问题
Recently I ran into codePointAt method of String in Java. I found also a few other codePoint methods: codePointBefore, codePointCount etc. They definitely have something to do with Unicode but I do not understand it.
Now I wonder when and how one should use codePointAt and similar methods.
回答1:
Short answer: it gives you the Unicode codepoint that starts at the specified index in String. i.e. the "unicode number" of the character at that position.
Longer answer: Java was created when 16 bit (aka a char) was enough to hold any Unicode character that existed (those parts are now known as the Basic Multilingual Plane or BMP). Later, Unicode was extended to include characters with a codepoint > 216. This means that a char could no longer hold all possible Unicode codepoints.
UTF-16 was the solution: it stores the "old" Unicode codepoints in 16 bit (i.e. exactly one char) and all the new ones in 32 bit (i.e. two char values). Those two 16 bit values are called a "surrogate pair". Now strictly speaking a char holds a "UTF-16 code unit" instead of "a Unicode character" as it used to.
Now all the "old" methods (handling only char) could be used just fine as long as you didn't use any of the "new" Unicode characters (or didn't really care about them), but if you cared about the new characters as well (or simply need to have complete Unicode support), then you'll need to use the "codepoint" versions that actually support all possible Unicode codepoints.
Note: A very well known example of unicode characters that are not in the BMP (i.e. work only when using the codepoint variant) are Emojis: Even the simple Grinning Face 😀 U+1F600 can't be represented in a single char.
回答2:
Code points support characters above 65535 which is Character.MAX_VALUE.
If you have text with such high characters you have to work with code points or int instead of chars.
It doesn't this by support UTF-16 which can use one or two 16-bit char and turn it into an int
AFAIK, Generally this is only required for Supplementary Multiligual and Supplementary Ideographic characters added recently such as non traditional Chinese.
回答3:
In short rarely as long you are using default charset in Java :) But for a more detailed explanation try these posts:
Comparing a char to a code-point? http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Character.html http://javarevisited.blogspot.com/2012/01/java-string-codepoint-get-unicode.html
Hope this helps clarify things for you :)
来源：https://stackoverflow.com/questions/12280801/what-exactly-does-string-codepointat-do