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SUMMARY: JavaDoc of java.io.RandomAccessFile JAVADOC | Sourc code of java.io.RandomAccessFile SOURCE | Download java.io.RandomAccessFile DOWNLOAD | NESTED | FIELD | CONSTR | METHOD DETAIL: FIELD | CONSTR | METHOD

java.io
Class RandomAccessFile  view RandomAccessFile download RandomAccessFile.java

java.lang.Object
  extended byjava.io.RandomAccessFile
All Implemented Interfaces:
DataInput, DataOutput
public class RandomAccessFile
extends java.lang.Object
implements DataOutput, DataInput

This class allows reading and writing of files at random locations. Most Java I/O classes are either pure sequential input or output. This class fulfills the need to be able to read the bytes of a file in an arbitrary order. In addition, this class implements the DataInput and DataOutput interfaces to allow the reading and writing of Java primitives.

Field Summary
private  gnu.java.nio.channels.FileChannelImpl ch
           
private  FileDescriptor fd
           
private  DataInputStream in
           
private  DataOutputStream out
           
 
Constructor Summary
RandomAccessFile(File file, java.lang.String mode)
          This method initializes a new instance of RandomAccessFile to read from the specified File object with the specified access mode.
RandomAccessFile(java.lang.String fileName, java.lang.String mode)
          This method initializes a new instance of RandomAccessFile to read from the specified file name with the specified access mode.
 
Method Summary
 void close()
          This method closes the file and frees up all file related system resources.
 java.nio.channels.FileChannel getChannel()
          This method creates a java.nio.channels.FileChannel.
 FileDescriptor getFD()
          This method returns a FileDescriptor object that represents the native file handle for this file.
 long getFilePointer()
          This method returns the current offset in the file at which the next read or write will occur
 long length()
          This method returns the length of the file in bytes
 int read()
          This method reads a single byte of data from the file and returns it as an integer.
 int read(byte[] buffer)
          This method reads bytes from the file into the specified array.
 int read(byte[] buffer, int offset, int len)
          This methods reads up to len bytes from the file into the specified array starting at position offset into the array.
 boolean readBoolean()
          This method reads a Java boolean value from an input stream.
 byte readByte()
          This method reads a Java byte value from an input stream.
 char readChar()
          This method reads a Java char value from an input stream.
 double readDouble()
          This method reads a Java double value from an input stream.
 float readFloat()
          This method reads a Java float value from an input stream.
 void readFully(byte[] buffer)
          This method reads raw bytes into the passed array until the array is full.
 void readFully(byte[] buffer, int offset, int count)
          This method reads raw bytes into the passed array buf starting offset bytes into the buffer.
 int readInt()
          This method reads a Java int value from an input stream It operates by reading four bytes from the stream and converting them to a single Java int The bytes are stored most significant byte first (i.e., "big endian") regardless of the native host byte ordering.
 java.lang.String readLine()
          This method reads the next line of text data from an input stream.
 long readLong()
          This method reads a Java long value from an input stream It operates by reading eight bytes from the stream and converting them to a single Java long The bytes are stored most significant byte first (i.e., "big endian") regardless of the native host byte ordering.
 short readShort()
          This method reads a signed 16-bit value into a Java in from the stream.
 int readUnsignedByte()
          This method reads 8 unsigned bits into a Java int value from the stream.
 int readUnsignedShort()
          This method reads 16 unsigned bits into a Java int value from the stream.
 java.lang.String readUTF()
          This method reads a String from an input stream that is encoded in a modified UTF-8 format.
 void seek(long pos)
          This method sets the current file position to the specified offset from the beginning of the file.
 void setLength(long newLen)
          This method sets the length of the file to the specified length.
 int skipBytes(int numBytes)
          This method attempts to skip and discard the specified number of bytes in the input stream.
 void write(byte[] buffer)
          This method writes all the bytes in the specified array to the file.
 void write(byte[] buffer, int offset, int len)
          This method writes len bytes to the file from the specified array starting at index offset into the array.
 void write(int oneByte)
          This method writes a single byte of data to the file.
 void writeBoolean(boolean val)
          This method writes a Java boolean to the underlying output stream.
 void writeByte(int val)
          This method writes a Java byte value to the underlying output stream.
 void writeBytes(java.lang.String val)
          This method writes all the bytes in a String out to the stream.
 void writeChar(int val)
          This method writes a single char value to the stream, high byte first.
 void writeChars(java.lang.String val)
          This method writes all the characters in a String to the stream.
 void writeDouble(double val)
          This method writes a Java double value to the stream.
 void writeFloat(float val)
          This method writes a Java float value to the stream.
 void writeInt(int val)
          This method writes a Java int to the stream, high bytes first.
 void writeLong(long val)
          This method writes a Java long to the stream, high bytes first.
 void writeShort(int val)
          This method writes a Java short to the stream, high byte first.
 void writeUTF(java.lang.String val)
          This method writes a Java String to the stream in a modified UTF-8 format.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

