How do I make take an object out of a vector as it's original class?

Question

In my program I have a class called Circles and a class called CircleList which is a vector of Circles.

I can add and take away Circles to the list fine but when I try to use a circle from the list like so.

Circle test = CircleList.elementAt(0);

It fails as a Circle is expected but an object is found. Is there anyway I can fix this so it comes out of the vector back as a Circle rather than as an Object?

here is the start of the CircleList class with the constructor

import java.util.Vector;
public class CircleList
{
    private Vector CircleList;

    public CircleList()
    {
    CircleList = new Vector();
    }

Is `CircleList` a class that you wrote (as you claim), or an object of type `Vector`? — nhgrif, Dec 08 '13 at 16:59
Show us the code for CircleList, especially it's definition. Does it extend List? Vector? What generic does it declare? — vptheron, Dec 08 '13 at 17:00
It seems you've learnt Java with a book or teacher which hasn't changed/who hasn't learnt anything since Java 1.1. We're at Java 7. Don't use Vector anymore. And use generic types. — JB Nizet, Dec 08 '13 at 17:00
I think CircleList is a variable. He is just not following the naming conventions ;) — Blub, Dec 08 '13 at 17:00
@EdgarBoda I agree that it's probably a variable, but he claimed it to be a class. If it's a class, then he's probably simply got a problem with his `elementAt` method. — nhgrif, Dec 08 '13 at 17:01
OP said it's a vector of circles. The question is whether they declared it as `Vector` or as `Vector`. Almost certainly, the problem is that they wrote `Vector`, and they could fix the code by writing `Vector` instead. — Dawood ibn Kareem, Dec 08 '13 at 17:04

score 4 · Accepted Answer · edited May 23 '17 at 12:13

Try it like this

public class CircleList {
    private Vector<Circle> circleList;

    public CircleList() {
        circleList = new Vector<Circle>();
    }

If you do this, the compiler knows that you'll only use circleList for storing Circle objects, not some other kind of object, and will therefore allow you to refer to anything that comes out of circleList with a Circle variable.

You might also consider using ArrayList in place of Vector. The Vector class does some additional synchronisation which you almost certainly don't need. For more details on this particular type of synchronisation and why you don't want it, I would suggest reading Jon Skeet's answer to Why is Java Vector class considered obsolete or deprecated?

Thankyou, this was really helpful. I'd also like to mention I would be using arraylists where possible but the exercise on my course asked to use a vector which is why I did. Thanks again. — Sam, Dec 08 '13 at 17:11

score 0 · Answer 2 · answered Dec 08 '13 at 17:09

Well the problem is occuring when you are defining vector.

Please share the insertion code.

However first try below code snippet for inserting elements into vector.

 Vector<Circle> CircleList = new Vector<Circle>(50); 
 CircleList.add(circle1);
 CircleList.add(circle2);
 ........................

Then retrieve the element.

Circle test = CircleList.elementAt(0);

How do I make take an object out of a vector as it's original class?

2 Answers2