高级点用这个

int len=40;
String str="I have a dream";
System.out.println("┌"+"-".repeat(len)+"┐");
System.out.println("|"+" ".repeat(len)+"|");
int rl=(len-str.length())/2;
System.out.printf("|%"+(rl+str.length())+"s%s|\n",str," ".repeat(rl));
System.out.println("|"+" ".repeat(len)+"|");
System.out.println("└"+"-".repeat(len)+"┘");