ch

private gnu.java.nio.channels.FileChannelImpl ch

fd

private FileDescriptor fd

out

private DataOutputStream out

in

private DataInputStream in
Constructor Detail

RandomAccessFile

public RandomAccessFile(File file,
                        java.lang.String mode)
                 throws FileNotFoundException
This method initializes a new instance of RandomAccessFile to read from the specified File object with the specified access mode. The access mode is either "r" for read only access or "rw" for read-write access.

Note that a SecurityManager check is made prior to opening the file to determine whether or not this file is allowed to be read or written.

RandomAccessFile

public RandomAccessFile(java.lang.String fileName,
                        java.lang.String mode)
                 throws FileNotFoundException
This method initializes a new instance of RandomAccessFile to read from the specified file name with the specified access mode. The access mode is either "r" for read only access, "rw" for read write access, "rws" for synchronized read/write access of both content and metadata, or "rwd" for read/write access where only content is required to be synchronous.

Note that a SecurityManager check is made prior to opening the file to determine whether or not this file is allowed to be read or written.

Method Detail

close

public void close()
           throws IOException
This method closes the file and frees up all file related system resources. Since most operating systems put a limit on how many files may be opened at any given time, it is a good idea to close all files when no longer needed to avoid hitting this limit

getFD

public final FileDescriptor getFD()
                           throws IOException
This method returns a FileDescriptor object that represents the native file handle for this file.

getFilePointer

public long getFilePointer()
                    throws IOException
This method returns the current offset in the file at which the next read or write will occur

setLength

public void setLength(long newLen)
               throws IOException
This method sets the length of the file to the specified length. If the currently length of the file is longer than the specified length, then the file is truncated to the specified length (the file position is set to the end of file in this case). If the current length of the file is shorter than the specified length, the file is extended with bytes of an undefined value (the file position is unchanged in this case).

The file must be open for write access for this operation to succeed.

length

public long length()
            throws IOException
This method returns the length of the file in bytes

read

public int read()
         throws IOException
This method reads a single byte of data from the file and returns it as an integer.

read

public int read(byte[] buffer)
         throws IOException
This method reads bytes from the file into the specified array. The bytes are stored starting at the beginning of the array and up to buf.length bytes can be read.

read

public int read(byte[] buffer,
                int offset,
                int len)
         throws IOException
This methods reads up to len bytes from the file into the specified array starting at position offset into the array.

readBoolean

public final boolean readBoolean()
                          throws IOException
This method reads a Java boolean value from an input stream. It does so by reading a single byte of data. If that byte is zero, then the value returned is false If the byte is non-zero, then the value returned is true

This method can read a boolean written by an object implementing the writeBoolean() method in the DataOutput interface.

Specified by:
readBoolean in interface DataInput

readByte

public final byte readByte()
                    throws IOException
This method reads a Java byte value from an input stream. The value is in the range of -128 to 127.

This method can read a byte written by an object implementing the writeByte() method in the DataOutput interface.

Specified by:
readByte in interface DataInput

readChar

public final char readChar()
                    throws IOException
This method reads a Java char value from an input stream. It operates by reading two bytes from the stream and converting them to a single 16-bit Java char The two bytes are stored most significant byte first (i.e., "big endian") regardless of the native host byte ordering.

