RandomAccessFile

public class RandomAccessFile extends Object
implements DataOutput DataInput Closeable

Instances of this class support both reading and writing to a random access file. A random access file behaves like a large array of bytes stored in the file system. There is a kind of cursor, or index into the implied array, called the file pointer; input operations read bytes starting at the file pointer and advance the file pointer past the bytes read. If the random access file is created in read/write mode, then output operations are also available; output operations write bytes starting at the file pointer and advance the file pointer past the bytes written. Output operations that write past the current end of the implied array cause the array to be extended. The file pointer can be read by the getFilePointer method and set by the seek method.

It is generally true of all the reading routines in this class that if end-of-file is reached before the desired number of bytes has been read, an EOFException (which is a kind of IOException) is thrown. If any byte cannot be read for any reason other than end-of-file, an IOException other than EOFException is thrown. In particular, an IOException may be thrown if the stream has been closed.

Public Constructor Summary

RandomAccessFile(String name, String mode)
Creates a random access file stream to read from, and optionally to write to, a file with the specified name.
RandomAccessFile(File file, String mode)
Creates a random access file stream to read from, and optionally to write to, the file specified by the File argument.

Public Method Summary

void
close()
Closes this random access file stream and releases any system resources associated with the stream.
final FileChannel
getChannel()
Returns the unique FileChannel object associated with this file.
final FileDescriptor
getFD()
Returns the opaque file descriptor object associated with this stream.
long
getFilePointer()
Returns the current offset in this file.
long
length()
Returns the length of this file.
int
read()
Reads a byte of data from this file.
int
read(byte[] b)
Reads up to b.length bytes of data from this file into an array of bytes.
int
read(byte[] b, int off, int len)
Reads up to len bytes of data from this file into an array of bytes.
final boolean
readBoolean()
Reads a boolean from this file.
final byte
readByte()
Reads a signed eight-bit value from this file.
final char
readChar()
Reads a character from this file.
final double
readDouble()
Reads a double from this file.
final float
readFloat()
Reads a float from this file.
final void
readFully(byte[] b)
Reads b.length bytes from this file into the byte array, starting at the current file pointer.
final void
readFully(byte[] b, int off, int len)
Reads exactly len bytes from this file into the byte array, starting at the current file pointer.
final int
readInt()
Reads a signed 32-bit integer from this file.
final String
readLine()
Reads the next line of text from this file.
final long
readLong()
Reads a signed 64-bit integer from this file.
final short
readShort()
Reads a signed 16-bit number from this file.
final String
readUTF()
Reads in a string from this file.
final int
readUnsignedByte()
Reads an unsigned eight-bit number from this file.
final int
readUnsignedShort()
Reads an unsigned 16-bit number from this file.
void
seek(long pos)
Sets the file-pointer offset, measured from the beginning of this file, at which the next read or write occurs.
void
setLength(long newLength)
Sets the length of this file.
int
skipBytes(int n)
Attempts to skip over n bytes of input discarding the skipped bytes.
void
write(byte[] b)
Writes b.length bytes from the specified byte array to this file, starting at the current file pointer.
void
write(byte[] b, int off, int len)
Writes len bytes from the specified byte array starting at offset off to this file.
void
write(int b)
Writes the specified byte to this file.
final void
writeBoolean(boolean v)
Writes a boolean to the file as a one-byte value.
final void
writeByte(int v)
Writes a byte to the file as a one-byte value.
final void
writeBytes(String s)
Writes the string to the file as a sequence of bytes.
final void
writeChar(int v)
Writes a char to the file as a two-byte value, high byte first.
final void
writeChars(String s)
Writes a string to the file as a sequence of characters.
final void
writeDouble(double v)
Converts the double argument to a long using the doubleToLongBits method in class Double, and then writes that long value to the file as an eight-byte quantity, high byte first.
final void
writeFloat(float v)
Converts the float argument to an int using the floatToIntBits method in class Float, and then writes that int value to the file as a four-byte quantity, high byte first.
final void
writeInt(int v)
Writes an int to the file as four bytes, high byte first.
final void
writeLong(long v)
Writes a long to the file as eight bytes, high byte first.
final void
writeShort(int v)
Writes a short to the file as two bytes, high byte first.
final void
writeUTF(String str)
Writes a string to the file using modified UTF-8 encoding in a machine-independent manner.

Protected Method Summary

void

Inherited Method Summary

Public Constructors

public RandomAccessFile (String name, String mode)

Creates a random access file stream to read from, and optionally to write to, a file with the specified name. A new FileDescriptor object is created to represent the connection to the file.

