Java charAt used with characters that have two code units

问题

From Core Java, vol. 1, 9th ed., p. 69:

The character ℤ requires two code units in the UTF-16 encoding. Calling

String sentence = "ℤ is the set of integers"; // for clarity; not in book
char ch = sentence.charAt(1)

doesn't return a space but the second code unit of ℤ.

But it seems that sentence.charAt(1) does return a space. For example, the if statement in the following code evaluates to true.

String sentence = "ℤ is the set of integers";
if (sentence.charAt(1) == ' ')
    System.out.println("sentence.charAt(1) returns a space");

Why?

I'm using JDK SE 1.7.0_09 on Ubuntu 12.10, if it's relevant.

回答1:

It sounds like tho book is saying that 'ℤ' is not a UTF-16 character in the basic multilingual plane, but in fact it is.

Java uses UTF-16 with surrogate pairs for characters that are not in the basic multilingual plane. Since 'ℤ' (0x2124) is in the basic multilingual plane it is represented by a single code unit. In your example sentence.charAt(0) will return 'ℤ', and sentence.charAt(1) will return ' '.

A character represented by surrogate pairs has two code units making up the character. sentence.charAt(0) would return the first code unit, and sentence.charAt(1) would return the second code unit.

See http://docs.oracle.com/javase/6/docs/api/java/lang/String.html:

A String represents a string in the UTF-16 format in which supplementary characters are represented by surrogate pairs (see the section Unicode Character Representations in the Character class for more information). Index values refer to char code units, so a supplementary character uses two positions in a String.

回答2:

According to the documentation String is represented internally as utf-16, so charAt() is giving you two code points. If you are interested in seeing the individual code points you can use this code (from this answer):

final int length = sentence.length();
for (int offset = 0; offset < length; ) {
   final int codepoint = sentence.codePointAt(offset);

   // do something with the codepoint

   offset += Character.charCount(codepoint);
}

回答3:

The Javadocs Explain this:

A String represents a string in the UTF-16 format in which supplementary characters are represented by surrogate pairs (see the section Unicode Character Representations in the Character class for more information). Index values refer to char code units, so a supplementary character uses two positions in a String.

In short, the book is wrong.

Edit to add from comments below: Something I didn't think of last night that was that the character you used in your question isn't actually the one they're talking about, and what they're really getting at is when you have have a character that required four bytes rather than two. The paragraph above in the Javadoc links to another javadoc; Unicode Character Representations which talks about the ramifications of this.

回答4:

Horstmann was talking about the 'Z' which need two UTF-16 code units. Take a look at this code:

public class Main {
    public static void main(String[] args)
    {
        String a = "\uD83D\uDE02 is String";
        System.out.println("Length: " + a.length());
        System.out.println(a.charAt(0));
        System.out.println(a.charAt(1));
        System.out.println(a.charAt(2));
        System.out.println(a.charAt(3));
    }
}

in IntelliJ Idea I can't even paste the 4 byte character as one character because while pasting this emoji: 😂 IDE automatically converts it to: "\uD83D\uDE02". Notice that this emoji is counted as 2 characters.

If you want to count the 'real length' then should use: System.out.println("Real length: " + a.codePointCount(0, a.length()));

Take a look at: What are the most common non-BMP Unicode characters in actual use?

来源：https://stackoverflow.com/questions/14150530/java-charat-used-with-characters-that-have-two-code-units