Internally, Java always represents Unicode characters with 16 bits. However, this is an inefficient use of bits when most of the characters being used only need eight bits or less to be represented, which is the case for text written in English and a number of other languages. The UTF-8 encoding provides a more compact way of representing sequences of Unicode when most of the characters are 7-bit ASCII characters. Therefore, UTF-8 is often a more efficient way of storing or transmitting text than using 16 bits for every character.
The UTF-8 encoding is a variable-width encoding of Unicode characters. Seven-bit ASCII characters (\u0000-\u007F) are represented in one byte, so they remain untouched by the encoding (i.e., a string of ASCII characters is a legal UTF-8 string). Characters in the range \u0080-\u07FF are represented in two bytes, and characters in the range \u0800-\uFFFF are represented in three bytes. Java actually uses a slightly modified version of UTF-8, since it encodes \u0000 using two bytes. The advantage of this approach is that a UTF-8 encoded string never contains a null character.
Java provides support for reading characters in the UTF-8 encoding with the readUTF() methods in RandomAccessFile, DataInputStream, and ObjectInputStream . The writeUTF() methods in RandomAccessFile, DataOutputStream, and ObjectOutputStream handle writing characters in the UTF-8 encoding.
The UTF-8 encoding begins with an unsigned 16-bit quantity that indicates the number of bytes of data that follow. This length value is in the format read by the readUnsignedShort() methods the above input classes and written by the writeUnsignedShort() methods in the above output classes.
The rest of the bytes are variable-length characters. A 1-byte character always has its high-order bit set to 0. A 2-byte character always begins with the high-order bits 110, while a 3-byte character starts with the high-order bits 1110. The second and third bytes of 2- and 3-byte characters always have their high-order bits set to 10, which makes them easy to distinguish from 1-byte characters and the initial bytes of 2- and 3-byte characters. This encoding scheme leaves room for seven bits of data in 1-byte characters, 11 bits of data in 2-byte characters, and 16 bits of data in 3-byte characters.
The table below summarizes the UTF-8 encoding: