What is meant by "escape sequence" in the definition of Java string literals?

From the Java Language Specification, Section 3.10.5 String Literals:

Characters may be represented by escape sequences - one escape sequence for characters in the range U+0000 to U+FFFF, two escape sequences for the UTF-16 surrogate code units of c开发者_开发百科haracters in the range U+010000 to U+10FFFF.

What does this mean? If a character falls within the range U+0000 to U+FFFF, then one escape sequence may be used. How different is one escape sequence from two escape sequences?

By escape sequence, does it refer to \n, \r and similar? Are these one sequence or two escape sequences?

From u+0000 to u+ffff, each number (if you will) represents a characters. However, some unicode characters (called surrogate pairs) are combination of two numbers in the u+010000 to u+10ffff. So if you have a number u+010000 to u+10ffff, then a second one is required to represent a valid character.

By escape sequence, they mean stuff like \u0000 (which you can use in a String literal to represent a unicode character).