Question

I developed an app with a GUI, with buttons, relative actionListeners, and exceptions.
Today I had this problem. In an actionEvent relative to a button of my GUI, I inserted this code, with some JOptionPane.showInputDialog:

public void actionPerformed(ActionEvent ae){

 if(ae.getSource()==b1){
 try{//FIRST `JOptionPane.showInputDialog`
   int load = Integer.parseInt(JOptionPane.showInputDialog(null,"Insert current load value: "));
   auto.setCurrentLoad(load);
   //other `JOptionPane.showInputDialog`
 int choiceDep = Integer.parseInt(JOptionPane.showInputDialog(null, "Does the truck transport perishable goods? 1: YES 2: NO"));
 if(choiceDep==1) {
   //here we have to insert expiration date
   int day = Integer.parseInt(JOptionPane.showInputDialog(null,"Insert value"));
  int month = Integer.parseInt(JOptionPane.showInputDialog(null,"Insert value"));
  int year = Integer.parseInt(JOptionPane.showInputDialog(null,"Insert value"));
  auto.setPerishable(day,month,year);
 }
 else if(choiceDep==2) 
                 auto.setNotPerishable();

String choiceAv = JOptionPane.showInputDialog(null, "Available Truck? Yes or no?");
 if(choiceAv.equals("Yes")) auto.setAvailable();
 else auto.setNotAvailable();

}
        //the exception      
    catch (Exception e) { System.out.println("Exception!");}             
  }

Where setAvailable, setNotAvailable,setPerishable,setCurrentLoad are methods of the external class, with reference auto.

When I execute this code, it appears the GUI, then I click on button b1. It appears the first JOptionPane.showInputDialog, to insert a value stored in a int load.

I entered a value, but no other JOptionPane.showInputDialog appeared (but there are other input dialog) and I got the exception in the command-line.
I noticed that the value inserted in the JOptionPane.showInputDialog is never passed to the line auto.setCurrentLoad(load);.

Why does it happen? Never seen this error before. Why do I always get the exception immediately after the first JOptionPane.showInputDialog Maybe the JVM doesn’t accept many of this JOptionPane.showInputDialog in the same statement/method? Or maybe(as I think) is a programming error of mine?

Thanks for your help. Cheers.

EDIT: I forgot to insert the exception I got in the command-line:

java.lang.NullPointerException
        at AutoCom.actionPerformed(AutoCom.java:50)
        at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
        at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
        at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
        at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown So
ce)
        at java.awt.Component.processMouseEvent(Unknown Source)
        at javax.swing.JComponent.processMouseEvent(Unknown Source)
        at java.awt.Component.processEvent(Unknown Source)
        at java.awt.Container.processEvent(Unknown Source)
        at java.awt.Component.dispatchEventImpl(Unknown Source)
        at java.awt.Container.dispatchEventImpl(Unknown Source)
        at java.awt.Component.dispatchEvent(Unknown Source)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
        at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
        at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
        at java.awt.Container.dispatchEventImpl(Unknown Source)
        at java.awt.Window.dispatchEventImpl(Unknown Source)
        at java.awt.Component.dispatchEvent(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

Most likely, the auto object is not initialized before you pressed the button. I’m assuming auto is a member variable of the AutoCom class. In that case, you should probably change the auto definition to:

protected <TypeOfAutoHere> auto = new <TypeOfAutoHere>();