Why some int from 0x0000 to 0xFFFF is not a defined unicode character

Question

I read from the Java doc of Character, that

The set of characters from U+0000 to U+FFFF is sometimes referred to as the Basic Multilingual Plane (BMP)

But I tried the following code, and found there is 2492 int is not defined! Is there any thing wrong? Or I have some misunderstanding? Thanks!

public static void main( String[] args )
{
    int count=0;
    for(int i = 0x0000; i<0xFFFF;i++)
    {
        if(!Character.isDefined(i))
        {
            count++;
        }
    }
    System.out.println(count);
}

Output :

2492

Answer 1

The documentation for isDefined() states that a character "is defined" if it has an entry or is in a range in the UnicodeData file. This identifies the set of code points that have been assigned to characters (and it might've been better named isAssigned()). As you discovered, not all of the code points in the Basic Multilingual Plane have been assigned to characters yet (this map shows where some of the empty spaces are).

However, even if a code point has not been assigned (that is, isDefined() is false), it may be assigned in a future version of Unicode, and is still a valid code point. Encoding/decoding and working with unassigned code points is perfectly valid (although, it is a little strange).