How do you iterate through Unicode codepoints in Java Strings?

Iterating through Unicode Codepoints in Java Strings

Introduction

Iterating through the Unicode codepoints of a Java String requires a unique approach as Java uses a UTF-16-esque encoding. This article explores different strategies and addresses concerns regarding the encoding of characters outside the Basic Multilingual Plane (BMP).

Approaching the Problem

Initially, one might consider using String#codePointAt(int) indexed by character offset. However, this approach presents two concerns: it's not indexed by codepoint offset, and handling codepoints outside the BMP poses challenges.

An alternative approach involves using String#charAt(int) to obtain characters and testing their membership in the high-surrogates range. While this method provides a way to determine if a codepoint is outside the BMP, it comes with the following drawbacks:

• Uncertainty about the representation of BMP-range codepoints
• High computational cost

The Optimal Solution

Fortunately, Java provides the canonical way to iterate over codepoints using String#codePointAt(int):

<code class="java">for (int offset = 0; offset < length; ) {
   final int codepoint = s.codePointAt(offset);

   // do something with the codepoint

   offset += Character.charCount(codepoint);
}</code>

Addressing Concerns

• Java indeed uses a UTF-16-esque encoding, storing characters outside the BMP as surrogates.
• The code provided above handles BMP-range codepoints correctly.
• Increasing the offset by Character.charCount(codepoint) correctly navigates surrogate pairs.

Conclusion

To summarize, iterating through Unicode codepoints in Java Strings requires a deeper understanding of the underlying encoding. However, using the canonical approach outlined in this article provides a correct and efficient solution for this common need.

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