I/O & System

System.out prints normal output, System.err prints error output (unbuffered), and System.in reads input from the console. In production, use a logging framework instead.

Use Scanner for simple input (nextLine, nextInt), BufferedReader for faster bulk reading, and Console for password input that hides characters.

Byte streams handle raw bytes (InputStream/OutputStream). Character streams handle text with encoding (Reader/Writer). Use character streams for text, byte streams for binary data.