Tuesday, July 26, 2022

Compact Strings in Java

In this post we’ll learn about a new feature Compact Strings in Java, added in Java 9, which adopts a more space-efficient internal representation for strings.

Motivation for Compact Strings in Java

Implementation of Java String class before Java 9 stored characters in a char array, using two bytes for each character - UTF-16 encoding. Since String is one of the most used class, String instances constitute a major component of heap usage. It has been observed that most String objects contain only Latin-1 characters which requires only one byte of storage. So internal storage always as UTF-16 means half of the storage is going unused.

Changes for Compact Strings

In order to make Strings more space efficient Java 9 onward internal representation of the String class has been modified from a UTF-16 char array to a byte array plus an encoding-flag field.

As per the Java Compact String feature, based upon the contents of the string characters are stored either as-

  • ISO-8859-1/Latin-1 (one byte per character), or
  • UTF-16 (two bytes per character)

The encoding-flag field indicates which encoding is used.

In the String class you can see the changes for the same-

Storage from char[] array, before Java 9

/** The value is used for character storage. */
 private final char value[]; 

has been changed to byte[] array

private final byte[] value;

Encoding-flag field is named as coder and is of type byte-

private final byte coder;

coder can have either of these two values-

@Native static final byte LATIN1 = 0;
@Native static final byte UTF16  = 1;

Based on whether the storage is Latin-1 or UTF-16 methods of the String class have different implementations too. In fact even the String class has two variants-

final class StringLatin1

final class StringUTF16

Based on the value of the encoding-flag field (coder) specific implementation is called by the methods of the String class.

public int compareTo(String anotherString) {
  byte v1[] = value;
  byte v2[] = anotherString.value;
  if (coder() == anotherString.coder()) {
    return isLatin1() ? StringLatin1.compareTo(v1, v2)
                        : StringUTF16.compareTo(v1, v2);
  return isLatin1() ? StringLatin1.compareToUTF16(v1, v2)
                    : StringUTF16.compareToLatin1(v1, v2);

That's all for this topic Compact Strings in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!

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