Interface DataInput
- All Known Subinterfaces:
-
ImageInputStream
,ImageOutputStream
,ObjectInput
- All Known Implementing Classes:
-
DataInputStream
,FileCacheImageInputStream
,FileCacheImageOutputStream
,FileImageInputStream
,FileImageOutputStream
,ImageInputStreamImpl
,ImageOutputStreamImpl
,MemoryCacheImageInputStream
,MemoryCacheImageOutputStream
,ObjectInputStream
,RandomAccessFile
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.
Modified UTF-8
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)
- Characters in the range
'\u0001'
to'\u007F'
are represented by a single byte. - The null character
'\u0000'
and characters in the range'\u0080'
to'\u07FF'
are represented by a pair of bytes. - Characters in the range
'\u0800'
to'\uFFFF'
are represented by three bytes.
Value | Byte | Bit Values | |||||||
---|---|---|---|---|---|---|---|---|---|
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ||
\u0001 to \u007F | 1 | 0 | bits 6-0 | ||||||
\u0000 ,\u0080 to \u07FF | 1 | 1 | 1 | 0 | bits 10-6 | ||||
2 | 1 | 0 | bits 5-0 | ||||||
\u0800 to \uFFFF | 1 | 1 | 1 | 1 | 0 | bits 15-12 | |||
2 | 1 | 0 | bits 11-6 | ||||||
3 | 1 | 0 | bits 5-0 |
The differences between this format and the standard UTF-8 format are the following:
- The null byte
'\u0000'
is encoded in 2-byte format rather than 1-byte, so that the encoded strings never have embedded nulls. - Only the 1-byte, 2-byte, and 3-byte formats are used.
- Supplementary characters are represented in the form of surrogate pairs.
- Since:
- 1.0
- See Also:
-
DataInputStream
,DataOutput
Method Summary
Modifier and Type | Method | Description |
---|---|---|
boolean | readBoolean() | Reads one input byte and returns |
byte | readByte() | Reads and returns one input byte. |
char | readChar() | Reads two input bytes and returns a |
double | readDouble() | Reads eight input bytes and returns a |
float | readFloat() | Reads four input bytes and returns a |
void | readFully(byte[] b) | Reads some bytes from an input stream and stores them into the buffer array |
void | readFully(byte[] b,
int off,
int len) | Reads |
int | readInt() | Reads four input bytes and returns an |
String | readLine() | Reads the next line of text from the input stream. |
long | readLong() | Reads eight input bytes and returns a |
short | readShort() | Reads two input bytes and returns a |
int | readUnsignedByte() | Reads one input byte, zero-extends it to type |
int | readUnsignedShort() | Reads two input bytes and returns an |
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 |
Method Detail
readFully
void readFully(byte[] b) throws IOException
Reads some bytes from an input stream and stores them into the buffer array b
. The number of bytes read is equal to the length of b
.
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. - End of file is detected, in which case an
EOFException
is thrown. - An I/O error occurs, in which case an
IOException
other thanEOFException
is thrown.
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.
- Parameters:
-
b
- the buffer into which the data is read. - Throws:
-
NullPointerException
- ifb
isnull
. -
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readFully
void readFully(byte[] b, int off, int len) throws IOException
Reads 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. - End of file is detected, in which case an
EOFException
is thrown. - An I/O error occurs, in which case an
IOException
other thanEOFException
is thrown.
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
.
- Parameters:
-
b
- the buffer into which the data is read. -
off
- an int specifying the offset in the data arrayb
. -
len
- an int specifying the number of bytes to read. - Throws:
-
NullPointerException
- ifb
isnull
. -
IndexOutOfBoundsException
- ifoff
is negative,len
is negative, orlen
is greater thanb.length - off
. -
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
skipBytes
int skipBytes(int n) throws IOException
Makes an attempt to skip over 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 n
bytes have been skipped is only one possibility. This method never throws an EOFException
. The actual number of bytes skipped is returned.
- Parameters:
-
n
- the number of bytes to be skipped. - Returns:
- the number of bytes actually skipped.
- Throws:
-
IOException
- if an I/O error occurs.
readBoolean
boolean readBoolean() throws IOException
Reads one input byte and returns true
if that byte is nonzero, false
if that byte is zero. This method is suitable for reading the byte written by the writeBoolean
method of interface DataOutput
.
- Returns:
- the
boolean
value read. - Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readByte
byte readByte() throws IOException
Reads and returns one input byte. The byte is treated as a signed value in the range -128
through 127
, inclusive. This method is suitable for reading the byte written by the writeByte
method of interface DataOutput
.
- Returns:
- the 8-bit value read.
- Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readUnsignedByte
int readUnsignedByte() throws IOException
Reads one input byte, zero-extends it to type int
, and returns the result, which is therefore in the range 0
through 255
. This method is suitable for reading the byte written by the writeByte
method of interface DataOutput
if the argument to writeByte
was intended to be a value in the range 0
through 255
.
- Returns:
- the unsigned 8-bit value read.
- Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readShort
short readShort() throws IOException
Reads two input bytes and returns a short
value. Let a
be the first byte read and b
be the second byte. The value returned is:
(short)((a << 8) | (b & 0xff))This method is suitable for reading the bytes written by the
writeShort
method of interface DataOutput
. - Returns:
- the 16-bit value read.
- Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readUnsignedShort
int readUnsignedShort() throws IOException
Reads two input bytes and returns an int
value in the range 0
through 65535
. Let a
be the first byte read and b
be the second byte. The value returned is:
(((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
. - Returns:
- the unsigned 16-bit value read.
- Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readChar
char readChar() throws IOException
Reads two input bytes and returns a char
value. Let a
be the first byte read and b
be the second byte. The value returned is:
(char)((a << 8) | (b & 0xff))This method is suitable for reading bytes written by the
writeChar
method of interface DataOutput
. - Returns:
- the
char
value read. - Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readInt
int readInt() throws IOException
Reads four input bytes and returns an int
value. Let a-d
be the first through fourth bytes read. The value returned is:
(((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
. - Returns:
- the
int
value read. - Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readLong
long readLong() throws IOException
Reads eight input bytes and returns a long
value. Let a-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 the writeLong
method of interface DataOutput
.
- Returns:
- the
long
value read. - Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readFloat
float readFloat() throws IOException
Reads four input bytes and returns a float
value. It does this by first constructing an int
value in exactly the manner of the readInt
method, then converting this int
value to a float
in exactly the manner of the method Float.intBitsToFloat
. This method is suitable for reading bytes written by the writeFloat
method of interface DataOutput
.
- Returns:
- the
float
value read. - Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readDouble
double readDouble() throws IOException
Reads eight input bytes and returns a double
value. It does this by first constructing a long
value in exactly the manner of the readLong
method, then converting this long
value to a double
in exactly the manner of the method Double.longBitsToDouble
. This method is suitable for reading bytes written by the writeDouble
method of interface DataOutput
.
- Returns:
- the
double
value read. - Throws:
-
EOFException
- if this stream reaches the end before reading all the bytes. -
IOException
- if an I/O error occurs.
readLine
String readLine() 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 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 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
.
- Returns:
- the next line of text from the input stream, or
null
if the end of file is encountered before a byte can be read. - Throws:
-
IOException
- if an I/O error occurs.
readUTF
String readUTF() throws IOException
Reads in a string that has been encoded using a modified UTF-8 format. The general contract of 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 String
.
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.
- Returns:
- a Unicode string.
- Throws:
-
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.