public interface DataInput
The 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 String from data in modified 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, an EOFException (which is a kind of IOException) is thrown. If any byte cannot be read for any reason other than end of file, an IOException other than EOFException is thrown. In particular, an 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 tables, 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
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:
Method Summary | |
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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 ![]() ![]() ![]() ![]() ![]() ![]() |
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 range 0 through 255. |
int |
readUnsignedShort
() Reads two input bytes and returns an int value in the range 0 through 65535. |
String |
readUTF
() Reads in a string that has been encoded using a modified 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. |
Method Detail |
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void readFully(byte[] b) throws IOException
This method blocks until one of the following conditions occurs:
If b is null, a NullPointerException is thrown. If b.length is zero, then no bytes are read. Otherwise, the first byte read is stored into element b[0], the next one into b[1], and so on. If an exception is thrown from this method, then it may be that some but not all bytes of b have been updated with data from the input stream.
void readFully(byte[] b, int off, int len) throws IOException
This method blocks until one of the following conditions occurs:
If b is null, a NullPointerException is thrown. If off is negative, or len is negative, or off+len is greater than the length of the array b, then an IndexOutOfBoundsException is thrown. If len is zero, then no bytes are read. Otherwise, the first byte read is stored into element b[off], the next one into b[off+1], and so on. The number of bytes read is, at most, equal to len.
int skipBytes(int n) throws IOException
boolean readBoolean() throws IOException
byte readByte() throws IOException
int readUnsignedByte() throws IOException
short readShort() throws IOException
(short)((a << 8) | (b & 0xff))This method is suitable for reading the bytes written by the writeShort method of interface DataOutput.
int readUnsignedShort() throws IOException
(((a & 0xff) << 8) | (b & 0xff))This method is suitable for reading the bytes written by the writeShort method of interface DataOutput if the argument to writeShort was intended to be a value in the range 0 through 65535.
char readChar() throws IOException
(char)((a << 8) | (b & 0xff))This method is suitable for reading bytes written by the writeChar method of interface DataOutput.
int readInt() throws IOException
(((a & 0xff) << 24) | ((b & 0xff) << 16) | ((c & 0xff) << 8) | (d & 0xff))This method is suitable for reading bytes written by the writeInt method of interface DataOutput.
long readLong() throws IOException
(((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 writeLong method of interface DataOutput.
float readFloat() throws IOException
double readDouble() throws IOException
String readLine() throws IOException
If end of file is encountered before even one byte can be read, then null is returned. Otherwise, each byte that is read is converted to type char 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, a 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 \u0100, that is, (char)256.
String readUTF() throws IOException
First, two bytes are read and used to construct an unsigned 16-bit integer in exactly the manner of the 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 0xxxxxxx (where x means "may be 0 or 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 110xxxxx, then the group consists of that byte a and a second byte b. If there is no byte b (because byte a was the last of the bytes to be read), or if byte b does not match the bit pattern 10xxxxxx, then a UTFDataFormatException is thrown. Otherwise, the group is converted to the character:
(char)(((a& 0x1F) << 6) | (b & 0x3F))If the first byte of a group matches the bit pattern 1110xxxx, then the group consists of that byte a and two more bytes b and c. If there is no byte c (because byte a was one of the last two of the bytes to be read), or either byte b or byte c does not match the bit pattern 10xxxxxx, then a UTFDataFormatException is thrown. Otherwise, the group is converted to the character:
(char)(((a & 0x0F) << 12) | ((b & 0x3F) << 6) | (c & 0x3F))If the first byte of a group matches the pattern 1111xxxx or the pattern 10xxxxxx, then a UTFDataFormatException is thrown.
If end of file is encountered at any time during this entire process, then an 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 String, which is returned.
The writeUTF method of interface DataOutput may be used to write data that is suitable for reading by this method.