The mode argument specifies the access mode with which the file is to be opened. The permitted values and their meanings are as specified for the RandomAccessFile(File,String) constructor.

If there is a security manager, its checkRead method is called with the name argument as its argument to see if read access to the file is allowed. If the mode allows writing, the security manager's checkWrite method is also called with the name argument as its argument to see if write access to the file is allowed.

Parameters
name the system-dependent filename
mode the access mode
Throws
IllegalArgumentException if the mode argument is not equal to one of "r", "rw", "rws", or "rwd"
FileNotFoundException if the mode is "r" but the given string does not denote an existing regular file, or if the mode begins with "rw" but the given string does not denote an existing, writable regular file and a new regular file of that name cannot be created, or if some other error occurs while opening or creating the file
SecurityException if a security manager exists and its checkRead method denies read access to the file or the mode is "rw" and the security manager's checkWrite method denies write access to the file

public RandomAccessFile (File file, String mode)

Creates a random access file stream to read from, and optionally to write to, the file specified by the File argument. A new FileDescriptor object is created to represent this file connection.

The mode argument specifies the access mode in which the file is to be opened. The permitted values and their meanings are:

ValueMeaning
"r" Open for reading only. Invoking any of the write methods of the resulting object will cause an IOException to be thrown.
"rw" Open for reading and writing. If the file does not already exist then an attempt will be made to create it.
"rws" Open for reading and writing, as with "rw", and also require that every update to the file's content or metadata be written synchronously to the underlying storage device.
"rwd"   Open for reading and writing, as with "rw", and also require that every update to the file's content be written synchronously to the underlying storage device.
The "rws" and "rwd" modes work much like the force(boolean) method of the FileChannel class, passing arguments of true and false, respectively, except that they always apply to every I/O operation and are therefore often more efficient. If the file resides on a local storage device then when an invocation of a method of this class returns it is guaranteed that all changes made to the file by that invocation will have been written to that device. This is useful for ensuring that critical information is not lost in the event of a system crash. If the file does not reside on a local device then no such guarantee is made.

The "rwd" mode can be used to reduce the number of I/O operations performed. Using "rwd" only requires updates to the file's content to be written to storage; using "rws" requires updates to both the file's content and its metadata to be written, which generally requires at least one more low-level I/O operation.

If there is a security manager, its checkRead method is called with the pathname of the file argument as its argument to see if read access to the file is allowed. If the mode allows writing, the security manager's checkWrite method is also called with the path argument to see if write access to the file is allowed.

Parameters
file the file object
mode the access mode, as described above
Throws
IllegalArgumentException if the mode argument is not equal to one of "r", "rw", "rws", or "rwd"
FileNotFoundException if the mode is "r" but the given file object does not denote an existing regular file, or if the mode begins with "rw" but the given file object does not denote an existing, writable regular file and a new regular file of that name cannot be created, or if some other error occurs while opening or creating the file
SecurityException if a security manager exists and its checkRead method denies read access to the file or the mode is "rw" and the security manager's checkWrite method denies write access to the file

Public Methods

public void close ()

Closes this random access file stream and releases any system resources associated with the stream. A closed random access file cannot perform input or output operations and cannot be reopened.

If this file has an associated channel then the channel is closed as well.

Throws
IOException if an I/O error occurs.

public final FileChannel getChannel ()

Returns the unique FileChannel object associated with this file.

The position of the returned channel will always be equal to this object's file-pointer offset as returned by the getFilePointer method. Changing this object's file-pointer offset, whether explicitly or by reading or writing bytes, will change the position of the channel, and vice versa. Changing the file's length via this object will change the length seen via the file channel, and vice versa.

Returns
  • the file channel associated with this file

public final FileDescriptor getFD ()

Returns the opaque file descriptor object associated with this stream.

Returns
  • the file descriptor object associated with this stream.
Throws
IOException if an I/O error occurs.
See Also

public long getFilePointer ()

Returns the current offset in this file.

Returns
  • the offset from the beginning of the file, in bytes, at which the next read or write occurs.
Throws
IOException if an I/O error occurs.

public long length ()

Returns the length of this file.

Returns
  • the length of this file, measured in bytes.
Throws
IOException if an I/O error occurs.

public int read ()

Reads a byte of data from this file. The byte is returned as an integer in the range 0 to 255 (0x00-0x0ff). This method blocks if no input is yet available.

