Java IO.pdf - Nguyen Dang Binh

0 0 0 0 0 x10 x9 x8
x7 x6 x5 x4 x3 x2 x1 x0
Java I/O
These characters are encoded into two bytes, but not in the most obvious fashion. The 11
significant bits of the character are broken up:
1 1 0 x10 x9 x8 x7 x6
1 0 x5 x4 x3 x2 x1 x0
Neither of the bytes that make up this number begins with a bit. Thus, you can distinguish
between bytes that are part of a two-byte character and bytes that represent one-byte
characters (which all begin with 0).
The remaining characters have values between 2048 and 65,535. Any or all of the bits in these
characters may take on either value or 1. Thus, they are encoded in three bytes, like this:
1 1 1 0 x15 x14 x13 x12
1 0 x11 x10 x9 x8 x7 x6
1 0 x5 x4 x3 x2 x1 x0
Within this scheme, any byte beginning with a bit must be a single-byte ASCII character
between 1 and 127. Any byte beginning with the three bits 110 must be the first byte of a twobyte
character. Any byte beginning with the four bits 1110 must be the first byte of a threebyte
character. Finally, any byte beginning with the two bits 10 must be the second or third
byte of a multibyte character.
The DataOutputStream class provides a writeUTF() method that encodes a string into UTF-
8 format. It first writes the number of encoded bytes in the string (as an unsigned short)
followed by the UTF-8 encoded format of the string onto the underlying output stream:
public final void writeUTF(String s) throws IOException
The DataInputStream class provides two corresponding readUTF() methods to read a UTF-
8 encoded string from its underlying input stream:
public final String readUTF() throws IOException
public static final String readUTF(DataInput in) throws IOException
Each of these first reads a two-byte, unsigned short that tells it how many more bytes to read.
These bytes are then read and decoded from UTF-8 into a Java Unicode string. An
EOFException is thrown if the stream ends before all the expected bytes have been read. If
the bytes read cannot be interpreted as a valid UTF-8 string, then a
UTFDataFormatException is thrown.
DataInputStream and DataOutputStream actually read and write a slight modification of
the official UTF-8 format. They encode the null character (0x00) in two bytes rather than one.
This makes it slightly easier for C code that expects null-terminated strings to parse Java
.class files. The Reader and Writer classes discussed in the next chapter read and write true
UTF-8 with one-byte nulls.
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