* <th colspan="8" id="bit_a">Bit Values * </tr> * <tr> * <th id="byte1_a">Byte 1 * <td>0 * <td colspan="7">bits 6-0 * </tr> * <tr> * <th colspan="9"> * The null character {@code '\u005Cu0000'} and characters * in the range {@code '\u005Cu0080'} to {@code '\u005Cu07FF'} are * represented by a pair of bytes: * </tr> * <tr> * <td> * <th colspan="8" id="bit_b">Bit Values * </tr> * <tr> * <th id="byte1_b">Byte 1 * <td>1 * <td>1 * <td>0 * <td colspan="5">bits 10-6 * </tr> * <tr> * <th id="byte2_a">Byte 2 * <td>1 * <td>0 * <td colspan="6">bits 5-0 * </tr> * <tr> * <th colspan="9"> * {@code char} values in the range {@code '\u005Cu0800'} * to {@code '\u005CuFFFF'} are represented by three bytes: * </tr> * <tr> * <td> * <th colspan="8"id="bit_c">Bit Values * </tr> * <tr> * <th id="byte1_c">Byte 1 * <td>1 * <td>1 * <td>1 * <td>0 * <td colspan="4">bits 15-12 * </tr> * <tr> * <th id="byte2_b">Byte 2 * <td>1 * <td>0 * <td colspan="6">bits 11-6 * </tr> * <tr> * <th id="byte3">Byte 3 * <td>1 * <td>0 * <td colspan="6">bits 5-0 * </tr> * </table> * </blockquote> * * The differences between this format and the * standard UTF-8 format are the following: * <ul> * <li>The null byte {@code '\u005Cu0000'} is encoded in 2-byte format * rather than 1-byte, so that the encoded strings never have * embedded nulls. * <li>Only the 1-byte, 2-byte, and 3-byte formats are used. * <li>Supplementary characters * are represented in the form of surrogate pairs. * </ul> * @author Frank Yellin * @see java.io.DataInputStream * @see java.io.DataOutput * @since JDK1.0 */ public interface DataInput { /** * Reads some bytes from an input * stream and stores them into the buffer * array {@code b}. The number of bytes * read is equal * to the length of {@code b}. * * This method blocks until one of the * following conditions occurs: * <ul> * <li>{@code b.length} * bytes of input data are available, in which * case a normal return is made. * * <li>End of * file is detected, in which case an {@code EOFException} * is thrown. * * <li>An I/O error occurs, in * which case an {@code IOException} other * than {@code EOFException} is thrown. * </ul> * * 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. * @exception EOFException if this stream reaches the end before reading * all the bytes. * @exception 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: * <ul> * <li>{@code len} bytes * of input data are available, in which case * a normal return is made. * * <li>End of file * is detected, in which case an {@code EOFException} * is thrown. * * <li>An I/O error occurs, in * which case an {@code IOException} other * than {@code EOFException} is thrown. * </ul> * * 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 into the data. * @param len an int specifying the number of bytes to read. * @exception EOFException if this stream reaches the end before reading * all the bytes. * @exception 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: * <pre>{@code (short)((a << 8) | (b & 0xff)) * }</pre> * 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: * <pre>{@code (((a & 0xff) << 8) | (b & 0xff)) * }</pre> * 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: * <pre>{@code (char)((a << 8) | (b & 0xff)) * }</pre> * 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: * <pre>{@code * (((a & 0xff) << 24) | ((b & 0xff) << 16) | * ((c & 0xff) << 8) | (d & 0xff)) * }</pre> * 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: * <pre>{@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))) * }</pre> * * 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 * <a href="#modified-utf-8">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: * <pre>{@code (char)(((a & 0x1F) << 6) | (b & 0x3F)) * }</pre> * 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: * <pre>{@code * (char)(((a & 0x0F) << 12) | ((b & 0x3F) << 6) | (c & 0x3F)) * }</pre> * 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; }

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The DataInput.java Java example source code

/*
 * Copyright (c) 1995, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.io;

/**
 * The {@code DataInput} interface provides
 * for reading bytes from a binary stream and
 * reconstructing from them data in any of
 * the Java primitive types. There is also
 * a
 * facility for reconstructing a {@code String}
 * from data in
 * <a href="#modified-utf-8">modified UTF-8
 * format.
 * <p>
 * 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.
 *
 * <h3>Modified UTF-8
 * <p>
 * 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
 * <i>3.9 Unicode Encoding Forms of The Unicode Standard, Version
 * 4.0</i>).
 * Note that in the following table, the most significant bit appears in the
 * far left-hand column.
 *
 * <blockquote>
 *   <table border="1" cellspacing="0" cellpadding="8"
 *          summary="Bit values and bytes">
 *     <tr>
 *       <th colspan="9">
 *         All characters in the range {@code '\u005Cu0001'} to
 *         {@code '\u005Cu007F'} are represented by a single byte:</span>
 *     </tr>
 *     <tr>
 *       <td>

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