Although RandomAccessFile is not a subclass of InputStream, this method behaves in exactly the same way as the read() method of InputStream.

Returns
  • the next byte of data, or -1 if the end of the file has been reached.
Throws
IOException if an I/O error occurs. Not thrown if end-of-file has been reached.

public int read (byte[] b)

Reads up to b.length bytes of data from this file into an array of bytes. This method blocks until at least one byte of input is available.

Although RandomAccessFile is not a subclass of InputStream, this method behaves in exactly the same way as the read(byte[]) method of InputStream.

Parameters
b the buffer into which the data is read.
Returns
  • the total number of bytes read into the buffer, or -1 if there is no more data because the end of this file has been reached.
Throws
IOException If the first byte cannot be read for any reason other than end of file, or if the random access file has been closed, or if some other I/O error occurs.
NullPointerException If b is null.

public int read (byte[] b, int off, int len)

Reads up to len bytes of data from this file into an array of bytes. This method blocks until at least one byte of input is available.

Although RandomAccessFile is not a subclass of InputStream, this method behaves in exactly the same way as the read(byte[], int, int) method of InputStream.

Parameters
b the buffer into which the data is read.
off the start offset in array b at which the data is written.
len the maximum number of bytes read.
Returns
  • the total number of bytes read into the buffer, or -1 if there is no more data because the end of the file has been reached.
Throws
IOException If the first byte cannot be read for any reason other than end of file, or if the random access file has been closed, or if some other I/O error occurs.
NullPointerException If b is null.
IndexOutOfBoundsException If off is negative, len is negative, or len is greater than b.length - off

public final boolean readBoolean ()

Reads a boolean from this file. This method reads a single byte from the file, starting at the current file pointer. A value of 0 represents false. Any other value represents true. This method blocks until the byte is read, the end of the stream is detected, or an exception is thrown.

Returns
  • the boolean value read.
Throws
EOFException if this file has reached the end.
IOException if an I/O error occurs.

public final byte readByte ()

Reads a signed eight-bit value from this file. This method reads a byte from the file, starting from the current file pointer. If the byte read is b, where 0 <= b <= 255, then the result is:

     (byte)(b)
 

This method blocks until the byte is read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next byte of this file as a signed eight-bit byte.
Throws
EOFException if this file has reached the end.
IOException if an I/O error occurs.

public final char readChar ()

Reads a character from this file. This method reads two bytes from the file, starting at the current file pointer. If the bytes read, in order, are b1 and b2, where 0 <= b1, b2 <= 255, then the result is equal to:

     (char)((b1 << 8) | b2)
 

This method blocks until the two bytes are read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next two bytes of this file, interpreted as a char.
Throws
EOFException if this file reaches the end before reading two bytes.
IOException if an I/O error occurs.

public final double readDouble ()

Reads a double from this file. This method reads a long value, starting at the current file pointer, as if by the readLong method and then converts that long to a double using the longBitsToDouble method in class Double.

This method blocks until the eight bytes are read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next eight bytes of this file, interpreted as a double.
Throws
EOFException if this file reaches the end before reading eight bytes.
IOException if an I/O error occurs.

public final float readFloat ()

Reads a float from this file. This method reads an int value, starting at the current file pointer, as if by the readInt method and then converts that int to a float using the intBitsToFloat method in class Float.

This method blocks until the four bytes are read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next four bytes of this file, interpreted as a float.
Throws
EOFException if this file reaches the end before reading four bytes.
IOException if an I/O error occurs.

public final void readFully (byte[] b)

Reads b.length bytes from this file into the byte array, starting at the current file pointer. This method reads repeatedly from the file until the requested number of bytes are read. This method blocks until the requested number of bytes are read, the end of the stream is detected, or an exception is thrown.

Parameters
b the buffer into which the data is read.
Throws
EOFException if this file reaches the end before reading all the bytes.
IOException if an I/O error occurs.

public final void readFully (byte[] b, int off, int len)

Reads exactly len bytes from this file into the byte array, starting at the current file pointer. This method reads repeatedly from the file until the requested number of bytes are read. This method blocks until the requested number of bytes are read, the end of the stream is detected, or an exception is thrown.

