public interfaceDataInputDataInput 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 aString from data inmodified UTF-8 format. 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, anEOFException (which is a kind ofIOException) is thrown. If any byte cannot be read for any reason other than end of file, anIOException other thanEOFException is thrown. In particular, anIOException 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 section3.9 Unicode Encoding Forms ofThe 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 '\u0001'to'\u007F'are represented by a single byte:Bit Values Byte 1 0 bits 6-0 The null character '\u0000'and characters in the range'\u0080'to'\u07FF'are represented by a pair of bytes:Bit Values Byte 1 1 1 0 bits 10-6 Byte 2 1 0 bits 5-0 charvalues in the range'\u0800'to'\uFFFF'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:
'\u0000' is encoded in 2-byte format rather than 1-byte, so that the encoded strings never have embedded nulls.DataInputStream,DataOutput| Modifier and Type | Method | Description |
|---|---|---|
boolean | readBoolean() | Reads one input byte and returns true if that byte is nonzero,false if that byte is zero. |
byte | readByte() | Reads and returns one input byte. |
char | readChar() | Reads two input bytes and returns a char value. |
double | readDouble() | Reads eight input bytes and returns a double value. |
float | readFloat() | Reads four input bytes and returns a float value. |
void | readFully(byte[] b) | Reads some bytes from an input stream and stores them into the buffer array b. |
void | readFully(byte[] b, int off, int len) | Reads len bytes from an input stream. |
int | readInt() | Reads four input bytes and returns an int value. |
String | readLine() | Reads the next line of text from the input stream. |
long | readLong() | Reads eight input bytes and returns a long value. |
short | readShort() | Reads two input bytes and returns a short value. |
int | readUnsignedByte() | Reads one input byte, zero-extends it to type int, and returns the result, which is therefore in the range0 through255. |
int | readUnsignedShort() | Reads two input bytes and returns an int value in the range0 through65535. |
String | readUTF() | Reads in a string that has been encoded using amodified UTF-8 format. |
int | skipBytes(int n) | Makes an attempt to skip over n bytes of data from the input stream, discarding the skipped bytes. |
void readFully(byte[] b) throwsIOException
b. The number of bytes read is equal to the length ofb.This method blocks until one of the following conditions occurs:
b.length bytes of input data are available, in which case a normal return is made.EOFException is thrown.IOException other thanEOFException is thrown. Ifb isnull, aNullPointerException is thrown. Ifb.length is zero, then no bytes are read. Otherwise, the first byte read is stored into elementb[0], the next one intob[1], and so on. If an exception is thrown from this method, then it may be that some but not all bytes ofb have been updated with data from the input stream.
b - the buffer into which the data is read.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.void readFully(byte[] b, int off, int len) throwsIOException
len bytes from an input stream.This method blocks until one of the following conditions occurs:
len bytes of input data are available, in which case a normal return is made.EOFException is thrown.IOException other thanEOFException is thrown. Ifb isnull, aNullPointerException is thrown. Ifoff is negative, orlen is negative, oroff+len is greater than the length of the arrayb, then anIndexOutOfBoundsException is thrown. Iflen is zero, then no bytes are read. Otherwise, the first byte read is stored into elementb[off], the next one intob[off+1], and so on. The number of bytes read is, at most, equal tolen.
