How to use remove() method on an ArrayList

[Robin Sharma]

Hi!

I get the UnsupportedOperationException exception when i try to run this code:

 
public class Question21 {
 
private int countryId = Integer.MIN_VALUE;
 
public static void main(String[] args) {
Question21[] q1 = new Question21[2];
q1[0] = new Question21();
q1[0].setCountryId(12);
q1[1] = new Question21();
q1[1].setCountryId(15);
Question21[] q2 = new Question21[1];
q2[0] = new Question21();
q2[0].setCountryId(15);
Question21[] q3 = null;
 
List list = Arrays.asList(q1);
System.out.println("Size of the list: " + list.size());
boolean removed = list.remove(q2[0]);
if(removed) {
q3 = (Question21[]) list.toArray(new Question21[list.size()]);
}
 
 
}
 
public int getCountryId() {
return this.countryId;
}
public void setCountryId(int countryId) {
this.countryId = countryId;
}
 
    /** Class-specific implementation of equals(). */
    public boolean equals(Object o)
    {
        boolean result = false;
 
        // Quick check to enhance performance.
        if (this == (Question21) o)
        {
            result = true;
        }
        else if (this.getClass().getName().equals(o.getClass().getName()))
        {
            if (this.getCountryId() == ((Question21) o).getCountryId())
            {
                result = true;
            }
        }
 
        return result;
    }
 
    /** Class-specific implementation of hashCode(). */
    public int hashCode()
    {
        return String.valueOf(this.getCountryId()).hashCode();
    }
 
}

Please help.

DW
[ December 07, 2005: Message edited by: Jim Yingst ]

[Tony Morris]

There are a couple of nasties that you are observing here. First, arrays are fixed length, therefore, the List returned by Arrays.asList will throw an UnsupportedOperationException if a "resizing" operation is invoked (such as remove). That this behaviour is not clearly documented is a limitation of the API Specification.

Most importantly, that you can even invoke such an operation is a horrible flaw that is intrinsic to the List interface. In fact, I became so annoyed at having to put up with this and many other flaws of Java 2 Collections, that I set about writing them "more correctly" (within the confines of the broken language).

For your case, I'd use:
http://www.contractualj.com/api/net/tmorris/adt/sequence/ArraySequence.html

[Jesper de Jong]

When you call Arrays.asList(), it will return a List object which is really only a wrapper around the array. The List will be read-only. You can't insert or remove items into that list.

To make it work, create a new ArrayList object (which is a real, modifiable implementation of List) to which you pass the List returned by Arrays.asList():

List list = new ArrayList(Arrays.asList(q1));

[Jim Yingst]

[Jesper]: When you call Arrays.asList(), it will return a List object which is really only a wrapper around the array. The List will be read-only.

It's not actually read-only. As Tony notes, you can't use operations which change the length, such as add() or remove(). But you can use set().
[ December 07, 2005: Message edited by: Jim Yingst ]

[Robin Sharma]

Originally posted by Jesper de Jong:
When you call Arrays.asList(), it will return a List object which is really only a wrapper around the array. The List will be read-only. You can't insert or remove items into that list.

To make it work, create a new ArrayList object (which is a real, modifiable implementation of List) to which you pass the List returned by Arrays.asList():

List list = new ArrayList(Arrays.asList(q1));

Thanks Jesper. Your solution is working fine.