concat():
String has concat method, remember string is immutable.
It adds a string to another string.
It will create the new object after concatenation done, since it is a immutable.
append():
StringBuilder and StringBuffer has append method, remember these two are mutable.
It appends a char or char sequence to a string.
It will not create a new object, since it is a mutable one.
http://pydoing.blogspot.tw/2011/01/java-concat.html
http://javadomain.in/ifference-between-concat-and-append-in-java/
http://stackoverflow.com/questions/8962482/yet-again-on-string-append-vs-concat-vs
http://stackoverflow.com/questions/8755063/concatenation-operator-vs-concat?lq=1
http://stackoverflow.com/questions/8962482/yet-again-on-string-append-vs-concat-vs