b - the buffer into which the data is read.off - an int specifying the offset into the data.len - an int specifying the number of bytes to read.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.int skipBytes(int n) throwsIOException
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 beforen bytes have been skipped is only one possibility. This method never throws anEOFException. The actual number of bytes skipped is returned.n - the number of bytes to be skipped.IOException - if an I/O error occurs.boolean readBoolean() throwsIOException
true if that byte is nonzero,false if that byte is zero. This method is suitable for reading the byte written by thewriteBoolean method of interfaceDataOutput.boolean value read.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.byte readByte() throwsIOException
-128 through127, inclusive. This method is suitable for reading the byte written by thewriteByte method of interfaceDataOutput.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.int readUnsignedByte() throwsIOException
int, and returns the result, which is therefore in the range0 through255. This method is suitable for reading the byte written by thewriteByte method of interfaceDataOutput if the argument towriteByte was intended to be a value in the range0 through255.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.short readShort() throwsIOException
short value. Leta be the first byte read andb be the second byte. The value returned is: (short)((a << 8) | (b & 0xff))writeShort method of interfaceDataOutput.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.int readUnsignedShort() throwsIOException
int value in the range0 through65535. Leta be the first byte read andb be the second byte. The value returned is: (((a & 0xff) << 8) | (b & 0xff))writeShort method of interfaceDataOutput if the argument towriteShort was intended to be a value in the range0 through65535.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.char readChar() throwsIOException
char value. Leta be the first byte read andb be the second byte. The value returned is: (char)((a << 8) | (b & 0xff))writeChar method of interfaceDataOutput.char value read.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.int readInt() throwsIOException
int value. Leta-d be the first through fourth bytes read. The value returned is: (((a & 0xff) << 24) | ((b & 0xff) << 16) | ((c & 0xff) << 8) | (d & 0xff))writeInt method of interfaceDataOutput.int value read.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.long readLong() throwsIOException
long value. Leta-h be the first through eighth bytes read. The value returned is: (((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 thewriteLong method of interfaceDataOutput.
long value read.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.float readFloat() throwsIOException
float value. It does this by first constructing anint value in exactly the manner of thereadInt method, then converting thisint value to afloat in exactly the manner of the methodFloat.intBitsToFloat. This method is suitable for reading bytes written by thewriteFloat method of interfaceDataOutput.float value read.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.double readDouble() throwsIOException
double value. It does this by first constructing along value in exactly the manner of thereadLong method, then converting thislong value to adouble in exactly the manner of the methodDouble.longBitsToDouble. This method is suitable for reading bytes written by thewriteDouble method of interfaceDataOutput.double value read.EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.String readLine() throwsIOException
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, thennull is returned. Otherwise, each byte that is read is converted to typechar by zero-extension. If the character'\n' is encountered, it is discarded and reading ceases. If the character'\r' is encountered, it is discarded and, if the following byte converts to the character'\n', then that is discarded also; reading then ceases. If end of file is encountered before either of the characters'\n' and'\r' is encountered, reading ceases. Once reading has ceased, aString 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\u0100, that is,(char)256.
null if the end of file is encountered before a byte can be read.IOException - if an I/O error occurs.String readUTF() throwsIOException
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 aString. First, two bytes are read and used to construct an unsigned 16-bit integer in exactly the manner of thereadUnsignedShort method . This integer value is called theUTF 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 pattern0xxxxxxx (wherex means "may be0 or1"), 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 pattern110xxxxx, then the group consists of that bytea and a second byteb. If there is no byteb (because bytea was the last of the bytes to be read), or if byteb does not match the bit pattern10xxxxxx, then aUTFDataFormatException is thrown. Otherwise, the group is converted to the character:
(char)(((a & 0x1F) << 6) | (b & 0x3F))1110xxxx, then the group consists of that bytea and two more bytesb andc. If there is no bytec (because bytea was one of the last two of the bytes to be read), or either byteb or bytec does not match the bit pattern10xxxxxx, then aUTFDataFormatException is thrown. Otherwise, the group is converted to the character: (char)(((a & 0x0F) << 12) | ((b & 0x3F) << 6) | (c & 0x3F))1111xxxx or the pattern10xxxxxx, then aUTFDataFormatException is thrown. If end of file is encountered at any time during this entire process, then anEOFException 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 aString, which is returned.
ThewriteUTF method of interfaceDataOutput may be used to write data that is suitable for reading by this method.
EOFException - if this stream reaches the end before reading all the bytes.IOException - if an I/O error occurs.UTFDataFormatException - if the bytes do not represent a valid modified UTF-8 encoding of a string.