Java 继承 - 查找总数

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

我想找到付款总额，但是当我编译并运行代码时，出现此错误

Exception in thread "main" java.lang.NullPointerException.

支付类别

package tutorial3;
    public class Payment {
        private double amount;
        public Payment()
        {
         this(0.0);
        }
        public Payment(double amount)
        {
         setAmount(amount);
        }
        public void setAmount(double amount)
        {
            this.amount = amount;
        }

        public double getAmount()
        {
            return amount;
        }

        public String toString()
        {
            return "amount paid is " + getAmount();
        }
    }

<小时/>

主类:

 public class main {
        public static void main(String[] args){   
            Payment [] p = new Payment[2];
            Scanner sales = new Scanner (System.in);
            double total = 0;
            for(int i=0; i<3; i++)
            {
             System.out.print("Sales amount? ");
             double amt = sales.nextInt();
             Payment cash = new Payment(amt);
            }        
            for( Payment pv : p  ){
                total += pv.getAmount();
            }
        }      
    }

Answer 1

您忘记将创建的 Payment 实例分配给您的数组(此外，循环中的索引是错误的):

        for(int i=0; i<pv.length; i++)
        {
          System.out.print("Sales amount? ");
          double amt = sales.nextDouble(); // it makes more sense to use nextDouble if
                                           // you are storing the result in a double
          pv[i] = new Payment(amt);
        }

顺便说一句，我假设您的 Payment 类有一个采用 double 的构造函数，否则您的代码将无法通过编译。