question: public java . lang . StringBuffer append ( char [ ] str , int offset , int len ) { internal . append ( str , offset , len ) ; return this ; }
target:  Appends the string representation of a subarray of the char array argument to this string buffer. Characters of the character array str, starting at index offset, are appended, in order, to the contents of this string buffer. The length of this string buffer increases by the value of len. The overall effect is exactly as if the arguments were converted to a string by the method String.valueOf(char[],int,int) and the characters of that string were then appended to this StringBuffer object.