Interface DataInput

All Known Subinterfaces:
ImageInputStream,ImageOutputStream,ObjectInput
All Known Implementing Classes:
DataInputStream,FileCacheImageInputStream,FileCacheImageOutputStream,FileImageInputStream,FileImageOutputStream,ImageInputStreamImpl,ImageOutputStreamImpl,MemoryCacheImageInputStream,MemoryCacheImageOutputStream,ObjectInputStream,RandomAccessFile
public interfaceDataInput
TheDataInput 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.

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 section3.9 Unicode Encoding Forms ofThe 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.
Encoding of UTF-8 values
ValueByteBit Values
7 6 5 4 3 2 1 0
\u0001 to\u007F 10bits 6-0
\u0000,
\u0080 to\u07FF		 1110bits 10-6
210bits 5-0
\u0800 to\uFFFF 11110bits 15-12
210bits 11-6
310bits 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:

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    Reads one input byte and returnstrue if that byte is nonzero,false if that byte is zero.
    byte
    Reads and returns one input byte.
    char
    Reads two input bytes and returns achar value.
    double
    Reads eight input bytes and returns adouble value.
    float
    Reads four input bytes and returns afloat value.
    void
    readFully(byte[] b)
    Reads some bytes from an input stream and stores them into the buffer arrayb.
    void
    readFully(byte[] b, int off, int len)
    Readslen bytes from an input stream.
    int
    Reads four input bytes and returns anint value.
    Reads the next line of text from the input stream.
    long
    Reads eight input bytes and returns along value.
    short
    Reads two input bytes and returns ashort value.
    int
    Reads one input byte, zero-extends it to typeint, and returns the result, which is therefore in the range0 through255.
    int
    Reads two input bytes and returns anint value in the range0 through65535.
    Reads in a string that has been encoded using amodified UTF-8 format.
    int
    skipBytes(int n)
    Makes an attempt to skip overn bytes of data from the input stream, discarding the skipped bytes.

  • Method Details

    • readFully

      void readFully(byte[] b) throwsIOException
      Reads some bytes from an input stream and stores them into the buffer arrayb. 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.
      • End of file is detected, in which case anEOFException is thrown.
      • An I/O error occurs, in which case anIOException 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.

      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) throwsIOException
      Readslen 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 anEOFException is thrown.
      • An I/O error occurs, in which case anIOException 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.

      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) throwsIOException
      Makes an attempt to skip overn 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.
      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() throwsIOException
      Reads one input byte and returnstrue 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.
      Returns:
      theboolean 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() throwsIOException
      Reads and returns one input byte. The byte is treated as a signed value in the range-128 through127, inclusive. This method is suitable for reading the byte written by thewriteByte method of interfaceDataOutput.
      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() throwsIOException
      Reads one input byte, zero-extends it to typeint, 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.
      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() throwsIOException
      Reads two input bytes and returns ashort value. Leta be the first byte read andb be the second byte. The value returned is:
      (short)((a << 8) | (b & 0xff))
      This method is suitable for reading the bytes written by thewriteShort method of interfaceDataOutput.
      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() throwsIOException
      Reads two input bytes and returns anint 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))
      This method is suitable for reading the bytes written by thewriteShort method of interfaceDataOutput if the argument towriteShort was intended to be a value in the range0 through65535.
      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() throwsIOException
      Reads two input bytes and returns achar value. Leta be the first byte read andb be the second byte. The value returned is:
      (char)((a << 8) | (b & 0xff))
      This method is suitable for reading bytes written by thewriteChar method of interfaceDataOutput.
      Returns:
      thechar 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() throwsIOException
      Reads four input bytes and returns anint 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))
      This method is suitable for reading bytes written by thewriteInt method of interfaceDataOutput.
      Returns:
      theint 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() throwsIOException
      Reads eight input bytes and returns along 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.

      Returns:
      thelong 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() throwsIOException
      Reads four input bytes and returns afloat 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.
      Returns:
      thefloat 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() throwsIOException
      Reads eight input bytes and returns adouble 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.
      Returns:
      thedouble 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() throwsIOException
      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 aString. 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.

      Returns:
      the next line of text from the input stream, ornull if the end of file is encountered before a byte can be read.
      Throws:
      IOException - if an I/O error occurs.

    • readUTF

      String readUTF() throwsIOException
      Reads in a string that has been encoded using amodified UTF-8 format. The general contract ofreadUTF 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))
      If the first byte of a group matches the bit pattern1110xxxx, 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))
      If the first byte of a group matches the pattern1111xxxx 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.

      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.