Exception in thread "main" java.lang.NullPointerException

at java.util.ArrayList.addAll(ArrayList.java:559)

at com.iflytek.epdcloud.recruit.utils.quartz.Acool.main(Acool.java:16)

 

import java.util.ArrayList;import java.util.List;public class Acool {    public static void main(String[] args) {        List 
     
       b = new ArrayList<>();        b.add("1");        b.add("1");        b.add("1");                List 
      
        a = new ArrayList<>();        a.addAll(b);        a.addAll(null);        a.toString();        System.out.println(a);    }}