在“ JOptionPane.showInputDialog”中，如果用户按下转义或X按钮（Java Swing），则会显示错误

Question

i am new to java, i just want to show an error message if user hit escape key from the keyboard or click on X button of showInputDialog or press cancel the program closes normally,

like now if i close or cancels the inputDialog it gives following error

 Exception in thread "main" java.lang.NullPointerException at Main.main(Main.java:11) 

i also tried to throw exception JVM but it does not works as i expected, here is my code:

String userInput;
BankAccount myAccount = new BankAccount();

while (true){
   userInput = JOptionPane.showInputDialog("1. Withdraw\n2. Deposit\n3. View Balance\n4. Exit");
    switch (userInput){

        case "1":
            myAccount.withdraw(Integer.parseInt(JOptionPane.showInputDialog("Please Enter ID: ")),Double.parseDouble(JOptionPane.showInputDialog("Please Enter Amount to Withdraw: ")));
            break;
        case "2":
            myAccount.deposit(Integer.parseInt(JOptionPane.showInputDialog("Please Enter ID: ")),Double.parseDouble(JOptionPane.showInputDialog("Please enter Amount to Deposit: ")));
            break;
        case "3":
            myAccount.viewBalance(Integer.parseInt(JOptionPane.showInputDialog("Please Enter ID: ")));
            break;
        case "4":
            myAccount.exit();
            System.exit(0);
        default:
            JOptionPane.showMessageDialog(null,"Invalid Input\nPlease Try Again");
            break;
    }
}

i just want to show an error message if user click X or cancels the prompt, how can i catch this? so i'll implement my logic there

Answer 1

JOptionPane.showInputDialog returns null, instead of a string, if the user clicks the 'x' or the Cancel button. So instead of:

while (true){
  userInput = JOptionPane.showInputDialog("1. Withdraw\n2. Deposit\n3. View Balance\n4. Exit");
  switch (userInput){

    case "1": ...

You would want to do something like:

while (true){
  userInput = JOptionPane.showInputDialog("1. Withdraw\n2. Deposit\n3. View Balance\n4. Exit");
  if (userInput == null) {
    JOptionPane.showMessageDialog(null, "Invalid Input\nPlease Try Again", "Cannot Cancel", JOptionPane.ERROR_MESSAGE);
    continue;
  }
  switch (userInput){

    case "1": ...

This will catch the cancel/'x' case, and the continue will have it skip to the next iteration of the while loop rather than throwing an error when it tries to use a switch statement with null.