Narrowing conversion from byte to char Java

Question

I read that although type byte occupies 8 bits of memory, and type char occupies 16 bits of memory, the conversion from byte to char is considered narrowing because byte is signed, while char is unsigned. I think this means that when we convert a type byte with a negative value to char, we lose the negative and so that is our loss of information and so the conversion is narrowing.

I am trying to test this in a Java program.

public class Test
{
  public static void main (String[] args)
  {
    byte num = -1;
    char test = (char)num;
    System.out.println(test);
  }
}

I expected that printing test would output:

This would be -1 without the negative sign because char is unsigned. This in fact outputs:

Is there something wrong with my program or with my idea about converting byte to char?

Just use a debugger and look what `test` "contains", then you'll understand. — Tom, Sep 14 '16 at 23:33
https://docs.oracle.com/javase/specs/jls/se8/html/jls-5.html#jls-5.1.4. — Sotirios Delimanolis, Sep 14 '16 at 23:35
(the duplicate question also contains the underflow during this conversion which also happens here) — Tom, Sep 14 '16 at 23:39
In binary: byte: 11111111 (-1) char: 1111111111111111 (65535) — Michael Krause, Sep 14 '16 at 23:42

score 0 · Accepted Answer · edited May 23 '17 at 10:33

That's not how signs or char works.

Bear in mind that System.out.println will show you the UTF-8 character (which is the same as ascii for the first 8 bits of values) for a char. So System.out.println((char)71); will print "G". So when you expect to see "1" to be printed, what you're saying is that you expect (char)-1 == 41. If you want the numeric value, you should cast it to int before printing.
System.out.println will substitute a question mark for unprintable characters.
byte to char first is converted to an int. The doc (linked by Sotirios Delimanolis) says;

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).

Where

A narrowing conversion of a signed integer to an integral type T simply discards all but the n lowest order bits, where n is the number of bits used to represent type T

Narrowing conversion from byte to char Java

1 Answers1