class Human
{
    String name;
    String sex;
    int age;
    String addr;
    void work(){
    System.out.println("我在工作");
    }
    void eat(){
    System.out.println("我在吃饭");
    }
    String getState(int time){
        String state=null;
        if(time>8&&24>time){
        if(time>=0&&time<17)
            state="我在工作";
        else if(time>17&&time<22)
            state="我在学习";
        else if(time>22||time<7)
            state="我在睡觉";
        }else
            state="错误的时间";
        return state;
    }
}
public class MethodDemo
{
    public static void main(String[] args)
    {
        Human wangming=new Human();
        wangming.name="王明";
        wangming.age=25;
        wangming.sex="男";
        wangming.addr="中国北京";
        System.out.println(wangming.name+"晚上23点钟你在干嘛");
        System.out.println(wangming.getState(23));
        System.out.println("下午15点呢");
        System.out.println(wangming.getState(15));
    }
}
C:\java>javac Human.java
Human.java:27: 错误: 类MethodDemo是公共的, 应在名为 MethodDemo.java 的文件中声明

public class MethodDemo
       ^
1 个错误

谢谢，找到原因了，类名只能是MethodDemo