如何避免空指针异常

Question

当我尝试输入存款，取款，余额查询和帐户信息的金额时，当我尝试选择"我已经有一个帐户，但如果我选择创建一个新帐户，则运行顺利"时，我的程序输出null。

public class BankAtm {
public static SavingsAccount accnt;

public static void main(String[] args) {
JTextField name = new JTextField();
JTextField pin = new JTextField();
JTextField aage = new JTextField();
JTextField contactNo = new JTextField();

String aNo;
String aName;
String aPin;
int age;
String aContactNo;

String[] choices = {"Yes", "I already have an account", "Exit"};

int y = JOptionPane.showOptionDialog(null, "Would you like to open a new acount?" , 
"Bank ATM", 
JOptionPane.DEFAULT_OPTION, JOptionPane.INFORMATION_MESSAGE, null, choices, choices[0]);

switch (y) {
case 0:
int max = 500;
int min = 100;

Object[] a = {"Account Name: ", name, "Age: ", aage, "Account Pin Number: ", pin, "Contact Number (please input parent's or guardian's contact number if below 18): ", contactNo};

JOptionPane op= new JOptionPane(a, JOptionPane.QUESTION_MESSAGE, JOptionPane.OK_CANCEL_OPTION, null, null);
JDialog dialog= op.createDialog(null, "Create New Account");
dialog.setVisible(true);

aName = name.getText();             
age = Integer.parseInt(aage.getText());
aPin = pin.getText();
aContactNo=contactNo.getText();

int no = (int) (Math.random()*(max-min+1)+max);         
JOptionPane.showMessageDialog(null, "Your account number is "+no);
aNo = Integer.toString(no);

accnt = new SavingsAccount (aName, aNo, aPin);
accnt.accntName=aName;
accnt.accntNo= aNo;
accnt.accntAge= age;
accnt.accntPin= aPin;
accnt.accntContactNo=aContactNo;

case 1:
int trans;
String php;
double amt;

do {

String s = JOptionPane.showInputDialog("1. Deposit"
+ "n2. Withdraw"
+ "n3. Balance Inquiry"
+ "n4. Display Account Info"
+ "n5. Exit");
trans = Integer.parseInt(s);

switch (trans) {
case 1:
php = JOptionPane.showInputDialog("Enter amount to deposit: ");
amt = Double.parseDouble(php);
accnt.deposit(amt);

break;

case 2:
php = JOptionPane.showInputDialog("Enter amount to withdraw: ");
amt = Double.parseDouble(php);
accnt.withdraw(amt);

break;

case 3:
accnt.balInquiry();

break;

case 4:
accnt.displayAccntInfo();

break;

case 5:
JOptionPane.showMessageDialog(null, "Thank you for using our service!~");

break;

}



} while (trans!=5);

}

return;
}
}

程序在线程"main"中输出Exceptionjava.lang.NullPointerException:无法调用SavingsAccount.displayAccntInfo()"因为"BankAtm.accnt"为空

Answer 1

在switch的情况0中，您通过其构造函数创建了一个新的SavingsAccount，从而给了他一个值，但如果您输入switch的情况1，则帐户构造函数从未被调用，因为您只是声明它:

public static SavingsAccount accnt;

当您初始化一个变量而不给它赋值时，它会得到一个默认值，即null。您不能访问null对象的属性，因为它没有任何属性。在对象上使用任何方法之前，必须从某处调用构造函数。