* It is generally true of all the reading * routines in this interface that if end of * file is reached before the desired number * of bytes has been read, an {@code EOFException} * (which is a kind of {@code IOException}) * is thrown. If any byte cannot be read for * any reason other than end of file, an {@code IOException} * other than {@code EOFException} is * thrown. In particular, an {@code IOException} * may be thrown if the input stream has been * closed. * *
* Implementations of the DataInput and DataOutput interfaces represent * Unicode strings in a format that is a slight modification of UTF-8. * (For information regarding the standard UTF-8 format, see section * 3.9 Unicode Encoding Forms of The Unicode Standard, Version * 4.0). * Note that in the following table, the most significant bit appears in the * far left-hand column. * *
*** * *
** ** All characters in the range {@code '\u005Cu0001'} to * {@code '\u005Cu007F'} are represented by a single byte: ** ** Bit Values ** *Byte 1 *0 * bits 6-0 * * ** The null character {@code '\u005Cu0000'} and characters * in the range {@code '\u005Cu0080'} to {@code '\u005Cu07FF'} are * represented by a pair of bytes: ** ** Bit Values ** *Byte 1 *1 * 1 * 0 * bits 10-6 * * *Byte 2 *1 * 0 * bits 5-0 * * ** {@code char} values in the range {@code '\u005Cu0800'} * to {@code '\u005CuFFFF'} are represented by three bytes: ** ** Bit Values ** *Byte 1 *1 * 1 * 1 * 0 * bits 15-12 * * *Byte 2 *1 * 0 * bits 11-6 * * * *Byte 3 *1 * 0 * bits 5-0 *
* The differences between this format and the * standard UTF-8 format are the following: *
* This method blocks until one of the * following conditions occurs: *
* If {@code b} is {@code null}, * a {@code NullPointerException} is thrown. * If {@code b.length} is zero, then * no bytes are read. Otherwise, the first * byte read is stored into element {@code b[0]}, * the next one into {@code b[1]}, and * so on. * If an exception is thrown from * this method, then it may be that some but * not all bytes of {@code b} have been * updated with data from the input stream. * * @param b the buffer into which the data is read. * @throws NullPointerException if {@code b} is {@code null}. * @throws EOFException if this stream reaches the end before reading * all the bytes. * @throws IOException if an I/O error occurs. */ void readFully(byte b[]) throws IOException; /** * * Reads {@code len} * bytes from * an input stream. *
* This method * blocks until one of the following conditions * occurs: *
* If {@code b} is {@code null}, * a {@code NullPointerException} is thrown. * If {@code off} is negative, or {@code len} * is negative, or {@code off+len} is * greater than the length of the array {@code b}, * then an {@code IndexOutOfBoundsException} * is thrown. * If {@code len} is zero, * then no bytes are read. Otherwise, the first * byte read is stored into element {@code b[off]}, * the next one into {@code b[off+1]}, * and so on. The number of bytes read is, * at most, equal to {@code len}. * * @param b the buffer into which the data is read. * @param off an int specifying the offset in the data array {@code b}. * @param len an int specifying the number of bytes to read. * @throws NullPointerException if {@code b} is {@code null}. * @throws IndexOutOfBoundsException if {@code off} is negative, * {@code len} is negative, or {@code len} is greater than * {@code b.length - off}. * @throws EOFException if this stream reaches the end before reading * all the bytes. * @throws IOException if an I/O error occurs. */ void readFully(byte b[], int off, int len) throws IOException; /** * Makes an attempt to skip over * {@code n} bytes * of data from the input * stream, discarding the skipped bytes. However, * it 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 {@code n} bytes * have been skipped is * only one possibility. * This method never throws an {@code EOFException}. * The actual * number of bytes skipped is returned. * * @param n the number of bytes to be skipped. * @return the number of bytes actually skipped. * @exception IOException if an I/O error occurs. */ int skipBytes(int n) throws IOException; /** * Reads one input byte and returns * {@code true} if that byte is nonzero, * {@code false} if that byte is zero. * This method is suitable for reading * the byte written by the {@code writeBoolean} * method of interface {@code DataOutput}. * * @return the {@code boolean} value read. * @exception EOFException if this stream reaches the end before reading * all the bytes. * @exception IOException if an I/O error occurs. */ boolean readBoolean() throws IOException; /** * Reads and returns one input byte. * The byte is treated as a signed value in * the range {@code -128} through {@code 127}, * inclusive. * This method is suitable for * reading the byte written by the {@code writeByte} * method of interface {@code DataOutput}. * * @return the 8-bit value read. * @exception EOFException if this stream reaches the end before reading * all the bytes. * @exception IOException if an I/O error occurs. */ byte readByte() throws IOException; /** * Reads one input byte, zero-extends * it to type {@code int}, and returns * the result, which is therefore in the range * {@code 0} * through {@code 255}. * This method is suitable for reading * the byte written by the {@code writeByte} * method of interface {@code DataOutput} * if the argument to {@code writeByte} * was intended to be a value in the range * {@code 0} through {@code 255}. * * @return the unsigned 8-bit value read. * @exception EOFException if this stream reaches the end before reading * all the bytes. * @exception IOException if an I/O error occurs. */ int readUnsignedByte() throws IOException; /** * Reads two input bytes and returns * a {@code short} value. Let {@code a} * be the first byte read and {@code b} * be the second byte. The value * returned * is: *
{@code (short)((a << 8) | (b & 0xff))
* }
* This method
* is suitable for reading the bytes written
* by the {@code writeShort} method of
* interface {@code DataOutput}.