As an example, if byte1 and byte2 represent the first and second byte read from the stream respectively, they will be transformed to a char in the following manner:

(char)(((byte1 & 0xFF) << 8) | (byte2 & 0xFF)

This method can read a char written by an object implementing the writeChar() method in the DataOutput interface.

Specified by:
readChar in interface DataInput

readDouble

public final double readDouble()
                        throws IOException
This method reads a Java double value from an input stream. It operates by first reading a logn value from the stream by calling the readLong() method in this interface, then converts that long to a double using the longBitsToDouble method in the class java.lang.Double

This method can read a double written by an object implementing the writeDouble() method in the DataOutput interface.

Specified by:
readDouble in interface DataInput

readFloat

public final float readFloat()
                      throws IOException
This method reads a Java float value from an input stream. It operates by first reading an int value from the stream by calling the readInt() method in this interface, then converts that int to a float using the intBitsToFloat method in the class java.lang.Float

This method can read a float written by an object implementing the writeFloat() method in the DataOutput interface.

Specified by:
readFloat in interface DataInput

readFully

public final void readFully(byte[] buffer)
                     throws IOException
This method reads raw bytes into the passed array until the array is full. Note that this method blocks until the data is available and throws an exception if there is not enough data left in the stream to fill the buffer

Specified by:
readFully in interface DataInput

readFully

public final void readFully(byte[] buffer,
                            int offset,
                            int count)
                     throws IOException
This method reads raw bytes into the passed array buf starting offset bytes into the buffer. The number of bytes read will be exactly len Note that this method blocks until the data is available and throws an exception if there is not enough data left in the stream to read len bytes.

Specified by:
readFully in interface DataInput

readInt

public final int readInt()
                  throws IOException
This method reads a Java int value from an input stream It operates by reading four bytes from the stream and converting them to a single Java int The bytes are stored most significant byte first (i.e., "big endian") regardless of the native host byte ordering.

As an example, if byte1 through byte4 represent the first four bytes read from the stream, they will be transformed to an int in the following manner:

(int)(((byte1 & 0xFF) << 24) + ((byte2 & 0xFF) << 16) + ((byte3 & 0xFF) << 8) + (byte4 & 0xFF)))

The value returned is in the range of 0 to 65535.

This method can read an int written by an object implementing the writeInt() method in the DataOutput interface.

Specified by:
readInt in interface DataInput

readLine

public final java.lang.String readLine()
                                throws IOException
This method reads the next line of text data from an input stream. It operates by reading bytes and converting those bytes to char values by treating the byte read as the low eight bits of the char and using 0 as the high eight bits. Because of this, it does not support the full 16-bit Unicode character set.

The reading of bytes ends when either the end of file or a line terminator is encountered. The bytes read are then returned as a String A line terminator is a byte sequence consisting of either \r \n or \r\n These termination charaters are discarded and are not returned as part of the string.

This method can read data that was written by an object implementing the writeLine() method in DataOutput

Specified by:
readLine in interface DataInput

readLong

public final long readLong()
                    throws IOException
This method reads a Java long value from an input stream It operates by reading eight bytes from the stream and converting them to a single Java long The bytes are stored most significant byte first (i.e., "big endian") regardless of the native host byte ordering.

As an example, if byte1 through byte8 represent the first eight bytes read from the stream, they will be transformed to an long in the following manner:

(long)((((long)byte1 & 0xFF) << 56) + (((long)byte2 & 0xFF) << 48) + (((long)byte3 & 0xFF) << 40) + (((long)byte4 & 0xFF) << 32) + (((long)byte5 & 0xFF) << 24) + (((long)byte6 & 0xFF) << 16) + (((long)byte7 & 0xFF) << 8) + ((long)byte9 & 0xFF)))

The value returned is in the range of 0 to 65535.

This method can read an long written by an object implementing the writeLong() method in the DataOutput interface.

Specified by:
readLong in interface DataInput

readShort

public final short readShort()
                      throws IOException
This method reads a signed 16-bit value into a Java in from the stream. It operates by reading two bytes from the stream and converting them to a single 16-bit Java short The two bytes are stored most significant byte first (i.e., "big endian") regardless of the native host byte ordering.

