

method, close( ), which is shown here:

void close( ) throws IOException

This method closes an open stream. Once closed, the stream cannot be used again. All I/O classes that

open a stream implement Closeable.

The Flushable interface also specifies only one method, flush( ), which is shown here:

void flush( ) throws IOException

Calling flush( ) causes any buffered output to be physically written to the underlying device. This interface is

implemented by the I/O classes that write to a stream.

FileFilter and FilenameFilter are used to filter directory listings.

The ObjectInput and ObjectOutput interfaces are used when serializing (saving and restoring) objects.

The Compressed File Streams

In java.util.zip, Java provides a very powerful set of specialized file streams that handle the compression and

decompression of data. All are subclasses of either InputStream or OutputStream, described earlier. The

compressed file streams are shown here:

DeflaterInputStream Reads data, compressing the data in the process.

DeflaterOutputStream Writes data, compressing the data in the process.

GZIPInputStream Reads a GZIP file.

GZIPOutputStream Writes a GZIP file.

InflaterInputStream Reads data, decompressing the data in the process.

InflaterOutputStream Writes data, decompressing the data in the process.

ZipInputStream Reads a ZIP file.

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ZipOutputStream Writes a ZIP file.

Using the compressed file streams, it is possible to automatically compress data while writing to a file or to

automatically decompress data when reading from a file. You can also create compressed files that arecompatible with the standard ZIP or GZIP formats, and you can decompress files in those formats. The

actual compression is provided by the Inflater and Deflater classes, also packaged

in java.util.zip. They use the ZLIB compression library. You wont usually need to deal with these classes

directly when compressing or decompressing files because their default operation is sufficient.

Tips For Handling ErrorsFile I/O poses a special challenge when it comes to error handling. There are two reasons for this. First, I/Ofailures are a very real possibility when reading or writing files. Despite the fact that computer hardware (andthe Internet) is much more reliable than in the past, it still fails at a fairly high rate, and any such failure mustbe handled in a manner consistent with the needs of yourapplication. The second reason that error handlingpresents a challenge when working with files is that nearly all file operations can generate one or moreexceptions. This means that nearly all file handling code must take place within a try block.

The most common I/O exception is IOException. This exception can be thrown by many of the constructorsand methods in the I/O system. As a general rule, it is generated when something goes wrong when readingor writing data, or when opening a file. Other common I/O-related exceptions, such asFileNotFoundException and ZipException, are subclasses of IOException.

There is another common exception related to file handling: SecurityException. Many constructors ormethods will throw a SecurityException if the invoking application does not have permission to access a fileor perform a specific operation. You will need to handlethis exception in a manner appropriate to yourapplication. For simplicity, the examples inthis chapter do not handle security exceptions, but it may be necessary for your applications to do so.

Because so many constructors and methods can generate an IOException, it is not uncommon to see codethat simply wraps all I/O operations within a single try block and then catches any IOException that may

occur. While adequate for experimenting with file I/O or possibly for simple utility programs that are for yourown personal use, this approach is not usually suitable for commercial code. This is because it does not letyou easily deal individually with each potential error. Instead, for detailed control, it is better to put eachoperation within its own try block. This way, you can precisely report and respond to the error that occurred.This is the approach demonstrated by the examples in this chapter.

Another way that IOExceptions are sometimes handled is by throwing them out of the method in which theyoccur. To do this you must include a throws IOException clause in the methods declaration. This approachis fine in some cases, because it reports an I/O failure back to the caller. However, in other situations it is adissatisfying shortcut because it causes all users of the method to handle the exception. The examples inthis chapter do not use this approach. Rather, they handle all IOExceptions explicitly. This allows each errorhandler to report precisely the error that occurred.

If you do handle IOExceptions by throwing them out of the method in which they occur, you must take extracare to close any files that have been opened by the method. The easiest way to do this is to wrap your

methods code in a try block and then use a finally clause to close the files(s) prior to the method returning.

In the examples in this chapter, any I/O exceptions that do occur are handled by simply displaying amessage. While this approach is acceptable for the example programs, real applications will usually need toprovide a more sophisticated response to an I/O error. For example, you might want to give the user theability to retry the operation, specify an alternative operation, or otherwise gracefully handle the problem.Preventing the loss or corruption of data is a primary goal. Part of being a great programmer is knowing howto effectively manage the things that might go wrong when an I/O operation fails.

One final point: a common mistake that occurs when handling files is forgetting to close a file when you aredone with it. Open files use system resources. Thus, there are limits to the

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number of files that can be open at any one time. Closing a file also ensures that any data written to the fileis actually written to the physical device. Therefore, the rule is very simple: if you open a file, close the file.Although files are typically closed automatically when an application ends, its best not to rely on thisbecause it can lead to sloppy programming and bad habits. It is better to explicitly close each file, properlyhandling any exceptions that might occur. For this reason, all files are explicitly closed by the examples inthis chapter, even when the program is ending.

Write to file

At this point we have a writer object and we can send real content to the file. You can do this

using the write() method, which has more variant but the most commonly used requires a

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string as input parameter.

Calling the write() method doesn't mean that it immediately writes the data into the file. The

output is maybe cached so if you want to send your data immediately to the file you need to

call the flush() method.

As last step you should close the file with the close() method and you are done.

The basic write method looks like this:

Code:

1. publicvoidwriteFile(){

2. String fileName = "c:\\test.txt";3.

4. try{

5. FileWriter writer = newFileWriter(fileName,true);

6. writer.write("Test text.");

7. writer.close();

8. }catch(IOException e){

9. e.printStackTrace();

10. }

11.}

12.

Java F1

However in a real world situation the FileWriter usually not used directly. Instead of FileWriter

the BufferedWriter or from Java 1.5 the PrintWriter are used. These writer objects gives you

more flexibility to handle your IO. Here is a simple BufferedWriter example:

Code:

1. publicvoidwriteFile(){

2. String fileName = "c:\\test.txt";3.

4. try{

5. BufferedWriter writer = newBufferedWriter(newFileWriter(fileName,true));

6. writer.write("Test text.");

7. writer.newLine();

8. writer.write("Line2");

9.writer.close();

10. }catch(IOException e){

11. e.printStackTrace();

12. }

13.}

Java F1

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