reading ArrayList JTextFields

Question

My program starts by prompting the user how many text fields they would like to have

public class textEvent1 implements ActionListener {    //action listener for "how many masses?"
            public void actionPerformed (ActionEvent e) {
                n = (int)(Double.parseDouble(massNumField.getText()));

next I create a for loop to create labels and textfields (which I have created lists for because I dont know how many there will be). There are a couple of lists but I will give an example of just one.

ArrayList masses = new ArrayList();

    for(int i=1; i<=n; i++) {                                  //adds text event 2 text to the screen
                        massLabel = new JLabel("How much mass does Mass " +i+ " have? ");
                        massField = new JTextField(5);
                        masses.add(massField);

Now my problem appears to come when I try to assign an element of the masses list to a variable like so.

for(int i=1; i<=n; i++) {
                        mass =  Double.parseDouble(((JTextComponent) masses.get(i)).getText());

I have tried a couple of things...mass = masses.get(i).....mass = masses.get(i).getText()) and so on and so on. I either keep getting errors such as Null pointer exceptions or things saying I cant parseDouble an Object.

There errors that arrise for this example are as below

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 3 >= 3
    at java.util.Vector.elementAt(Unknown Source)
    at acmx.export.java.util.ArrayList.get(ArrayList.java:54)
    at Orbit$textEvent2.actionPerformed(Orbit.java:151)

line 151 is

mass =  Double.parseDouble(((JTextComponent) masses.get(i)).getText());

Could you edit your question to include how you're creating and setting up the masses variable? I suspect that may be the issue. (Hit the edit button in the lower-left of your question) — Michael0x2a
– Michael0x2a, Commented Sep 17, 2013 at 16:21
"Any suggestions would be very useful." I suggest you ask a question.. Also: 1) For better help sooner, post an SSCCE. 2) Always copy/paste error & exception output. — Andrew Thompson
– Andrew Thompson, Commented Sep 17, 2013 at 17:02

kiheru · Accepted Answer · 2013-09-17 19:15:34Z

2

When creating the JTextFields, you do:

for(int i=1; i<=n; i++) {
    ...

Note that List indices start at 0, so when you retrieve the items with a similar loop, using i as the index, you are trying to access one past the last item. Change the reading loop indices to:

for (int i = 0; i < n; i++) {
    ...

Or you could use an enhance for loop, unless you need to use an ancient java version:

for (Object massField : masses) {
    mass =  Double.parseDouble(((JTextComponent) massField).getText());
    ...

(Then you should really use generics too, if the java version supports them).

edited Sep 17, 2013 at 19:15

answered Sep 17, 2013 at 19:07

kiheru

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3 Comments

kiheru Over a year ago

Btw, is there a particular reason to not use ArrayList<JTextField>? That would save the cast and be good for type safety.

David Jarrin Over a year ago

ya I tried that and for some strange reason it didn't accept it. it came up with a warning saying "The type ArrayList is not generic; it cannot be parameterized with arguments <JTextField>"

kiheru Over a year ago

@DavidJarrin looks like you're importing the wrong ArrayList. It should be java.util.ArrayList

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