As an example, if byte1 and byte2 represent the first and second byte read from the stream respectively, they will be transformed to a short in the following manner:

(short)(((byte1 & 0xFF) << 8) | (byte2 & 0xFF)

The value returned is in the range of -32768 to 32767.

This method can read a short written by an object implementing the writeShort() method in the DataOutput interface.

Specified by:
readShort in interface DataInput

readUnsignedByte

public final int readUnsignedByte()
                           throws IOException
This method reads 8 unsigned bits into a Java int value from the stream. The value returned is in the range of 0 to 255.

This method can read an unsigned byte written by an object implementing the writeUnsignedByte() method in the DataOutput interface.

Specified by:
readUnsignedByte in interface DataInput

readUnsignedShort

public final int readUnsignedShort()
                            throws IOException
This method reads 16 unsigned bits into a Java int value from the stream. It operates by reading two bytes from the stream and converting them to a single Java int The two bytes are stored most significant byte first (i.e., "big endian") regardless of the native host byte ordering.

As an example, if byte1 and byte2 represent the first and second byte read from the stream respectively, they will be transformed to an int in the following manner:

(int)(((byte1 & 0xFF) << 8) + (byte2 & 0xFF))

The value returned is in the range of 0 to 65535.

This method can read an unsigned short written by an object implementing the writeUnsignedShort() method in the DataOutput interface.

Specified by:
readUnsignedShort in interface DataInput

readUTF

public final java.lang.String readUTF()
                               throws IOException
This method reads a String from an input stream that is encoded in a modified UTF-8 format. This format has a leading two byte sequence that contains the remaining number of bytes to read. This two byte sequence is read using the readUnsignedShort() method of this interface.

After the number of remaining bytes have been determined, these bytes are read an transformed into char values. These char values are encoded in the stream using either a one, two, or three byte format. The particular format in use can be determined by examining the first byte read.

If the first byte has a high order bit of 0 then that character consists on only one byte. This character value consists of seven bits that are at positions 0 through 6 of the byte. As an example, if byte1 is the byte read from the stream, it would be converted to a char like so:

(char)byte1

If the first byte has 110 as its high order bits, then the character consists of two bytes. The bits that make up the character value are in positions 0 through 4 of the first byte and bit positions 0 through 5 of the second byte. (The second byte should have 10 as its high order bits). These values are in most significant byte first (i.e., "big endian") order.

As an example, if byte1 and byte2 are the first two bytes read respectively, and the high order bits of them match the patterns which indicate a two byte character encoding, then they would be converted to a Java char like so:

(char)(((byte1 & 0x1F) << 6) | (byte2 & 0x3F))

If the first byte has a 1110 as its high order bits, then the character consists of three bytes. The bits that make up the character value are in positions 0 through 3 of the first byte and bit positions 0 through 5 of the other two bytes. (The second and third bytes should have 10 as their high order bits). These values are in most significant byte first (i.e., "big endian") order.

As an example, if byte1 byte2 and byte3 are the three bytes read, and the high order bits of them match the patterns which indicate a three byte character encoding, then they would be converted to a Java char like so:

(char)(((byte1 & 0x0F) << 12) | ((byte2 & 0x3F) << 6) | (byte3 & 0x3F))

Note that all characters are encoded in the method that requires the fewest number of bytes with the exception of the character with the value of \u0000 which is encoded as two bytes. This is a modification of the UTF standard used to prevent C language style NUL values from appearing in the byte stream.

This method can read data that was written by an object implementing the writeUTF() method in DataOutput

Specified by:
readUTF in interface DataInput

seek

public void seek(long pos)
          throws IOException
This method sets the current file position to the specified offset from the beginning of the file. Note that some operating systems will allow the file pointer to be set past the current end of the file.

skipBytes

public int skipBytes(int numBytes)
              throws IOException
This method attempts to skip and discard the specified number of bytes in the input stream. It may actually skip fewer bytes than requested. The actual number of bytes skipped is returned. This method will not skip any bytes if passed a negative number of bytes to skip.

Specified by:
skipBytes in interface DataInput

write

public void write(int oneByte)
           throws IOException
This method writes a single byte of data to the file. The file must be open for read-write in order for this operation to succeed.

