Question

嘿伙计们我是java的初学者（整理），我遇到了麻烦。我试图通过Finances类中的变量来获取我在Budget类中设置的日期，但我不断获得NullPointerException。

我尝试在财务类中使用dueDate变量，但即使在我实例化之后，logCat仍然会声明它a null object reference。

public class Budget {
    private Date mDate = new Date();
    public static double budgetAmount;

    private static Budget instance = new Budget();
    private Budget(){

    }

    public double getBudget(){
        return budgetAmount;
    }

    public void setBudget(double budget){
        budgetAmount = budget;
    }

    public static Budget getInstance(){
        return instance;
    }


    public Date getDate() {
        return mDate;
    }

    public void setDate(Date date) {
        mDate = date;
    }

}

这是财务类

public class Finances extends Activity{
    int amtFood; int amtCar; int amtGas; int amtTvBill; int amtInBill;
    int amtTronics; int amtWear; int amtLight; int amtWater; int amtOthers;

    Date nowDate;
    Date dueDate = new Date();

    double MainAmount = Budget.budgetAmount;
    static int SpentAmount;

    Budget budgetDate;

    public Finances(){
        HashMap<String,Integer> amtAdd = ExpenditureLab.get(this).getExpendMap();
        int food= amtAdd.get("Groceries/Food");
        int car = amtAdd.get("Car servicing");
        int gas = amtAdd.get("Gas Money");
        int tvBill = amtAdd.get("Television Bill");
        int inBill = amtAdd.get("Internet Bill");
        int tronics = amtAdd.get("Electronics");
        int wear = amtAdd.get("Clothes/Wearables");
        int light = amtAdd.get("Light Bill");
        int water = amtAdd.get("Water Bill");
        int others = amtAdd.get("Others");



        dueDate = budgetDate.getDate();
    }

    public void addSpent(int amt){
        SpentAmount += amt;
    }

}

Answer 1

budgetDate永远不会被启动。

Budget budgetDate; #=> this is null

然后尝试在null对象上调用getDate（）。

dueDate = budgetDate.getDate(); #=> causing your nullPointerException?

因此，您需要首先实例化您的budgetDate对象。

Budget budgetDate = new Budget();

然后设置mDate对象

budgetDate.setDate(someDate); #=> someDate is a Date object

之前打电话

dueDate = budgetDate.getDate();

Answer 2

是budgetDate未初始化，导致空指针异常。另外，不要在Activity中使用构造函数，您可以在Activity的onCreate方法中进行所有初始化。

Answer 3

dueDate不是空对象，您将其设置为budgetDate.getDate()，而budgetDate尚未实例化。

Answer 4

我没有看到您为budgetDate分配值的地方（永远不会被启动）。调用dueDate = budgetDate.getDate();

时，它是空指针异常

解决方案：将budgetDate = new Budget()放在dueDate = budgetDate.getDate();之前：

@Override
public void onCreate(Bundle bundle) {
    HashMap<String,Integer> amtAdd = ExpenditureLab.get(this).getExpendMap();
    int food= amtAdd.get("Groceries/Food");
    int car = amtAdd.get("Car servicing");
    int gas = amtAdd.get("Gas Money");
    int tvBill = amtAdd.get("Television Bill");
    int inBill = amtAdd.get("Internet Bill");
    int tronics = amtAdd.get("Electronics");
    int wear = amtAdd.get("Clothes/Wearables");
    int light = amtAdd.get("Light Bill");
    int water = amtAdd.get("Water Bill");
    int others = amtAdd.get("Others");

    budgetDate = new Budget();
    dueDate = budgetDate.getDate();
}

NullPointerException问题请协助

4 个答案: