StringBuilder.appendCodePoint(int CodePoint)

appendCodePoint
public StringBuilder appendCodePoint(int codePoint)
Appends the string representation of the codePoint argument to this sequence.
The argument is appended to the contents of this sequence. The length of this sequence increases by Character.charCount(codePoint).
The overall effect is exactly as if the argument were converted to a char array by the method Character.toChars(int) and the character in that array were then appended to this character sequence.
Parameters:
codePoint - a Unicode code point
Returns:
a reference to this object.
Since:
1.5

这个函数的意思其实就是向StringBuilder中添加一个Unicode code point 为 Codepoint的字符 即appendCodePoint(97) == append(Character.toChars(97)) 两个语句的意思都是向已有的序列中添加一个字符

下面是一个简单的演示

String str = "hello";
StringBuilder strbud = new StringBuilder();
strbud.append(str);
System.out.println(strbud.appendCodePoint(97));

输出为

helloa

参考资料:
Document