Readers and Writers deal with character streams. These are abstract classes. A program must use classes derived from them. For example, a BufferedReader is a Reader.

Character streams are optimized for handling character data. They also translate between the internal format used by Java programs and an external format used for text files. Inside a Java program character data is represented with the 16-bit char data type. The characters of a String also use this 16-bit code. On a disk file, characters are represented in a format called UTF. This format uses one to four bytes per character and is intended to be a universal format—one format for all text files in any language anywhere in the world.

UTF stands for "Unicode Transformation Format". Usually a UTF text file is identical to an ASCII text file. ASCII is the standard way to represent characters that most computers have used for the past forty years. A file created with a text editor is usually an ASCII text file.

A UTF text file can include non-ASCII characters such as Cyrillic, Greek, and Asian characters. By reading and writing UTF files, Java programs can process text from any of the World's languages.