程序员自己编写的类和JDK类是一种合作关系。（因为多态的存在，可能存在我们调用JDK类的情况，也可能存在JDK自动调用我们的类的情况。）-Programmers write their own class and the JDK class is a partnership. (because polymorphism existed, there may be we call the JDK class, there may also