A character set is a set of textual and graphic symbols, each of which is mapped to a set of nonnegative integers.

The first character set used in computing was US-ASCII. It is limited in that it can represent only American English. US-ASCII contains uppercase and lowercase Latin alphabets, numerals, punctuation, a set of control codes, and a few miscellaneous symbols.

Unicode defines a standardized, universal character set that can be extended to accommodate additions. When the Java program source file encoding doesn’t support Unicode, you can represent Unicode characters as escape sequences by using the notation `\u`XXXX, where XXXX is the character’s 16-bit representation in hexadecimal. For example, the Spanish version of a message file could use Unicode for non-ASCII characters, as follows:

A character encoding maps a character set to units of a specific width and defines byte serialization and ordering rules. Many character sets have more than one encoding. For example, Java programs can represent Japanese character sets using the EUC-JP or Shift-JIS encodings, among others. Each encoding has rules for representing and serializing a character set.

The ISO 8859 series defines 13 character encodings that can represent texts in dozens of languages. Each ISO 8859 character encoding can have up to 256 characters. ISO-8859-1 (Latin-1) comprises the ASCII character set, characters with diacritics (accents, diaereses, cedillas, circumflexes, and so on), and additional symbols.

UTF-8 (Unicode Transformation Format, 8-bit form) is a variable-width character encoding that encodes 16-bit Unicode characters as one to four bytes. A byte in UTF-8 is equivalent to 7-bit ASCII if its high-order bit is zero; otherwise, the character comprises a variable number of bytes.

UTF-8 is compatible with the majority of existing web content and provides access to the Unicode character set. Current versions of browsers and email clients support UTF-8. In addition, many web standards specify UTF-8 as their character encoding. For example, UTF-8 is one of the two required encodings for XML documents (the other is UTF-16).

Web components usually use PrintWriter to produce responses; PrintWriter automatically encodes using ISO-8859-1. Servlets can also output binary data using OutputStream classes, which perform no encoding. An application that uses a character set that cannot use the default encoding must explicitly set a different encoding.