package contact;

import java.util.Scanner;
public class HelloWorld
{
   
    public static void main(String[] args)
    {
        Scanner put =new Scanner(System.in);
        Person P1=new Person();
        
        P1.name="Tom";
        P1.age = 18;
        P1.sex = 1;        
        P1.study();
   
    }
     
}

public class Person ()
{
        String name;
        int age;
        int sex;
        
        public void study()
        {
            System.out.println("studying");
        }
        public void showAge()
        {
            System.out.println("age:"+age);
        }
        public int addAge(int i)
        {
            age+=i;
            return age;
        }
}

把class Person后面的括号去掉，
Person类的public去掉