Question

为什么JAVA编程里把主函数放在定义的类中　

Answer 1

The Java Virtual Machine (JVM) has to start the application somewhere. As Java does not have a concept of “things outside of a class” the method that is called by the JVM has to be in a class. And because it is static, no instance of that class is created yet.
根据官方文档，这个语法现象建立于java虚拟机设计。在建立虚拟机的时候就没有这个概念。虚拟机在执行程序时候首先在main方法，没有就报错。你可以试试建立一个类，然后注释main方法，会报错：在类中找不到main方法。