listeners2

[rob armstrong]

Have been trying to code a program and i am having problems with the listeners. I have a combobox, a bunch of textfields and a 'ok button'. User picks the combo box, and writes in some text into the textfields and hits the 'ok button'. For now i want to just get all the info in varialbes so i can write more code later to write the results to disk. However i cant get the listeners to work right. here are some of the problem points..
blah blah
ButtonListener BL=new ButtonListener();

blah blah`

buttoncreate=new JButton("Create .trn file");
buttoncreate.addActionListener(BL);
mainPanel.add(buttoncreate);
this.add(mainPanel);
buttonquit=new JButton("Quit");
mainPanel.add(buttonquit);

blah blah

public class ButtonListener implements ActionListener{

public void actionPerformed (ActionEvent e){


if(e.getSource()==buttoncreate){
String start=(String)carType.getSelectedItem();
}
else{

buttoncreate.setText("bad");
}

it will compile but when i enter a value or no value in textfields i just get an error on the Sring start line


thanks roba

[Michael Dunn]

> i just get an error on the Sring start line

if it's getting to that line, the listener is working.
what is the error message?

[rob armstrong]

if either i enter info in the box or just leave it emty (JTextField) the output pane would spit out info and it would not work. It complained about that line.
roba

[rob armstrong]

sorry here is the error
Exception in thread"AWT-EventQueue-0"java.lang.NullPointerException at robbox1$ButtonListener.actionPerformed(robbox.java:118


String start=(String)carType.getSelectedItem(); //line 118

sorry this time i am tring to get info from my combo box. But I have not got any listener to work yet. Thanks for your time.
roba

[Rachel Swailes]

What it could be doing is trying to get the selected item when there is no item selected. Maybe try to getSelectedIndex and check that the value is >=0 before you do the getSelectedItem line.

Try that and see.

Cheers,
Rachel

[Ulf Dittmer]

The NullPointerException would indicate that carType is null, not that there is trouble in getSelectedItem(). Is it null?

[rob armstrong]

I tried to make declare the variable right off the bat now to see what happens.
private JComboBox carType="Cabboose";

but it says 'imcompatible type' so i can not do it that way.
Do you want to have a quick look at my code? It is only about 100 lines and the gui parts work fine, just the listener problem. Thanks
roba
JComboBox carType= new JComboBox();
carType.addItem("Caboose");

blah blah

public class ButtonListener implements ActionListener{

public void actionPerformed (ActionEvent e){


if(e.getSource()==buttoncreate){

String start=(String)carType.getSelectedItem();

[Michael Dunn]

if only 100 or so lines, post the code

[Michael Dunn]

would you possibly have your code structured like this:

  
class SomeName
{
  JComboBox carType; //<--------------
  public SomeName()
  {
    ...
    JComboBox carType= new JComboBox(); //<--------------
    carType.addItem("Caboose");
    ...
  }
}

if you do, including the 'type' (JComboBox) in the constructor
makes that 'carType' local to the constructor. The other 'carType'
is then null when accessed from elswhere in your code

[rob armstrong]

Here is the whole thing...



import javax.swing.*;
import java.awt.event.*;
import java.util.Scanner.*;

public class robbox1 extends JFrame{

public static void main(String[] args){
new robbox1();
}
private JButton buttoncreate, buttonquit;
//public JComboBox carType;
private JLabel instuctions, carNumStart, carNumEnd, emailtext, twentyline, yardNumlab, tracklabel;
private JTextField carID, carIDend, origion, emailbox, twentylinebox, yardNumfield, trackfield;
public String start, end, email, origional, yardNum, trackNum, carTypeSelect;


public robbox1(){
this.setLocation(120,120);
this.setSize (450,420);
this.setTitle("Slim Car Builder 2.0");
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);


ButtonListener bl=new ButtonListener(); // Is this right???
JPanel mainPanel=new JPanel();

