unicode - What is a "surrogate pair" in Java?

Question

I was reading the documentation for StringBuffer, in particular the reverse() method. That documentation mentions something about surrogate pairs. What is a surrogate pair in this context? And what are low and high surrogates?

Answer 1

The term "surrogate pair" refers to a means of encoding Unicode characters with high code-points in the UTF-16 encoding scheme.

In the Unicode character encoding, characters are mapped to values between 0x0 and 0x10FFFF.

Internally, Java uses the UTF-16 encoding scheme to store strings of Unicode text. In UTF-16, 16-bit (two-byte) code units are used. Since 16 bits can only contain the range of characters from 0x0 to 0xFFFF, some additional complexity is used to store values above this range (0x10000 to 0x10FFFF). This is done using pairs of code units known as surrogates.

The surrogate code units are in two ranges known as "high surrogates" and "low surrogates", depending on whether they are allowed at the start or end of the two-code-unit sequence.