How Does Java's StringBuffer Handle Surrogate Pairs During String Reversal?

Understanding Surrogate Pairs in Java's StringBuffer

In the documentation for StringBuffer's reverse() method, the concept of "surrogate pairs" surfaces. This article delves into what surrogate pairs are and explains the significance of low and high surrogates in Java's UTF-16 encoding scheme.

What Are Surrogate Pairs?

Unicode, a widely adopted character encoding standard, assigns code points ranging from 0x0 to 0x10FFFF to characters. However, Java internally stores Unicode text using UTF-16, which uses 16-bit code units. To accommodate Unicode characters with higher code points (0x10000 to 0x10FFFF), surrogate pairs enter the picture.

The Role of Surrogates

UTF-16 handles high code point characters by utilizing two code units known as surrogate pairs. These pairs consist of two separate code units: high surrogates and low surrogates. High surrogates appear at the start of a pair, while low surrogates follow.

Implications for StringBuffer's reverse()

The reverse() method in StringBuffer, as suggested by its name, reverses the characters in a given string. This operation becomes crucial when dealing with surrogate pairs. Because these pairs are treated as single entities in UTF-16, reversing a string containing surrogate pairs requires preserving the correct order of the code units within each pair. Reversing the code units within a surrogate pair can result in malformed Unicode text.

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