Question

There is a String variable containing ascii characters and double bytes characters(for example, the Chinese, Japanese,…).

How to decide the total length of the String ? Also, I want to implement with the string substring/replace function.

Answer 1

As other’s have said, Java Strings are conceptually read-only arrays of Java characters, and the “length” of a String is the number of characters. However, there are complicating issues:

• A Java character is not necessarily what you think of as a character. In particular, there are more Unicode characters (code-points) than can be represented using Java characters. Some Unicode code-points require two Java characters to represent them. (This is the “extended plane” issue that Thilo refers to.)

• Some JVMs (with the appropriate JVM flags set at startup) will use a String representation where the characters are encoded in UTF-8. While the length of the String is the same (in this case, the number of Java characters represented by the UTF-8), the memory used can be significantly less.

Then there is the question of how many bytes are required to represent the String’s characters as UTF-8, or in some other encoding. As far as I know, the only JVM provided way to find that out is to do the conversion; e.g. using getBytes(charSet).

Finally, there is the question of how many bytes a String occupies in the heap. You can find out how many bytes are in the String object and its associated char[] backing object. However, predicting what that is going to be can be tricky, when you consider that substring and other String methods can create sets of strings that share a single backing array.