encoding - Byte and char conversion in Java

Question

If I convert a character to byte and then back to char, that character mysteriously disappears and becomes something else. How is this possible?

This is the code:

char a = 'è';       // line 1       
byte b = (byte)a;   // line 2       
char c = (char)b;   // line 3
System.out.println((char)c + " " + (int)c);

Until line 2 everything is fine:

• In line 1 I could print "a" in the console and it would show "è".

• In line 2 I could print "b" in the console and it would show -56, that is 200 because byte is signed. And 200 is "è". So it's still fine.

But what's wrong in line 3? "c" becomes something else and the program prints ? 65480. That's something completely different.

What I should write in line 3 in order to get the correct result?

Answer 1

A character in Java is a Unicode code-unit which is treated as an unsigned number. So if you perform c = (char)b the value you get is 2^16 - 56 or 65536 - 56.

Or more precisely, the byte is first converted to a signed integer with the value 0xFFFFFFC8 using sign extension in a widening conversion. This in turn is then narrowed down to 0xFFC8 when casting to a char, which translates to the positive number 65480.

From the language specification:

5.1.4. Widening and Narrowing Primitive Conversion

First, the byte is converted to an int via widening primitive conversion (§5.1.2), and then the resulting int is converted to a char by narrowing primitive conversion (§5.1.3).

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To get the right point use char c = (char) (b & 0xFF) which first converts the byte value of b to the positive integer 200 by using a mask, zeroing the top 24 bits after conversion: 0xFFFFFFC8 becomes 0x000000C8 or the positive number 200 in decimals.

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Above is a direct explanation of what happens during conversion between the byte, int and char primitive types.

If you want to encode/decode characters from bytes, use Charset, CharsetEncoder, CharsetDecoder or one of the convenience methods such as new String(byte[] bytes, Charset charset) or String#toBytes(Charset charset). You can get the character set (such as UTF-8 or Windows-1252) from StandardCharsets.