三 流程控制
3.1 scanner
psvm
new Scanner(System.in) alt+enter+enter 自动补全
Scanner scanner = new Scanner(System.in);
3.2 if
3.3 switch
3.4 while
3.5 for
psvm
new Scanner(System.in) alt+enter+enter 自动补全
Scanner scanner = new Scanner(System.in);