java.io

Class RandomAccessFile

public class RandomAccessFile extends Object implements DataOutput, DataInput, Closeable

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.

Constructor Summary
	RandomAccessFile(File file, String mode) This method initializes a new instance of `RandomAccessFile` to read from the specified `File` object with the specified access mode.
	RandomAccessFile(String fileName, 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.
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.
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.
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(int oneByte) This method writes a single byte of data to the file.
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	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(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(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(String val) This method writes a Java `String` to the stream in a modified UTF-8 format.

Constructor Detail

RandomAccessFile

public RandomAccessFile(File file, String mode)

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.

Parameters: file The File object to read and/or write. mode "r" for read only or "rw" for read-write access to the file

Throws: IllegalArgumentException If mode has an illegal value SecurityException If the requested access to the file is not allowed FileNotFoundException If the file is a directory, or any other error occurs

RandomAccessFile

public RandomAccessFile(String fileName, String mode)

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.

Parameters: fileName The name of the file to read and/or write mode "r", "rw", "rws", or "rwd"

Method Detail

close

public void close()

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

getChannel

public final 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.

getFD

public final FileDescriptor getFD()

This method returns a FileDescriptor object that represents the native file handle for this file.

Returns: The FileDescriptor object for this file

Throws: IOException If an error occurs

getFilePointer

public long getFilePointer()

This method returns the current offset in the file at which the next read or write will occur

Returns: The current file position

Throws: IOException If an error occurs

length

public long length()

This method returns the length of the file in bytes

Returns: The length of the file

Throws: IOException If an error occurs

read

public int read()

This method reads a single byte of data from the file and returns it as an integer.

Returns: The byte read as an int, or -1 if the end of the file was reached.

Throws: IOException If an error occurs

read

public int read(byte[] buffer)

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.

Parameters: buffer The buffer to read bytes from the file into

Returns: The actual number of bytes read or -1 if end of file

Throws: IOException If an error occurs

read

public 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.

Parameters: buffer The array to read the bytes into offset The index into the array to start storing bytes len The requested number of bytes to read

Returns: The actual number of bytes read, or -1 if end of file

Throws: IOException If an error occurs

readBoolean

public final boolean readBoolean()

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.

Returns: The boolean value read

Throws: EOFException If end of file is reached before reading the boolean IOException If any other error occurs

readByte

public final byte readByte()

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.

Returns: The byte value read

Throws: EOFException If end of file is reached before reading the byte IOException If any other error occurs

See Also: DataOutput

readChar

public final char readChar()

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.

Returns: The char value read

Throws: EOFException If end of file is reached before reading the char IOException If any other error occurs

See Also: DataOutput

readDouble

public final double readDouble()

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.

Returns: The double value read

Throws: EOFException If end of file is reached before reading the double IOException If any other error occurs

readFloat

public final float readFloat()

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.

Returns: The float value read

Throws: EOFException If end of file is reached before reading the float IOException If any other error occurs

readFully

public final void readFully(byte[] buffer)

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

Parameters: buffer The buffer into which to read the data

Throws: EOFException If end of file is reached before filling the buffer IOException If any other error occurs

readFully

public final void readFully(byte[] buffer, int offset, int count)

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.

Parameters: buffer The buffer into which to read the data offset The offset into the buffer to start storing data count The number of bytes to read into the buffer

Throws: EOFException If end of file is reached before filling the buffer IOException If any other error occurs

readInt

public final 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.

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.

Returns: The int value read

Throws: EOFException If end of file is reached before reading the int IOException If any other error occurs

See Also: DataOutput

readLine

public final String readLine()

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

Returns: The line read as a String

Throws: IOException If an error occurs

See Also: DataOutput

readLong

public final 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.

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.

Returns: The long value read

Throws: EOFException If end of file is reached before reading the long IOException If any other error occurs

See Also: DataOutput

readShort

public final short readShort()

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.

Returns: The short value read

Throws: EOFException If end of file is reached before reading the value IOException If any other error occurs

See Also: DataOutput

readUnsignedByte

public final int readUnsignedByte()

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.

Returns: The unsigned bytes value read as a Java int

Throws: EOFException If end of file is reached before reading the value IOException If any other error occurs

See Also: DataOutput

readUnsignedShort

public final int readUnsignedShort()

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.

Returns: The unsigned short value read as a Java int

Throws: EOFException If end of file is reached before reading the value IOException If any other error occurs

readUTF

public final String readUTF()

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

Returns: The String read

Throws: EOFException If end of file is reached before reading the String UTFDataFormatException If the data is not in UTF-8 format IOException If any other error occurs

See Also: DataOutput

seek

public void seek(long pos)

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.

Parameters: pos The offset from the beginning of the file at which to set the file pointer

Throws: IOException If an error occurs

setLength

public void setLength(long newLen)

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.

Parameters: newLen The new length of the file

Throws: IOException If an error occurs

skipBytes

public int skipBytes(int numBytes)

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.

Parameters: numBytes The requested number of bytes to skip.

Returns: The number of bytes actually skipped.

Throws: IOException If an error occurs.

write

public void write(int oneByte)

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.

Parameters: oneByte The byte of data to write, passed as an int.

Throws: IOException If an error occurs

write

public void write(byte[] buffer)

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.

Parameters: buffer The array of bytes to write to the file

write

public 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.

Parameters: buffer The array of bytes to write to the file offset The index into the array to start writing file len The number of bytes to write

Throws: IOException If an error occurs

writeBoolean

public final void writeBoolean(boolean val)

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.

Parameters: val The boolean value to write to the stream

Throws: IOException If an error occurs

writeByte

public final void writeByte(int val)

This method writes a Java byte value to the underlying output stream.

Parameters: val The byte to write to the stream, passed as an int.

Throws: IOException If an error occurs

writeBytes

public final void writeBytes(String val)

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.

Parameters: val The String to write to the stream

Throws: IOException If an error occurs

writeChar

public final void writeChar(int val)

This method writes a single char value to the stream, high byte first.

Parameters: val The char value to write, passed as an int.

Throws: IOException If an error occurs

writeChars

public final void writeChars(String val)

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.

Parameters: val The String to write to the stream.

Throws: IOException If an error occurs

writeDouble

public final void writeDouble(double val)

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.

Parameters: val The double precision floating point number to write to the stream.

Throws: IOException If an error occurs

writeFloat

public final void writeFloat(float val)

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.

Parameters: val The floating point number to write to the stream.

Throws: IOException If an error occurs

writeInt

public final void writeInt(int val)

This method writes a Java int to the stream, high bytes first. This method requires four bytes to encode the value.

Parameters: val The int value to write to the stream.

Throws: IOException If an error occurs

writeLong

public final void writeLong(long val)

This method writes a Java long to the stream, high bytes first. This method requires eight bytes to encode the value.

Parameters: val The long value to write to the stream.

Throws: IOException If an error occurs

writeShort

public final void writeShort(int val)

This method writes a Java short to the stream, high byte first. This method requires two bytes to encode the value.

Parameters: val The short value to write to the stream, passed as an int.

Throws: IOException If an error occurs

writeUTF

public final void writeUTF(String val)

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".

Parameters: val The String to write to the output in UTF format

Throws: IOException If an error occurs