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What are Compact Strings in Java 9?
Since Java 9, the JVM optimizes strings by using a new feature called Compact Strings. Instead of having a char[] array, a string can be represented as a byte[] array. We can use either UTF-16 or Latin-1 to produce either one or two bytes per character. If JVM detects the string contains only ISO-8859-1/Latin-1 characters, then string uses one byte per character internally.
The string can be represented with a compact string or not is detected when the string is created. This feature has enabled by default and switches off using the -XX:-CompactStrings. It doesn't revert to a char[] implementation and stores all strings as UTF-16.
// In Java 8
public class String {
private final char[] value; // Stores characters in the string
---------
}
// In Java 9
public class String {
private final byte[] value; // Stores characters in the string
private final byte coder; // a flag whether to use 1 byte per character or 2 bytes per characters for this string
---------
}
Updated on: 19-Mar-2020
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