Programming to support languages that use anything other than the Latin character set has always been a major problem. There are a variety of 8-bit character sets defined for many national languages, but if you want to combine the Latin character set and Cyrillic in the same context, for example, things can get difficult. If you want to handle Japanese as well, it becomes impossible with an 8-bit character set because with 8 bits you only have 256 different codes so there just aren't enough character codes to go round. Unicode is a standard character set that was developed to allow the characters necessary for almost all languages to be encoded. It uses a 16-bit code to represent a character (so each character occupies two bytes), and with 16 bits up to 65,535 non-zero character codes can be distinguished. With so many character codes available, there is enough to allocate each major national character set its own set of codes, including character sets such as Kanji which is used for Japanese, and which requires thousand of character codes. It doesn't end there though. Unicode supports three encoding forms that allow up to a million additional characters to be represented.
As we shall see in Chapter 2, Java sourcecode is in Unicode characters. Comments, identifiers (names – see Chapter 2), and character and string literals can all use any characters in the Unicode set that represent letters. Java also supports Unicode internally to represent characters and strings, so the framework is there for a comprehensive international language capability in a program. The normal ASCII set that you are probably familiar with corresponds to the first 128 characters of the Unicode set. Apart from being aware that each character occupies two bytes, you can ignore the fact that you are handling Unicode characters in the main, unless of course you are building an application that supports multiple languages from the outset.