Specified by:
write in interface DataOutput

write

public void write(byte[] buffer)
           throws IOException
This method writes all the bytes in the specified array to the file. The file must be open read-write in order for this operation to succeed.

Specified by:
write in interface DataOutput

write

public void write(byte[] buffer,
                  int offset,
                  int len)
           throws IOException
This method writes len bytes to the file from the specified array starting at index offset into the array.

Specified by:
write in interface DataOutput

writeBoolean

public final void writeBoolean(boolean val)
                        throws IOException
This method writes a Java boolean to the underlying output stream. For a value of true, 1 is written to the stream. For a value of false, 0 is written.

Specified by:
writeBoolean in interface DataOutput

writeByte

public final void writeByte(int val)
                     throws IOException
This method writes a Java byte value to the underlying output stream.

Specified by:
writeByte in interface DataOutput

writeShort

public final void writeShort(int val)
                      throws IOException
This method writes a Java short to the stream, high byte first. This method requires two bytes to encode the value.

Specified by:
writeShort in interface DataOutput

writeChar

public final void writeChar(int val)
                     throws IOException
This method writes a single char value to the stream, high byte first.

Specified by:
writeChar in interface DataOutput

writeInt

public final void writeInt(int val)
                    throws IOException
This method writes a Java int to the stream, high bytes first. This method requires four bytes to encode the value.

Specified by:
writeInt in interface DataOutput

writeLong

public final void writeLong(long val)
                     throws IOException
This method writes a Java long to the stream, high bytes first. This method requires eight bytes to encode the value.

Specified by:
writeLong in interface DataOutput

writeFloat

public final void writeFloat(float val)
                      throws IOException
This method writes a Java float value to the stream. This value is written by first calling the method Float.floatToIntBits to retrieve an int representing the floating point number, then writing this int value to the stream exactly the same as the writeInt() method does.

Specified by:
writeFloat in interface DataOutput

writeDouble

public final void writeDouble(double val)
                       throws IOException
This method writes a Java double value to the stream. This value is written by first calling the method Double.doubleToLongBits to retrieve an long representing the floating point number, then writing this long value to the stream exactly the same as the writeLong() method does.

Specified by:
writeDouble in interface DataOutput

writeBytes

public final void writeBytes(java.lang.String val)
                      throws IOException
This method writes all the bytes in a String out to the stream. One byte is written for each character in the String. The high eight bits of each character are discarded.

Specified by:
writeBytes in interface DataOutput

writeChars

public final void writeChars(java.lang.String val)
                      throws IOException
This method writes all the characters in a String to the stream. There will be two bytes for each character value. The high byte of the character will be written first.

Specified by:
writeChars in interface DataOutput

writeUTF

public final void writeUTF(java.lang.String val)
                    throws IOException
This method writes a Java String to the stream in a modified UTF-8 format. First, two bytes are written to the stream indicating the number of bytes to follow. Note that this is the number of bytes in the encoded String not the String length. Next come the encoded characters. Each character in the String is encoded as either one, two or three bytes. For characters in the range of \u0001 to \u007F, one byte is used. The character value goes into bits 0-7 and bit eight is 0. For characters in the range of \u0080 to \u007FF, two bytes are used. Bits 6-10 of the character value are encoded bits 0-4 of the first byte, with the high bytes having a value of "110". Bits 0-5 of the character value are stored in bits 0-5 of the second byte, with the high bits set to "10". This type of encoding is also done for the null character \u0000. This eliminates any C style NUL character values in the output. All remaining characters are stored as three bytes. Bits 12-15 of the character value are stored in bits 0-3 of the first byte. The high bits of the first bytes are set to "1110". Bits 6-11 of the character value are stored in bits 0-5 of the second byte. The high bits of the second byte are set to "10". And bits 0-5 of the character value are stored in bits 0-5 of byte three, with the high bits of that byte set to "10".

Specified by:
writeUTF in interface DataOutput

getChannel

public final java.nio.channels.FileChannel getChannel()
This method creates a java.nio.channels.FileChannel. Nio does not allow one to create a file channel directly. A file channel must be created by first creating an instance of Input/Output/RandomAccessFile and invoking the getChannel() method on it.

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