java.io

Class RandomAccessFile

java.lang.Object

java.io.RandomAccessFile

All Implemented Interfaces:

Closeable, DataInput, DataOutput, AutoCloseable

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.

Since:

JDK1.0

Author:

unascribed

Constructor Summary

Constructors

Constructor and Description
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.
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.

Method Summary

Modifier and Type	Method and Description
void	close() Closes this random access file stream and releases any system resources associated with the 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.
boolean	readBoolean() Reads a boolean from this file.
byte	readByte() Reads a signed eight-bit value from this file.
char	readChar() Reads a character from this file.
double	readDouble() Reads a double from this file.
float	readFloat() Reads a float from this file.
void	readFully(byte[] b) Reads b.length bytes from this file into the byte array, starting at the current file pointer.
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.
int	readInt() Reads a signed 32-bit integer from this file.
String	readLine() Reads the next line of text from this file.
long	readLong() Reads a signed 64-bit integer from this file.
short	readShort() Reads a signed 16-bit number from this file.
int	readUnsignedByte() Reads an unsigned eight-bit number from this file.
int	readUnsignedShort() Reads an unsigned 16-bit number from this file.
String	readUTF() Reads in a string 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 to the output stream the eight low-order bits of the argument b.
void	writeBoolean(boolean v) Writes a boolean to the file as a one-byte value.
void	writeByte(int v) Writes a byte to the file as a one-byte value.
void	writeBytes(String s) Writes the string to the file as a sequence of bytes.
void	writeChar(int v) Writes a char to the file as a two-byte value, high byte first.
void	writeChars(String s) Writes a string to the file as a sequence of characters.
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.
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.
void	writeInt(int v) Writes an int to the file as four bytes, high byte first.
void	writeLong(long v) Writes a long to the file as eight bytes, high byte first.
void	writeShort(int v) Writes a short to the file as two bytes, high byte first.
void	writeUTF(String str) Writes a string to the file using modified UTF-8 encoding in a machine-independent manner.

Methods inherited from class java.lang.Object

clone, equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

RandomAccessFile

public RandomAccessFile(File file,
                        String mode)
                 throws FileNotFoundException

Creates a random access file stream to read from, and optionally to write to, the file specified by the File argument.

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

Value	Meaning
"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" 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

RandomAccessFile

public RandomAccessFile(String name,
                        String mode)
                 throws FileNotFoundException

Creates a random access file stream to read from, and optionally to write to, a file with the specified name.

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

See Also:

SecurityException

Method Detail

close

public void close()
           throws IOException

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.

Specified by:

close in interface Closeable

Specified by:

close in interface AutoCloseable

Throws:

IOException - if an I/O error occurs.

getFilePointer

public long getFilePointer()
                    throws IOException

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.

length

public long length()
            throws IOException

Returns the length of this file.

Returns:

the length of this file, measured in bytes.

Throws:

IOException - if an I/O error occurs.

read

public int read()
         throws IOException

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 InputStream.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.

read

public int read(byte[] b)
         throws IOException

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 InputStream.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.

read

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

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 InputStream.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

readBoolean

public final boolean readBoolean()
                          throws IOException

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.

Specified by:

readBoolean in interface DataInput

Returns:

the boolean value read.

Throws:

EOFException - if this file has reached the end.

IOException - if an I/O error occurs.

readByte

public final byte readByte()
                    throws IOException

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.

Specified by:

readByte in interface DataInput

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.

readChar

public final char readChar()
                    throws IOException

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.

Specified by:

readChar in interface DataInput

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.

readDouble

public final double readDouble()
                        throws IOException

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.

Specified by:

readDouble in interface DataInput

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.

See Also:

readLong(), Double.longBitsToDouble(long)

readFloat

public final float readFloat()
                      throws IOException

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.

Specified by:

readFloat in interface DataInput

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.

See Also:

readInt(), Float.intBitsToFloat(int)

readFully

public final void readFully(byte[] b)
                     throws IOException

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.

Specified by:

readFully in interface DataInput

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.

readFully

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

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.

Specified by:

readFully in interface DataInput

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.

readInt

public final int readInt()
                  throws IOException

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.

