Answer 1

If you have a look in the documentation you will find different constructors for String Buffer.

* StringBuffer()
Constructs a string buffer with no characters in it and an initial capacity of 16 characters.

* StringBuffer(CharSequence seq)
Constructs a string buffer that contains the same characters as the specified CharSequence.

* StringBuffer(int capacity)
Constructs a string buffer with no characters in it and the specified initial capacity.

* StringBuffer(String str)
Constructs a string buffer initialized to the contents of the specified string.

https://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html

I think only the designer of this class knows why he/she chooses 16.

https://coderanch.com/t/455484/java/StringBuffer-size-charectors-java