Parameters
b the buffer into which the data is read.
off the start offset of the data.
len the number of bytes to read.
Throws
EOFException if this file reaches the end before reading all the bytes.
IOException if an I/O error occurs.

public final int readInt ()

Reads a signed 32-bit integer from this file. This method reads 4 bytes from the file, starting at the current file pointer. If the bytes read, in order, are b1, b2, b3, and b4, where 0 <= b1, b2, b3, b4 <= 255, then the result is equal to:

     (b1 << 24) | (b2 << 16) + (b3 << 8) + b4
 

This method blocks until the four bytes are read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next four bytes of this file, interpreted as an int.
Throws
EOFException if this file reaches the end before reading four bytes.
IOException if an I/O error occurs.

public final String readLine ()

Reads the next line of text from this file. This method successively reads bytes from the file, starting at the current file pointer, until it reaches a line terminator or the end of the file. Each byte is converted into a character by taking the byte's value for the lower eight bits of the character and setting the high eight bits of the character to zero. This method does not, therefore, support the full Unicode character set.

A line of text is terminated by a carriage-return character ('\r'), a newline character ('\n'), a carriage-return character immediately followed by a newline character, or the end of the file. Line-terminating characters are discarded and are not included as part of the string returned.

This method blocks until a newline character is read, a carriage return and the byte following it are read (to see if it is a newline), the end of the file is reached, or an exception is thrown.

Returns
  • the next line of text from this file, or null if end of file is encountered before even one byte is read.
Throws
IOException if an I/O error occurs.

public final long readLong ()

Reads a signed 64-bit integer from this file. This method reads eight bytes from the file, starting at the current file pointer. If the bytes read, in order, are b1, b2, b3, b4, b5, b6, b7, and b8, where:

     0 <= b1, b2, b3, b4, b5, b6, b7, b8 <=255,
 

then the result is equal to:

     ((long)b1 << 56) + ((long)b2 << 48)
     + ((long)b3 << 40) + ((long)b4 << 32)
     + ((long)b5 << 24) + ((long)b6 << 16)
     + ((long)b7 << 8) + b8
 

This method blocks until the eight bytes are read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next eight bytes of this file, interpreted as a long.
Throws
EOFException if this file reaches the end before reading eight bytes.
IOException if an I/O error occurs.

public final short readShort ()

Reads a signed 16-bit number from this file. The method reads two bytes from this file, starting at the current file pointer. If the two bytes read, in order, are b1 and b2, where each of the two values is between 0 and 255, inclusive, then the result is equal to:

     (short)((b1 << 8) | b2)
 

This method blocks until the two bytes are read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next two bytes of this file, interpreted as a signed 16-bit number.
Throws
EOFException if this file reaches the end before reading two bytes.
IOException if an I/O error occurs.

public final String readUTF ()

Reads in a string from this file. The string has been encoded using a modified UTF-8 format.

The first two bytes are read, starting from the current file pointer, as if by readUnsignedShort. This value gives the number of following bytes that are in the encoded string, not the length of the resulting string. The following bytes are then interpreted as bytes encoding characters in the modified UTF-8 format and are converted into characters.

This method blocks until all the bytes are read, the end of the stream is detected, or an exception is thrown.

Returns
  • a Unicode string.
Throws
EOFException if this file reaches the end before reading all the bytes.
IOException if an I/O error occurs.
UTFDataFormatException if the bytes do not represent valid modified UTF-8 encoding of a Unicode string.

public final int readUnsignedByte ()

Reads an unsigned eight-bit number from this file. This method reads a byte from this file, starting at the current file pointer, and returns that byte.

This method blocks until the byte is read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next byte of this file, interpreted as an unsigned eight-bit number.
Throws
EOFException if this file has reached the end.
IOException if an I/O error occurs.

public final int readUnsignedShort ()

Reads an unsigned 16-bit number from this file. This method reads two bytes from the file, starting at the current file pointer. If the bytes read, in order, are b1 and b2, where 0 <= b1, b2 <= 255, then the result is equal to:

     (b1 << 8) | b2
 

This method blocks until the two bytes are read, the end of the stream is detected, or an exception is thrown.

Returns
  • the next two bytes of this file, interpreted as an unsigned 16-bit integer.
Throws
EOFException if this file reaches the end before reading two bytes.
IOException if an I/O error occurs.

public void seek (long pos)

Sets the file-pointer offset, measured from the beginning of this file, at which the next read or write occurs. The offset may be set beyond the end of the file. Setting the offset beyond the end of the file does not change the file length. The file length will change only by writing after the offset has been set beyond the end of the file.

Parameters
pos the offset position, measured in bytes from the beginning of the file, at which to set the file pointer.
Throws
IOException if pos is less than 0 or if an I/O error occurs.

public void setLength (long newLength)

Sets the length of this file.