JLabel instructions=new JLabel("Please select the Car Type to construct:");
mainPanel.add(instructions);

JComboBox carType= new JComboBox();
carType.addItem("Caboose"); //3
carType.addItem("Passenger Car"); //4
carType.addItem("Well Car"); //5
carType.addItem("Spine Car"); //6
carType.addItem("AutoRack"); //7
carType.addItem("Woodchip Car"); //8
carType.addItem("Box Car"); //9
carType.addItem("Refer"); //10
carType.addItem("Coal Car"); //11
carType.addItem("Tank Car"); //12
carType.addItem("Grain Hopper"); //13
carType.addItem("Open Top Hopper"); //14
carType.addItem("Gondola"); //15
carType.addItem("Ore Jennie"); //16
carType.addItem("Flat Car"); //17
carType.addItem("Centerbeam Car"); //18
carType.addItem("Intermodal Flat"); //19
carType.addItem("Stock Car"); //20
carType.addItem("Steel Coil Car"); //21
carType.addItem("Bulkhead Flat"); //22
carType.addItem("Heavy Flat Car"); //23
carType.addItem("Covered Hopper"); //24


mainPanel.add(carType);

JLabel carNumStart=new JLabel("Enter Starting Car Number in the box:");
mainPanel.add(carNumStart);

JTextField carID=new JTextField(12);
mainPanel.add(carID);

JLabel carNumEnd=new JLabel("Enter Ending Car Number (200 car Max):");
mainPanel.add(carNumEnd);
JTextField carIDend=new JTextField("GATX 123457", 12);
mainPanel.add(carIDend);

JLabel origionator=new JLabel(" Enter Origionator");
mainPanel.add(origionator);
JTextField origion=new JTextField("Slim", 10);
mainPanel.add(origion);

JLabel emailtext=new JLabel("Enter email address or instuctions:");
mainPanel.add(emailtext);
JTextField emailbox=new JTextField("[e-mail]", 25);
mainPanel.add(emailbox);

JLabel twentyline=new JLabel("Enter extra instuctions shown with car on covering with mouse");
mainPanel.add(twentyline);
JTextField twentylinebox=new JTextField("bill to Zuni, NM when empty", 25);
mainPanel.add(twentylinebox);

JLabel yardNumlab=new JLabel(" Enter yard number for cut of cars to arrive at:");
mainPanel.add(yardNumlab);
JTextField yardNumfield=new JTextField("1100 ", 5);
mainPanel.add(yardNumfield);

JLabel tracklabel=new JLabel(" Enter yard exit number for cut to arrive on: (track #):");
mainPanel.add(tracklabel);
JTextField trackfield=new JTextField("1", 2);
mainPanel.add(trackfield);


buttoncreate=new JButton("Create .trn file");
buttoncreate.addActionListener(bl);
mainPanel.add(buttoncreate);
this.add(mainPanel);
buttonquit=new JButton("Quit");
mainPanel.add(buttonquit);


this.add(mainPanel);
this.setVisible(true);
}

public class ButtonListener implements ActionListener{

public void actionPerformed (ActionEvent e){


if(e.getSource()==buttoncreate){
String start=carID.getText(); //This is the line it complains about
if(name.length()==0);
buttoncreate.setText("no entry");
}


}
/else{


buttoncreate.setText("bad");
}

// basically i want to put all of the JTextFields and JComboBox into
// separate String variables that I can use to make another piece of the program (diskwriting)
// I think Micheal Dunn is right but I am unsure how to change it to make it work!
// Thanks for you time!!
// roba

[Michael Dunn]

> String start=carID.getText(); //This is the line it complains about

class members:
JTextField carID, carIDend,etc

in your constructor:
JTextField carID=new JTextField(12);

reasons given in earlier post.

to fix, change
JTextField carID=new JTextField(12);
to
carID=new JTextField(12);

[rob armstrong]

Thanks that is what my problem was. Thanks.
roba