I used RandomAccessFile to read a byte from a text file.
public static void readFile(RandomAccessFile fr) {
byte[] cbuff = new byte[1];
fr.read(cbuff,0,1);
System.out.println(new String(cbuff));
}
Why am I seeing one full character being read by this?
A char represents a character in Java (*). It is 2 bytes large (or 16 bits).
That doesn't necessarily mean that every representation of a character is 2 bytes long. In fact many character encodings only reserve 1 byte for every character (or use 1 byte for the most common characters).
When you call the String(byte[]) constructor you ask Java to convert the byte[] to a String using the platform's default charset (pre Java 18)(**). Since the platform default charset is usually a 1-byte encoding such as ISO-8859-1 or a variable-length encoding such as UTF-8, it can easily convert that 1 byte to a single character.
If you run that code on a platform that uses UTF-16 (or UTF-32 or UCS-2 or UCS-4 or ...) as the platform default encoding, then you will not get a valid result (you'll get a String containing the Unicode Replacement Character instead).
That's one of the reasons why you should not depend on the platform default encoding: when converting between byte[] and char[]/String or between InputStream and Reader or between OutputStream and Writer, you should always specify which encoding you want to use. If you don't, then your code will be platform-dependent.
(*) that's not entirely true: a char represents a UTF-16 code unit. Either one or two UTF-16 code units represent a Unicode code point. A Unicode code point usually represents a character, but sometimes multiple Unicode code points are used to make up a single character. But the approximation above is close enough to discuss the topic at hand.
(**) Note that on Android the default character set is always UTF-8 and starting with Java 18 the Java platform itself also switched to this default (but it can still be configured to act the legacy way)