我创建了一个带有两个子类的父类，这两个子类包含更改实例变量和.toString（）方法的方法。在另一个包含我的主要功能的类中，我试图创建一个对象数组，然后可以对其进行排序。问题是当我尝试使用设置器更改实例变量时。我收到“ java.lang.NullPointerException”错误，找不到在线原因。
我尝试过仔细检查父类和子类的拼写错误。我还尝试过将实例变量更改为常量所涉及的变量。我试图更改存储对象的阵列中的乳液。
System.out.println("Which customer is it?");
customerNumber = scan.nextInt(); //scan is a scanner that is setup
customer[index].setAcctNum(customerNumber); //This is where the error is. .setAcctNum() is a mutator that I made to change a variable called AccountNumber
arr[index] = customer.toString(); //This is supposed to be an array to store the .toString(); value which is the AccountNumber variable changed in the code above.```
What I want this code to do is take an input, change the instance variable, and then store it in an array. The program is looping so when I input the list of numbers on different runs 3 4 7 2 and then print out all the contents of the array it prints out 3 4 7 2 to be sorted later.