- Java文件中的类
- 一个Java源文件中可没有主类（即函数运行的入口，public static void main）或公共类（public修饰的类），但是有的话，对于公共类只能由一个，对于主类则可以有多个。
- 参考地址:https://blog.csdn.net/weixin_44984627/article/details/104765866
- 类、方法、成员变量的修饰符
- 参考链接：https://www.cnblogs.com/zyx110/p/10621592.html
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- null关键字
- Java中null关键字指的是空的意思，用于对象时是指没有使用new分配内存空间
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