Java String internal representation

Question

I understand that the internal representation of Java for String is UTF-16. What is java string representation?

Also, I know that in a UTF-16 String, each 'character' is encoded with one or two 16-bit code units.

However, when I debug the following java code

String hello = "Hello";

the variable hello is an array of 5 bytes 0x48, 0x101, 0x108, 0x108, 0x111 which is ASCII for "Hello".

How can this be?

Answer 1

I took a gcore dump of a mini java process with this code:

 class Hi {
    public static void main(String args[]) {
        String hello = "Hello";
        try {
            Thread.sleep(60_000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

And did a gcore memory dump on Ubuntu. (usign jps to get the pid and passed that to gcore)

If found this: 48 65 6C 6C 6F in the dump using a Hexeditor, so it is somewhere in the memory as ASCII.

But also 48 00 65 00 6C 00 6C which is part of the UTF-16 representation of the String

Answer 2

String internal representation is not specified, it's the implementation detail, so you cannot rely on it. It's very likely that in JDK-9 it will be changed to use double encoding (Latin-1 for strings which can be encoded in Latin-1, UTF-16 for other strings). See JEP-254 for details. This feature is already integrated in OpenJDK master codebase, so if you are using Java-9 early access builds, you will have actually 5 bytes.