What is the Java's internal represention for String? Modified UTF-8? UTF-16?
Java uses UTF-16 for the internal text representation
The representation for String and StringBuilder etc in Java is UTF-16
https://docs.oracle.com/javase/8/docs/technotes/guides/intl/overview.html
How is text represented in the Java platform?
The Java programming language is based on the Unicode character set, and several libraries implement the Unicode standard. The primitive data type char in the Java programming language is an unsigned 16-bit integer that can represent a Unicode code point in the range U+0000 to U+FFFF, or the code units of UTF-16. The various types and classes in the Java platform that represent character sequences - char[], implementations of java.lang.CharSequence (such as the String class), and implementations of java.text.CharacterIterator - are UTF-16 sequences.
At the JVM level, if you are using -XX:+UseCompressedStrings (which is default for some updates of Java 6) The actual in-memory representation can be 8-bit, ISO-8859-1 but only for strings which do not need UTF-16 encoding.
http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html
and supports a non-standard modification of UTF-8 for string serialization.
Serialized Strings use UTF-8 by default.
And how many bytes does Java use for a char in memory?
A char is always two bytes, if you ignore the need for padding in an Object.
Note: a code point (which allows character > 65535) can use one or two characters, i.e. 2 or 4 bytes.
Prior to Java 9, the standard in-memory representation for a Java String is UTF-16 code-units held in a char[]. Modified UTF-8 is used in other contexts; e.g. in ".class" files, and the object serialization format.
You can confirm this by looking at the source code of the java.lang.String class.
With Java 6 update 21 and later, there was a non-standard option (-XX:UseCompressedStrings) to enable compressed strings. This feature was removed in Java 7.
For Java 9 and later, the behavior if String has been changed to use a compact representation for Strings by default. The java command documentation now says this:
-XX:-CompactStrings
Disables the Compact Strings feature. By default, this option is enabled. When this option is enabled, Java Strings containing only single-byte characters are internally represented and stored as single-byte-per-character Strings using ISO-8859-1 / Latin-1 encoding. This reduces, by 50%, the amount of space required for Strings containing only single-byte characters. For Java Strings containing at least one multibyte character: these are represented and stored as 2 bytes per character using UTF-16 encoding. Disabling the Compact Strings feature forces the use of UTF-16 encoding as the internal representation for all Java Strings.
Note that neither "compressed" or "compact" strings used / use UTF-8 encoding.
See also:
- Java Platform, Standard Edition What’s New in Oracle JDK 9
- JEP 254: Compact Strings
- Difference between compact strings and compressed strings in Java 9
UTF-16.
From http://java.sun.com/javase/technologies/core/basic/intl/faq.jsp :
How is text represented in the Java platform?
The Java programming language is based on the Unicode character set, and several libraries implement the Unicode standard. The primitive data type char in the Java programming language is an unsigned 16-bit integer that can represent a Unicode code point in the range U+0000 to U+FFFF, or the code units of UTF-16. The various types and classes in the Java platform that represent character sequences - char[], implementations of java.lang.CharSequence (such as the String class), and implementations of java.text.CharacterIterator - are UTF-16 sequences.