通过 JFrame 添加到 ArrayList 时出现 NullPointerException 错误

Question

我的问题正如标题所说的那样。

我有一个 Tracker.class、一个 TrackerGUI.class 和一个 TrackerApp.class（JFrame 的驱动程序）。 Tracker 类在 TrackerGUI.class 中初始化为private Tracker tracker

代码本身编译正确（所以我猜这意味着没有语法错误，但是当我尝试添加一个对象时，终端弹出（我正在使用 BlueJ），并给我这个错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at TrackerGUI$Food.actionPerformed(TrackerGUI.java:121)

点击会引导我：

public class TrackerGUI extends JFrame {
    private Tracker tracker;
    double calories;
    double quantity;
    // more initialising of GUI elements; so all the JTextField things are initialised
    // setting up the GUI code, too

    public class Food implements ActionListener {
        public void actionPerformed (ActionEvent ae) {
            double calories = Double.parseDouble(caloriesField.getText().replaceAll(",",""));
            String newFood = activityField.getText();
            if(calories < 0) {
                textArea.append("Calories cannot be negative. \n\n"); 
            }
            else {
                tracker.addFood(newFood, calories); // this line!
                textArea.append(newFood + " with " + calories + " calories added to database! \n");
            }
        }
    }

在tracker类中，addFood的代码是这样的：

public class Tracker {
    ArrayList<Food> foodBase = new ArrayList<Food>();
    String foodName;
    Double calories;

public void addFood(String foodName, double calories)
{
    Food newFood = new Food(foodName, calories);
    foodBase.add(newFood);
}

我的代码中缺少什么？注意：我必须使用ArrayList，而我不能使用List 或Map 我觉得我错过了一个for 声明，它得到foodBase.size()，但我不确定它适合哪里？

Answer 1

ArrayList<Food> foodBase = new ArrayList<Food>();

您在构造函数中声明和初始化一个 local 变量，该变量恰好与 addFood() 方法中使用的字段同名。应该是

this.foodBase = new ArrayList<Food>();

编辑：再次阅读异常，您似乎也忘记在包含 Food 内部类 (TrackerGUI) 的类中初始化 tracker

Answer 2

改变这个：

    else {
        tracker.addFood(newFood, calories); // this line!
        textArea.append(newFood + " with " + calories + " calories added to database! \n");
    }

到：

    else if (tracker != null){
        tracker.addFood(newFood, calories); // this line!
        textArea.append(newFood + " with " + calories + " calories added to database! \n");
    }

确保在 tracker 为 null 时不执行此代码并确保在此之前正确初始化了跟踪器：

 private Tracker tracker =  new Tracker(); // initialize your tracker!!