Question

我正在进行一项练习，我建立了一个银行系统，我创建了一个帐户类，一个客户类和一个银行类，但是当我检查程序时，我得到了：

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

以下是我的课程：

package bank.elements;

public class Account {

    private int id;
    private float balance;

    //constructors
    public Account(int id, float balance){
        this.setId(id);
        this.setBalance(balance);
    }

    public Account (int id){
        this.id=id;
        this.setBalance(0);
    }


    //getters
    public int getId(){
        return this.id;

    }

    public float getBalance(){
        return this.balance;
    }


    //setters
    public void setBalance(float balance){
        this.balance+=balance;
    }

    public void setId(int id){
        this.id=id;
    }

}



package bank.elements;

public class Client {

    private int id;
    private String name;
    private String rank;
    private float balance;
    private Account[] accounts = new Account[100];

    public Client(){

    }

    //getters
    public int getId(){
        return id;
    }

    public String getName(){
        return name;
    }

    public String getRank(){
        return rank;
    }

    public float getBalance(){
        return balance;
    }

    public float getAccountBalance(int id){

        float balance = 0;
        for (int i = 0; i < accounts.length; i++) {
            if(accounts[i].getId()==id){
                balance = accounts[i].getBalance();
            }
        }
        return balance;
    }

    public float getFortune(){
        float sum = 0;
        for (int i = 0; i < accounts.length; i++) {
            sum+=getAccountBalance(i);
        }
        sum+=getBalance();
        return sum;
    }

    //setters
    public void setName(String name){
        this.name = name;
    }

    public void setBalance(float balance){
        this.balance+=balance;
    }

    public void setId(int id){
        this.id=id;
    }

    public void addAccount(int id){
        for (int i = 0; i < accounts.length; i++) {
            if(accounts[i]==null){
                accounts[i].setId(id);
            }
        }
    }

}



package bank.elements;

public class Bank {

    public Bank(){

    }

    public Client[] clients = new Client[100];

    public float getBalance(){
        float sum = 0;

        for (int i = 0; i < clients.length; i++) {
            sum+= clients[i].getFortune();
        }
        return sum;
    }

    public void addClient(){
        for (int i = 0; i < clients.length; i++) {
            if(clients[i] == null){
                clients[i].setId(i);
            }
        }

    }

    public void removeClient(int id){
        for (int i = 0; i < clients.length; i++) {
            if(clients[i].getId()==id){
                clients[i]=null;
                for (int j = (i+1); j < clients.length; j++) {
                    clients[j] = clients[j-1];
                }

            }
        }
    }
}

这是程序主题：

package bank.program;

import bank.elements.Account;
import bank.elements.Client;
import bank.elements.Bank;

public class Program {
    public static void main(String[] args) {
        Bank b = new Bank();
        b.addClient();
        b.clients[0].setBalance(100);
        b.clients[0].addAccount(2);
        System.out.println(b.clients[0].getFortune());
    }
}

我错过了什么？

Answer 1

在addClient()方法中：

for (int i = 0; i < clients.length; i++) {
      if(clients[i] == null){
           clients[i].setId(i);
      }
 }

因为你检查数组的第i个位置的Client对象是否为null然后你试图从这个对象访问一个方法，从而检查NPE，所以抛出NPE是完全正常的。

执行public Client[] clients = new Client[100];时，数组的100个插槽初始化为null（Java中对象的默认值初始化）。

我会在你的构造函数中初始化它：

public Bank(){
   for(int i = 0; i < clients.length; i++){
      clients[i] = new Client();
   }
}

在addClient()方法中更改您的if（执行if client[i] != null以阻止NPE），即：

for (int i = 0; i < clients.length; i++) {
      if (client[i] != null){
          clients[i].setId(i);
     }
 }

Client类的Account数组的相同评论。

另请考虑阅读：java-What is a Null Pointer Exception?

如何避免null属性？

1 个答案: