Question

您好我有以下代码：

    java.util.Date date = new java.util.Date();

    ArrayList<Worker> workerList = null;

            //Create Worker
            System.out.println ("You have selected create worker. \n" + "please enter a name for the worker.\n");

            //Create new worker object
            scanner.nextLine();
            String tName = scanner.nextLine();

            Worker newWorker = new Worker (tName, date, 0); 

            workerList.add(newWorker);

            wCurrent = newWorker;

和

Worker (String s, Date d, int i){
    workerName = s;
    dateEmployed = d;
    jobsCompleted = i;
}

然而它总是在workerList.add崩溃，我的错误在哪里？ Netbeans说"Null Pointer Defference"我不知道这意味着什么......请帮忙

修改！谢谢它修好了！

另一个快速问题

我也有

   public String toString(){
       return "The worker " + workerName + " has completed "+ jobsCompleted + " jobs";
    }

但是每当我调用toString方法时它都会崩溃。这是在我创建对象之后。

Answer 1

 ArrayList<Worker> workerList = null;

您将其初始化为null。因此崩溃。你需要正确初始化

 ArrayList<Worker> workerList = new ArrayList<Worker>();

使用界面编程更好（而不是左侧的ArrayList，声明一个List）

 List<Worker> workerList = new ArrayList<Worker>();

Answer 2

您只需要初始化列表。您创建变量ArrayList<Worker> workerList并将其填入null。请在声明时List<Worker> workerList = new ArrayList<>();进行正确初始化，如果要在声明后稍后初始化它，请workerList = new ArrayList<>();，这通常很有用。您可以阅读What is a NullPointerException, and how do I fix it?了解更多信息。

为什么我不能将此对象添加到数组列表中？

2 个答案: