java.io

Class DataInputStream

public class DataInputStream extends FilterInputStream implements DataInput

This subclass of FilteredInputStream implements the DataInput interface that provides method for reading primitive Java data types from a stream.

Constructor Summary
	DataInputStream(InputStream in) This constructor initializes a new `DataInputStream` to read from the specified subordinate stream.

Method Summary
int	read(byte[] b) This method reads bytes from the underlying stream into the specified byte array buffer.
int	read(byte[] b, int off, int len) This method reads bytes from the underlying stream into the specified byte array buffer.
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[] b) This method reads raw bytes into the passed array until the array is full.
void	readFully(byte[] buf, int offset, int len) 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`.
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`.
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.
static String	readUTF(DataInput in) This method reads a String encoded in UTF-8 format from the specified `DataInput` source.
int	skipBytes(int n) This method attempts to skip and discard the specified number of bytes in the input stream.

Constructor Detail

DataInputStream

public DataInputStream(InputStream in)

This constructor initializes a new DataInputStream to read from the specified subordinate stream.

Parameters: in The subordinate InputStream to read from

This method reads bytes from the underlying stream into the specified byte array buffer. It will attempt to fill the buffer completely, but may return a short count if there is insufficient data remaining to be read to fill the buffer.

Parameters: b The buffer into which bytes will be read.

Returns: The actual number of bytes read, or -1 if end of stream reached before reading any bytes.

Throws: IOException If an error occurs.

read

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

This method reads bytes from the underlying stream into the specified byte array buffer. It will attempt to read len bytes and will start storing them at position off into the buffer. This method can return a short count if there is insufficient data remaining to be read to complete the desired read length.

Parameters: b The buffer into which bytes will be read. off The offset into the buffer to start storing bytes. len The requested number of bytes to read.

Returns: The actual number of bytes read, or -1 if end of stream reached before reading any bytes.

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

See Also: DataOutput

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 long 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[] b)

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. Note also that zero length buffers are permitted. In this case, the method will return immediately without reading any bytes from the stream.

Parameters: b 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[] buf, int offset, int len)

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. Note also that zero length buffers are permitted. In this case, the method will return immediately without reading any bytes from the stream.

Parameters: buf The buffer into which to read the data offset The offset into the buffer to start storing data len 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 -2147483648 to 2147483647.

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()

Deprecated:

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

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)(((byte1 & 0xFF) << 56) + ((byte2 & 0xFF) << 48) + ((byte3 & 0xFF) << 40) + ((byte4 & 0xFF) << 32) + ((byte5 & 0xFF) << 24) + ((byte6 & 0xFF) << 16) + ((byte7 & 0xFF) << 8) + (byte8 & 0xFF)))

The value returned is in the range of -9223372036854775808 to 9223372036854775807.

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

See Also: DataOutput

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

readUTF

public static final String readUTF(DataInput in)

This method reads a String encoded in UTF-8 format from the specified DataInput source.

Parameters: in The DataInput source to read from

Returns: The String read from the source

Throws: IOException If an error occurs

skipBytes

public final int skipBytes(int n)

This method attempts to skip and discard the specified number of bytes in the input stream. It may actually skip fewer bytes than requested. This method will not skip any bytes if passed a negative number of bytes to skip.

Parameters: n The requested number of bytes to skip.

Returns: The requested number of bytes to skip.

Throws: IOException If an error occurs.

UNKNOWN: The JDK docs claim that this returns the number of bytes actually skipped. The JCL claims that this method can throw an EOFException. Neither of these appear to be true in the JDK 1.3's implementation. This tries to implement the actual JDK behaviour.