Exception raised while adding an element into an ArrayList

Question

I have an array list I am trying to add values into it and I am running into exception. I have used this code many times much still can't figure out what is creating this error, below is the code for your reference.

The line where I am using add method I get into a null pointer exception, All the above values are getting printed in console)

sid = new ArrayList<String>();
Enumeration e = Global.qtutlist.keys();
int qj=0; 
//iterate through Hashtable keys Enumeration
while(e.hasMoreElements())
{
    System.out.println("sid is key and its value id" );
    System.out.println(Integer.parseInt(e.nextElement().toString()));
    try
    {
        sid.add(e.nextElement().toString());
        System.out.println("lenght is "+ sid.size());
    }

    catch(Exception ex)
    {
        System.out.println("caught exception is"+ex.getMessage());

    }
}

You need to tell us which exception happens in which line.

RoToRa
– RoToRa

2011-08-11 11:57:12 +00:00
Commented Aug 11, 2011 at 11:57 — RoToRa
– RoToRa, Commented Aug 11, 2011 at 11:57
@all above Max have said it is nullpointerexception

TeaCupApp
– TeaCupApp

2011-08-11 11:58:47 +00:00
Commented Aug 11, 2011 at 11:58 — TeaCupApp
– TeaCupApp, Commented Aug 11, 2011 at 11:58

Jigar Joshi · Accepted Answer · 2011-08-11 11:58:02Z

5

you are calling nextElement() twice in loop and checking once

make it as follows

while(e.hasMoreElements())
        {   String item = e.nextElement().toString()
            System.out.println("sid is key and its value id" );
            System.out.println(Integer.parseInt(item));
            try{
            sid.add(item);
            System.out.println("lenght is "+ sid.size());
            }catch(Exception ex){
                System.out.println("caught exception is"+ex.getMessage());
            }
        }

If it shows NumberFormatException then one of the string isn't parsable to int

answered Aug 11, 2011 at 11:58

Jigar Joshi

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Comments

Lukas Eder · Accepted Answer · 2011-08-11 11:58:33Z

2

You're calling

e.nextElement()

Twice. Store it in a variable and operate on that variable instead

while(e.hasMoreElements()) {
    Object o = e.nextElement();
    // ...
}

answered Aug 11, 2011 at 11:58

Lukas Eder

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3 Comments

Thorn G Over a year ago

In addition, isn't Enumeration generic? Parameterize it and then you won't have to store each value in a basic Object reference.

Lukas Eder Over a year ago

@Tom: True. I thought that's not the issue here, though

Thorn G Over a year ago

No, but the fact that ArrayList was properly parameterized while Enumeration was not seemed inconsistent.

Sean Patrick Floyd · Accepted Answer · 2011-08-11 11:58:19Z

1

You are using e.nextElement() twice. This can't work. Enumerations use the Iterator design pattern, which means that you may only access each element once, before the internal counter advances to the next object. Note that hasMoreElements() does not advance the cursor, only nextElement() does.

Store the result in a local variable and re-use that:

System.out.println("sid is key and its value id" );
String str = e.nextElement().toString();
System.out.println(Integer.parseInt(str));
try{
    sid.add(str);
    System.out.println("lenght is "+ sid.size());
}catch(Exception ex){
    System.out.println("caught exception is"+ex.getMessage());
}

answered Aug 11, 2011 at 11:58

Sean Patrick Floyd

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Comments

Baz1nga · Accepted Answer · 2011-08-11 11:58:04Z

0

e.nextElement() is null that is the reason and you are doing an toString() operation on null

answered Aug 11, 2011 at 11:58

Baz1nga

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Comments

user784540 · Accepted Answer · 2011-08-11 11:58:52Z

0

You invoke nextElement twice when you check item presence only once.

answered Aug 11, 2011 at 11:58

user784540

Comments

Andreas Dolk · Accepted Answer · 2011-08-11 12:00:00Z

0

You are checking e.hasMoreElements() once and call e.nextElement() twice in the loop. Every call to nextElement() increments the internal marker, so every time, the number of elements in your enumeration is odd, you get an NPE.

answered Aug 11, 2011 at 12:00

Andreas Dolk

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