当你想阻止一个变量被改变，可能会用到const关键字。在你给一个变量加上const修饰符的同时，通常需要对它进行初始化，因为以后的任何时候你将没有机会再去改变它-When you want to be changed to prevent a variable may be used for const keyword. To a variable you add const modifier at the same time, usually need to initialize it, because any time you will not have a chance to change it again