Answer 1

If there's no need to use multithreating, It is better to use StringBuilder than StringBuffer since It is faster. Here all the details: https://www.journaldev.com/538/string-vs-stringbuffer-vs-stringbuilder

Answer 2

Here is an explanation: https://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html

Answer 3

StringBuffer + char[] buffer + FileReader
https://code.sololearn.com/cWmtMdPLUu85

Answer 4

Abduzcan