Java:NullPointerException,似乎无法找出我为什么会这样做

Question

所以,我会说我是编程的新手,我的大部分背景来自C++,我在处理指针和引用等等.我已经开始学习Java了,我不得不编写这个示例程序,但由于某种原因,我无法逃避程序中的NullPointerException.我试着寻找答案,我理解我见过的其他答案,但我不明白为什么我会有一个空指针.想到也许我编码完全错了,我用C++重新编写了程序,但它似乎没有任何问题,这让我的想法更加困难.我认为这可能是因为Java处理引用的方式,因为没有指针和地址的概念.

如果它会有所帮助,我也可以发布标头/类和堆栈转储.

package com.company;

import java.util.Random;

public class Main {

    public static void main(String[] args)
    {
        // Create some bank accounts.
        BankAccount[] accountsList = new BankAccount[5];
        // Create a list of random names.
        String[] namesList = {"Bob", "Alice", "John", "Matt", "Billy"};
        // Set all account names, and print initial balance.
        for (int x = 0; x < 5; x++)
        {
            accountsList[x].SetCustomerName(namesList[x]);
            System.out.println("Initial Balance // Account: " + accountsList[x].GetCustomerName());
            System.out.println("Balance: $" + accountsList[x].GetBalance());
            System.out.println();
        }
        // Conduct n rounds of simulated deposit/withdrawal transactions for each account.
        for (int rounds = 5; rounds < 0; rounds--)
        {
            for (BankAccount x : accountsList)
            {
                System.out.println("Transaction Inquiry // Account: " + x.GetCustomerName());
                x.DepositFunds(Math.abs(new Random().nextInt()));
                x.WithdrawFunds(Math.abs(new Random().nextInt()));
                System.out.println();
            }
        }
    }
}

用C++重写相同的程序(编译好并运行没问题)

#include "BankAccount.h"
#include <iostream>
#include <cstdlib>
#include <ctime>

int main(int nArgs, char* pszArgs[])
{
    // Seed the RNG.
    srand(time(NULL));
    // Create some bank accounts.
    BankAccount accountsList[5];
    // Create a list of random names.
    std::string namesList[] = { "Bob", "Alice", "John", "Matt", "Billy" };
    // Set all account names, and print initial balance.
    for (int x = 0; x < 5; x++)
    {
        accountsList[x].SetCustomerName(namesList[x]);
        std::cout << "Initial Balance // Account: " << accountsList[x].GetCustomerName() << "\n";
        std::cout << "Balance: $" << accountsList[x].GetBalance() << "\n\n";
    }
    // Conduct n rounds of simulated deposit/withdrawal transactions for each account.
    for (int rounds = 5; rounds > 0; rounds--)
    {
        for (BankAccount &x : accountsList)
        {
            std::cout << "Transaction Inquiry // Account: " << x.GetCustomerName() << "\n";
            x.DepositFunds(abs(rand()));
            x.WithdrawFunds(abs(rand()));
            std::cout << std::endl;
        }
    }
    return 0;
}

Answer 1

BankAccount[] accountsList = new BankAccount[5];

这不会创建填充数组的对象.

您必须遍历数组并创建对象以填充它.

for(int i = 0; i< accountList.length; i++) {
    accoutnList[i] = new BankAccount();
}

在其他编程语言中,数组的构造也意味着这个数组被填充,在java中你创建了一个数组,但这并不意味着数组是用适当的对象初始化的.实际上它持有空引用,因此观察到NPE.