*
* @return the 16-bit value read.
* @exception EOFException if this stream reaches the end before reading
* all the bytes.
* @exception IOException if an I/O error occurs.
*/
short readShort() throws IOException;
/**
* Reads two input bytes and returns
* an {@code int} value in the range {@code 0}
* through {@code 65535}. Let {@code a}
* be the first byte read and
* {@code b}
* be the second byte. The value returned is:
* {@code (((a & 0xff) << 8) | (b & 0xff))
* }
* This method is suitable for reading the bytes
* written by the {@code writeShort} method
* of interface {@code DataOutput} if
* the argument to {@code writeShort}
* was intended to be a value in the range
* {@code 0} through {@code 65535}.
*
* @return the unsigned 16-bit value read.
* @exception EOFException if this stream reaches the end before reading
* all the bytes.
* @exception IOException if an I/O error occurs.
*/
int readUnsignedShort() throws IOException;
/**
* Reads two input bytes and returns a {@code char} value.
* Let {@code a}
* be the first byte read and {@code b}
* be the second byte. The value
* returned is:
* {@code (char)((a << 8) | (b & 0xff))
* }
* This method
* is suitable for reading bytes written by
* the {@code writeChar} method of interface
* {@code DataOutput}.
*
* @return the {@code char} value read.
* @exception EOFException if this stream reaches the end before reading
* all the bytes.
* @exception IOException if an I/O error occurs.
*/
char readChar() throws IOException;
/**
* Reads four input bytes and returns an
* {@code int} value. Let {@code a-d}
* be the first through fourth bytes read. The value returned is:
* {@code
* (((a & 0xff) << 24) | ((b & 0xff) << 16) |
* ((c & 0xff) << 8) | (d & 0xff))
* }
* This method is suitable
* for reading bytes written by the {@code writeInt}
* method of interface {@code DataOutput}.
*
* @return the {@code int} value read.
* @exception EOFException if this stream reaches the end before reading
* all the bytes.
* @exception IOException if an I/O error occurs.
*/
int readInt() throws IOException;
/**
* Reads eight input bytes and returns
* a {@code long} value. Let {@code a-h}
* be the first through eighth bytes read.
