A DESCRIPTION OF THE REQUEST :
For string concatination str1 + str2 javac builds the following code

(new StringBuilder(String.valueOf(str1))).append(str2).toString()

I suggest a more efficient way

new StringBuilder(str1.length() + str2.length()).append(str1).append(str2).toString()


JUSTIFICATION :
With the current implementation of string concatination StringBuilder may need to expand its buffer capacity each time a next string is appended, which is very inefficient. Creating a StringBuilder with a pre-calculated capacity ensures that this will never happens.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
for str1 + str2 javac builds this code

new StringBuilder(str1.length() + str2.length()).append(str1).append(str2).toString()

ACTUAL -
for str1 + str2 javac builds this code

(new StringBuilder(String.valueOf(str1))).append(str2).toString()