If the present length of the file as returned by the length method is greater than the newLength argument then the file will be truncated. In this case, if the file offset as returned by the getFilePointer method is greater than newLength then after this method returns the offset will be equal to newLength.

If the present length of the file as returned by the length method is smaller than the newLength argument then the file will be extended. In this case, the contents of the extended portion of the file are not defined.

Parameters
newLength The desired length of the file
Throws
IOException If an I/O error occurs

public int skipBytes (int n)

Attempts to skip over n bytes of input discarding the skipped bytes.

This method may skip over some smaller number of bytes, possibly zero. This may result from any of a number of conditions; reaching end of file before n bytes have been skipped is only one possibility. This method never throws an EOFException. The actual number of bytes skipped is returned. If n is negative, no bytes are skipped.

Parameters
n the number of bytes to be skipped.
Returns
  • the actual number of bytes skipped.
Throws
IOException if an I/O error occurs.

public void write (byte[] b)

Writes b.length bytes from the specified byte array to this file, starting at the current file pointer.

Parameters
b the data.
Throws
IOException if an I/O error occurs.

public void write (byte[] b, int off, int len)

Writes len bytes from the specified byte array starting at offset off to this file.

Parameters
b the data.
off the start offset in the data.
len the number of bytes to write.
Throws
IOException if an I/O error occurs.

public void write (int b)

Writes the specified byte to this file. The write starts at the current file pointer.

Parameters
b the byte to be written.
Throws
IOException if an I/O error occurs.

public final void writeBoolean (boolean v)

Writes a boolean to the file as a one-byte value. The value true is written out as the value (byte)1; the value false is written out as the value (byte)0. The write starts at the current position of the file pointer.

Parameters
v a boolean value to be written.
Throws
IOException if an I/O error occurs.

public final void writeByte (int v)

Writes a byte to the file as a one-byte value. The write starts at the current position of the file pointer.

Parameters
v a byte value to be written.
Throws
IOException if an I/O error occurs.

public final void writeBytes (String s)

Writes the string to the file as a sequence of bytes. Each character in the string is written out, in sequence, by discarding its high eight bits. The write starts at the current position of the file pointer.

Parameters
s a string of bytes to be written.
Throws
IOException if an I/O error occurs.

public final void writeChar (int v)

Writes a char to the file as a two-byte value, high byte first. The write starts at the current position of the file pointer.

Parameters
v a char value to be written.
Throws
IOException if an I/O error occurs.

public final void writeChars (String s)

Writes a string to the file as a sequence of characters. Each character is written to the data output stream as if by the writeChar method. The write starts at the current position of the file pointer.

Parameters
s a String value to be written.
Throws
IOException if an I/O error occurs.
See Also

public final void writeDouble (double v)

Converts the double argument to a long using the doubleToLongBits method in class Double, and then writes that long value to the file as an eight-byte quantity, high byte first. The write starts at the current position of the file pointer.

Parameters
v a double value to be written.
Throws
IOException if an I/O error occurs.

public final void writeFloat (float v)

Converts the float argument to an int using the floatToIntBits method in class Float, and then writes that int value to the file as a four-byte quantity, high byte first. The write starts at the current position of the file pointer.

Parameters
v a float value to be written.
Throws
IOException if an I/O error occurs.

public final void writeInt (int v)

Writes an int to the file as four bytes, high byte first. The write starts at the current position of the file pointer.

Parameters
v an int to be written.
Throws
IOException if an I/O error occurs.

public final void writeLong (long v)

Writes a long to the file as eight bytes, high byte first. The write starts at the current position of the file pointer.

Parameters
v a long to be written.
Throws
IOException if an I/O error occurs.

public final void writeShort (int v)

Writes a short to the file as two bytes, high byte first. The write starts at the current position of the file pointer.

Parameters
v a short to be written.
Throws
IOException if an I/O error occurs.

public final void writeUTF (String str)

Writes a string to the file using modified UTF-8 encoding in a machine-independent manner.

First, two bytes are written to the file, starting at the current file pointer, as if by the writeShort method giving the number of bytes to follow. This value is the number of bytes actually written out, not the length of the string. Following the length, each character of the string is output, in sequence, using the modified UTF-8 encoding for each character.

Parameters
str a string to be written.
Throws
IOException if an I/O error occurs.

Protected Methods

protected void finalize ()

Throws
Throwable