我正在练习建立银行系统，创建帐户类，客户类和银行类的练习，但是当我检查程序时会得到：
Exception in thread "main" java.lang.NullPointerException.
这是我的课程：
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];
}
}
}
}
}
这是程序线程：
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());
}
}
我想念什么？
用你的addClient()方法：
for (int i = 0; i < clients.length; i++) {
if(clients[i] == null){
clients[i].setId(i);
}
}
因此，抛出NPE是完全正常的，因为您检查了位于阵列第i个位置的Client对象是否存在null，然后您试图从该对象访问方法，因此也要访问NPE。
执行此操作时public Client[] clients = new Client[100];，将数组的100个插槽初始化为null（Java中Object的默认值初始化）。
我会在您的构造函数中对其进行初始化：
public Bank(){
for(int i = 0; i < clients.length; i++){
clients[i] = new Client();
}
}
更改addClient()方法中的if （if client[i] != null以防止发生NPE），即：
for (int i = 0; i < clients.length; i++) {
if (client[i] != null){
clients[i].setId(i);
}
}
对您的Client班级使用相同的注释，数组为Account。
还可以考虑阅读以下内容：java-什么是空指针异常？
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