Specified by:

readInt in interface DataInput

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.

readLine

@Nullable
public final String readLine()
                                throws IOException

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.

readLong

public final long readLong()
                    throws IOException

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.

Specified by:

readLong in interface DataInput

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.

readShort

public final short readShort()
                      throws IOException

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.

Specified by:

readShort in interface DataInput

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.

readUnsignedByte

public final int readUnsignedByte()
                           throws IOException

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.

Specified by:

readUnsignedByte in interface DataInput

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.

readUnsignedShort

public final int readUnsignedShort()
                            throws IOException

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.

Specified by:

readUnsignedShort in interface DataInput

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.

readUTF

public final String readUTF()
                     throws IOException

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.

Specified by:

readUTF in interface DataInput

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.

See Also:

readUnsignedShort()

seek

public void seek(long pos)
          throws IOException

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.

setLength

public void setLength(long newLength)
               throws IOException

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

skipBytes

public int skipBytes(int n)
              throws IOException

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.

Specified by:

skipBytes in interface DataInput

Parameters:

n - the number of bytes to be skipped.

Returns:

the actual number of bytes skipped.

Throws:

IOException - if an I/O error occurs.

write

public void write(byte[] b)
           throws IOException

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

Specified by:

write in interface DataOutput

Parameters:

b - the data.

Throws:

IOException - if an I/O error occurs.

write

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

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

Specified by:

write in interface DataOutput

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.

write

public void write(int b)
           throws IOException

Description copied from interface: DataOutput

Writes to the output stream the eight low-order bits of the argument b. The 24 high-order bits of b are ignored.

Specified by:

write in interface DataOutput

Parameters:

b - the byte to be written.

Throws:

IOException - if an I/O error occurs.

writeBoolean

public final void writeBoolean(boolean v)
                        throws IOException

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.

Specified by:

writeBoolean in interface DataOutput

Parameters:

v - a boolean value to be written.

Throws:

IOException - if an I/O error occurs.

writeByte

public final void writeByte(int v)
                     throws IOException

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

Specified by:

writeByte in interface DataOutput

Parameters:

v - a byte value to be written.

Throws:

IOException - if an I/O error occurs.

writeBytes

public final void writeBytes(String s)
                      throws IOException

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.

writeChar

public final void writeChar(int v)
                     throws IOException

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.

Specified by:

writeChar in interface DataOutput

Parameters:

v - a char value to be written.

Throws:

IOException - if an I/O error occurs.

writeChars

public final void writeChars(String s)
                      throws IOException

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.

Specified by:

writeChars in interface DataOutput

Parameters:

s - a String value to be written.

Throws:

IOException - if an I/O error occurs.

See Also:

writeChar(int)

writeDouble

public final void writeDouble(double v)
                       throws IOException

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.

Specified by:

writeDouble in interface DataOutput

Parameters:

v - a double value to be written.

Throws:

IOException - if an I/O error occurs.

See Also:

Double.doubleToLongBits(double)

writeFloat

public final void writeFloat(float v)
                      throws IOException

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.

Specified by:

writeFloat in interface DataOutput

Parameters:

v - a float value to be written.

Throws:

IOException - if an I/O error occurs.

See Also:

Float.floatToIntBits(float)

writeInt

public final void writeInt(int v)
                    throws IOException

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

Specified by:

writeInt in interface DataOutput

Parameters:

v - an int to be written.

Throws:

IOException - if an I/O error occurs.

writeLong

public final void writeLong(long v)
                     throws IOException

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

Specified by:

writeLong in interface DataOutput

Parameters:

v - a long to be written.

Throws:

IOException - if an I/O error occurs.

writeShort

public final void writeShort(int v)
                      throws IOException

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

Specified by:

writeShort in interface DataOutput

Parameters:

v - a short to be written.

Throws:

IOException - if an I/O error occurs.

writeUTF

public final void writeUTF(String str)
                    throws IOException

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.

Specified by:

writeUTF in interface DataOutput

Parameters:

str - a string to be written.

Throws:

IOException - if an I/O error occurs.