StringBuilderって速いの?
結論：ちょっぱや import java . io .*; class StringAppend { public static void main ( String [] args ) { for ( int i = 0 ; i < 50 ; i += 5 ) { long start = System . currentTimeMillis (); String str = "" ; for ( int j = 0 ; j < 100 *( i + 1 ) ; j ++) { str += "aa" ; } System . out . println ( "" + i + " " +( System . currentTimeMillis ()- start )); } System . out . println ( "" ); for ( int i = 0 ; i < 50 ; i += 5 ) { long start = System . currentTimeMillis (); StringBuilder str = new StringBuilder (); for ( int j = 0 ; j < 100 *( i + 1 ) ; j ++) { str . append ( "aa" ); } System . out . println ( "" + i + " " +( System . currentTimeMillis ()- start )); } } }