* The value returned is:
* {@code
* (((long)(a & 0xff) << 56) |
* ((long)(b & 0xff) << 48) |
* ((long)(c & 0xff) << 40) |
* ((long)(d & 0xff) << 32) |
* ((long)(e & 0xff) << 24) |
* ((long)(f & 0xff) << 16) |
* ((long)(g & 0xff) << 8) |
* ((long)(h & 0xff)))
* }
* * This method is suitable * for reading bytes written by the {@code writeLong} * method of interface {@code DataOutput}. * * @return the {@code long} value read. * @exception EOFException if this stream reaches the end before reading * all the bytes. * @exception IOException if an I/O error occurs. */ long readLong() throws IOException; /** * Reads four input bytes and returns * a {@code float} value. It does this * by first constructing an {@code int} * value in exactly the manner * of the {@code readInt} * method, then converting this {@code int} * value to a {@code float} in * exactly the manner of the method {@code Float.intBitsToFloat}. * This method is suitable for reading * bytes written by the {@code writeFloat} * method of interface {@code DataOutput}. * * @return the {@code float} value read. * @exception EOFException if this stream reaches the end before reading * all the bytes. * @exception IOException if an I/O error occurs. */ float readFloat() throws IOException; /** * Reads eight input bytes and returns * a {@code double} value. It does this * by first constructing a {@code long} * value in exactly the manner * of the {@code readLong} * method, then converting this {@code long} * value to a {@code double} in exactly * the manner of the method {@code Double.longBitsToDouble}. * This method is suitable for reading * bytes written by the {@code writeDouble} * method of interface {@code DataOutput}. * * @return the {@code double} value read. * @exception EOFException if this stream reaches the end before reading * all the bytes. * @exception IOException if an I/O error occurs. */ double readDouble() throws IOException; /** * Reads the next line of text from the input stream. * It reads successive bytes, converting * each byte separately into a character, * until it encounters a line terminator or * end of * file; the characters read are then * returned as a {@code String}. Note * that because this * method processes bytes, * it does not support input of the full Unicode * character set. *
* If end of file is encountered * before even one byte can be read, then {@code null} * is returned. Otherwise, each byte that is * read is converted to type {@code char} * by zero-extension. If the character {@code '\n'} * is encountered, it is discarded and reading * ceases. If the character {@code '\r'} * is encountered, it is discarded and, if * the following byte converts to the * character {@code '\n'}, then that is * discarded also; reading then ceases. If * end of file is encountered before either * of the characters {@code '\n'} and * {@code '\r'} is encountered, reading * ceases. Once reading has ceased, a {@code String} * is returned that contains all the characters * read and not discarded, taken in order. * Note that every character in this string * will have a value less than {@code \u005Cu0100}, * that is, {@code (char)256}. * * @return the next line of text from the input stream, * or {@code null} if the end of file is * encountered before a byte can be read. * @exception IOException if an I/O error occurs. */ String readLine() throws IOException; /** * Reads in a string that has been encoded using a * modified UTF-8 * format. * The general contract of {@code readUTF} * is that it reads a representation of a Unicode * character string encoded in modified * UTF-8 format; this string of characters * is then returned as a {@code String}. *
* First, two bytes are read and used to * construct an unsigned 16-bit integer in * exactly the manner of the {@code readUnsignedShort} * method . This integer value is called the * UTF length and specifies the number * of additional bytes to be read. These bytes * are then converted to characters by considering * them in groups. The length of each group * is computed from the value of the first * byte of the group. The byte following a * group, if any, is the first byte of the * next group. *
* If the first byte of a group * matches the bit pattern {@code 0xxxxxxx} * (where {@code x} means "may be {@code 0} * or {@code 1}"), then the group consists * of just that byte. The byte is zero-extended * to form a character. *
* If the first byte * of a group matches the bit pattern {@code 110xxxxx}, * then the group consists of that byte {@code a} * and a second byte {@code b}. If there * is no byte {@code b} (because byte * {@code a} was the last of the bytes * to be read), or if byte {@code b} does * not match the bit pattern {@code 10xxxxxx}, * then a {@code UTFDataFormatException} * is thrown. Otherwise, the group is converted * to the character: *
{@code (char)(((a & 0x1F) << 6) | (b & 0x3F))
* }
* If the first byte of a group
* matches the bit pattern {@code 1110xxxx},
* then the group consists of that byte {@code a}
* and two more bytes {@code b} and {@code c}.
* If there is no byte {@code c} (because
* byte {@code a} was one of the last
* two of the bytes to be read), or either
* byte {@code b} or byte {@code c}
* does not match the bit pattern {@code 10xxxxxx},
* then a {@code UTFDataFormatException}
* is thrown. Otherwise, the group is converted
* to the character:
* {@code
* (char)(((a & 0x0F) << 12) | ((b & 0x3F) << 6) | (c & 0x3F))
* }
* If the first byte of a group matches the
* pattern {@code 1111xxxx} or the pattern
* {@code 10xxxxxx}, then a {@code UTFDataFormatException}
* is thrown.
* * If end of file is encountered * at any time during this entire process, * then an {@code EOFException} is thrown. *
* After every group has been converted to * a character by this process, the characters * are gathered, in the same order in which * their corresponding groups were read from * the input stream, to form a {@code String}, * which is returned. *
* The {@code writeUTF} * method of interface {@code DataOutput} * may be used to write data that is suitable * for reading by this method. * @return a Unicode string. * @exception EOFException if this stream reaches the end * before reading all the bytes. * @exception IOException if an I/O error occurs. * @exception UTFDataFormatException if the bytes do not represent a * valid modified UTF-8 encoding of a string. */ String readUTF() throws IOException; }