using codePointAt()

[Ken Sampson]

Hello,

I do not understand how to make use of codePointAt() method. Can anybody help me to understand with a simple code how and where we can make use of this method instead of using charAt() method?

[Campbell Ritchie]

Find a copy of Horstmann and Cornell (Core Java 2, Santa Clara: Sun Microsystems Press/Prentice-Hall). I have the 7th Edition (2005) and you find this on page 51, volume I.

[B]Strings[/B]
Conceptually, Java Strings are sequences of Unicode characters. . . .
[B]Code Points and Code Units[/B]
Java Strings are implements as sequences of [I]char[/I] values. As discussed
on page 41, the [I]char[/I] data type is a code unit for representing
Unicode code points in the UTF-16 encoding. The most commonly used Unicode
characters can be represented with a single code unit. The supplementary
characters require a pair of code units.
   The [I]length[/I] method yields the number of code units required for a
given string in the UTF-16 encoding. For example:
   [I]String greeting = "Hello";[/I]
   [I]int n = greeting.length(); // is 5[/I]
To get the true length, this is, the number of code points, call
   [I]int cpCount = greeting.codePointCount(0, greeting.length());[/I]
The call s.charAt(n) returns the code unit at position n where n is between
0 and s.length() - 1. For example:
   [I]char first = greeting.charAt(0); // first is 'H'[/I]
   [I]char last = greeting.charAt(4); // last is 'o'[/I]
To get at the [I]i[/I]th code point, use the statements
   [I]int index = greeting.offsetByCodePoints(0, i);
   int cp = greeting.codePointAt(index);[/I]
Why are we making a fuss about code units? Consider the sentence
   [B]Z[/B] is the set of integers.
The [B]Z[/B] character requires two code units in the UTF-16 encoding. Calling
   [I]char ch = sentence.charAt(1);[/I]
doesn't return a space but the second code unit of [B]Z[/B]. To avoid this
problem you should not use the char type. It is too low-level.
If your code traverses a string and you want to look at each code point in
turn use these statements:
   [I]int cp = sentence.codePointAt(i);
   if(Character.isSupplementaryCodePoint(cp) i += 2;
   else i++;[/I]
Fortunately the codePointAt method can tell whether a code unit is the first
or second half of a supplementary character, and it returns the right result
either way. That is, you can move backwards with the following statements:
   [I]i--;
   int cp = sentence.codePointAt(i);
   if(Character.isSupplementaryCodePoint(cp)) i--;[/I]

If you
go to Unicode, you find that European and
African languages and numbers and symbols fill up much of the range between
0 and 65535 (0xFFFF), but there are gaps, some of several hundred numbers.
These gaps were used as supplementary symbols, as the first half of a
character from Chinese, etc., or even the big-Z which means "all integers."
These won't fit into the 16 bits of a char, so the 1st half goes in
one char and the 2nd half in the next char. There are methods
in the Character class which deal with this sort of thing. By putting the
two halves together, you get an int, which has space in 32 bits to
hold all conceivable Earthling languages, and probably Klingon as well.

So rather than Unicode having space for 65000 characters it now has space
for 1.1 million (or 17 million; I can't remember which). But if you stick to
European languages you will probably never need to worry about code points.

[ May 04, 2008: Message edited by: Campbell Ritchie to improve formatting: hence the code tabs]
[ May 04, 2008: Message edited by: Campbell Ritchie ]

[dwarakanathan thiru]

If you need to convert the UTF-8 charset languages into a Unicode standard format, use this method.

STRING.codePointAt(INDEX);

The above will return the code point for the character you have keyed into the string.
Later you can paste it in browser and it is automatically decoded and displayed.

Try some chinese languages.