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Legacy software frequently assumes that every character in a string occupies 8 bits (a Java byte). The Java language assumes that every character in a string occupies 16 bits (a Java char). Unfortunately, neither the Java byte nor Java char data types can represent all possible Unicode characters. Many strings are stored or communicated using encodings such as UTF-8 that support characters with varying sizes.

While Java strings are stored as an array of characters and can be represented as an array of bytes, a single character in the string might be represented by two or more consecutive elements of type byte or of type char. Splitting a char or byte array risks splitting a multibyte character.

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Multibyte Characters

Multibyte encodings such as UTF-8 are used for character sets that require more than one byte to uniquely identify each constituent character. For example, the Japanese encoding Shift-JIS (shown below) , one of the Japanese encodings, supports multibyte encoding wherein where the maximum character length is two bytes (one leading and one trailing byte).

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The trailing byte ranges overlap the range of both the single -byte and lead -byte characters. When a multibyte character is separated across a buffer boundary, it can be interpreted differently than if it were not separated across the buffer boundary; this difference arises because of the ambiguity of its composing bytes \[[Phillips 2005|AA. Bibliography#Phillips 05]\]. 

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The char data type (and consequently the value that a Character object encapsulates) are sic based on the original Unicode specification, which defined characters as fixed-width 16-bit entities. The Unicode standard has since been changed to allow for characters whose representation requires more than 16 bits. The range of legal code points is now \u0000 to \u10FFFF, known as Unicode scalar value.

The Java 2 platform uses the UTF-16 representation in char arrays and in the String and StringBuffer classes. In this representation, supplementary characters are represented as a pair of char values, the first from the high-surrogates range, (\uD800-\uDBFF), the second from the low-surrogates range (\uDC00-\uDFFF).

An int value represents all Unicode code points, including supplementary code points. The lower (least significant) 21 bits of int are used to represent Unicode code points, and the upper (most significant) 11 bits must be zero. Unless otherwise specified, the behavior with respect to supplementary characters and surrogate char values is as follows:

• The methods that only accept a char value cannot support supplementary characters. They treat char values from the surrogate ranges as undefined characters. For example, Character.isLetter('\uD840') returns false, even though this specific value if followed by any low-surrogate value in a string would represent a letter.
• The methods that accept an int value support all Unicode characters, including supplementary characters. For example, Character.isLetter(0x2F81A) returns true because the code point value represents a letter (a CJK ideograph).

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This code fails to account for the interaction between characters represented with a multibyte encoding and the boundaries between the loop iterations. If the last byte read from the data stream in one read() operation is the leading byte of a multibyte character, the trailing bytes are not encountered until the next iteration of the while loop. However, multibyte encoding is resolved during construction of the new String within the loop. Consequently, the multibyte encoding is can be interpreted incorrectly.

Compliant Solution (Read)

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// Fails for combining characters
public static String trim_bad2(String string) {
  int ch;
  int i;
  for (i = 0; i < string.length(); i += Character.charCount(ch)) {
    ch = string.codePointAt(i);
    if (!Character.isLetter(ch)) {
      break;
    }
  }
  
  return string.substring(i);
}

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