将对象添加到数组列表。尝试将对象添加到ArrayList时出错

List

People

Main

List

ArrayList

People

Main

add

nullPointerException

public class Main {
    public static void main(String[] args) {

        List l = new List();       // making new List object 
        People p = new People();   // making new People object

        l.addPeople(p);           // calling from List object \"addPeople\" method and
    }            

                 // parsing People object \"p\"
 }




import java.util.ArrayList;

public class List {

        public List(){           //constructor
    }

        ArrayList<People>list;      // new ArrayList to hold objects of type \"People\"

    public void addPeople(People people){   
        list.add(people);               // I get error here
    }
} 

public class People {

    public People(){         // constructor
    }
}