Comparing a char to a code-point?

A little bit of background: When Java appeared in 1995, the char type was based on the original "Unicode 88" specification, which was limited to 16 bits. A year later, when Unicode 2.0 was implemented, the concept of surrogate characters was introduced to go beyond the 16 bit limit.

Java internally represents all Strings in UTF-16 format. For code points exceeding U+FFFF the code point is represented by a surrogate pair, i.e., two chars with the first being the high-surrogates code unit, (in the range \uD800-\uDBFF), the second being the low-surrogate code unit (in the range \uDC00-\uDFFF).

From the early days, all basic Character methods were based on the assumption that a code point could be represented in one char, so that's what the method signatures look like. I guess to preserve backward compatibility that was not changed when Unicode 2.0 came around and caution is needed when dealing with them. To quote from the Java documentation:

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).

Casting the char to an int, as you do in your sample, works fine though.

The Character class contains many useful methods for working with Unicode code points. Note methods like Character.toChars(int) that return an array of chars. If your codepoint lies in the supplementary range, then the array will be two chars in length.

How you want to compare the values depends on whether you want to support the full range of Unicode values. This sample code can be used to iterate through a String's codepoints, testing to see if there is a match for the supplementary character MATHEMATICAL_FRAKTUR_CAPITAL_G (𝔊 - U+1D50A):

public final class CodePointIterator {

  private final String sequence;
  private int index = 0;

  public CodePointIterator(String sequence) {
    this.sequence = sequence;
  }

  public boolean hasNext() {
    return index < sequence.length();
  }

  public int next() {
    int codePoint = sequence.codePointAt(index);
    index += Character.charCount(codePoint);
    return codePoint;
  }

  public static void main(String[] args) {
    String sample = "A" + "\uD835\uDD0A" + "B" + "C";
    int match = 0x1D50A;
    CodePointIterator pointIterator = new CodePointIterator(sample);
    while (pointIterator.hasNext()) {
      System.out.println(match == pointIterator.next());
    }
  }
}

For Java 8 onwards CharSequence.codePoints() can be used:

public static void main(String[] args) {
  String sample = "A" + "\uD835\uDD0A" + "B" + "C";
  int match = 0x1D50A;
  sample.codePoints()
        .forEach(cp -> System.out.println(cp == match));
}

I created a table to help get a handle on Unicode string length and comparison cases that sometimes need to be handled.

Comparing a char to a code-point?

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