Re: ArrayList help

From:

Knute Johnson <nospam@rabbitbrush.frazmtn.com>

Newsgroups:

comp.lang.java.help

Date:

Fri, 23 Feb 2007 16:14:24 -0800

Message-ID:

<BrLDh.340509$qy.148517@newsfe16.lga>

TonyB wrote:

I'm trying out some examples of ArrayList, but keep getting
nullpointerexception errors.
Bascially I'm creating a new ArrayList object in a class constructor.
code fragment is
public class TestList
{
private ArrayList rlist ;

You create a reference to an ArrayList here. It's default value is null.

    // Constructor for objects of class RouteList
    public TestList() {
        ArrayList rlist = new ArrayList() ;

You create a reference and an ArrayList here but it's scope is only to
the next line.

    }
    public boolean addItem(int type){
        if (rlist.size() == 0 ) //(or if(rlist.isEmpty() == true)

This line is accessing the instance variable at the top and it is null.

......}
}
In one of the methods to the class I add an item to the list, if the item is
not already in the list. In the method I first test to see if the list is
empty and this test causes the exception. I tried rlist.isEmpty(). Also
tried rlist.size() both have the same error.
It appears that using either of the methods cause an exception. Is there any
safe way of checking if the ArrayList is empty ?
Tony

class someClass {
     ArrayList al;

     public someClass() {
         al = new ArrayList();

Now this ArrayList is visible in your whole class and in the package for
that matter.

--

Knute Johnson
email s/nospam/knute/