Initializing an ArrayList within a constructor

Question

As part of an assignment I'm supposed to create a bunch of rectangles and put each of those objects within an ArrayList, which is to be initialized in a constructor. I think I've confused myself. Eclipse is telling me that I can't use .add(new_rec) with this object because it is of type (Double, Double). I assume because the ArrayList is simply <Double> this is causing some issue. Aside from that I think I'm misguided somewhere else, but I'm not sure where.

public class WhyWontThisWork {

        WhyWontThisWork(Double name, Double rec_name){
            Rectangle new_rec = new Rectangle(23.1,43.0);
            ArrayList<Double> name = new ArrayList<Double>();
            for(int i = 0; i < 10; i++){
                name.add(new_rec);
            }
        }
}

...and the Rectangle class looks like...

public class Rectangle {
    private double length;
    private double width;

    public Rectangle(double length, double width){
        this.length = length;
        this.width = width;
    }

the main reason is that a rectangle is not a double, additionally, you have to give different names for name, they cannot be the same — epoch
– epoch, Commented Jan 31, 2014 at 7:58
@epoch Oops, my mistake on the naming sloppiness. Thanks for pointing that out. — Matt
– Matt, Commented Jan 31, 2014 at 8:00

Elemental · Accepted Answer · 2014-01-31 08:05:56Z

2

This is really quite simple- the type you need for name should be:

ArrayList<Rectangle> name = new ArrayList<Rectangle>();

The generic type inside the <> represents what kinds of a thing you are adding to the ArrayList. If you need to store an array of double you can use ArrayList<Double> if they are strings then ArrayList<String>

In your case you need to store a list of Rectangles so ArrayList<Rectangle> is your solution.

edited Jan 31, 2014 at 8:05

answered Jan 31, 2014 at 7:58

Elemental

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5 Comments

Matt Over a year ago

So the <...> is to be filled with any object? I'm confused on why Rectangle works here.

Elemental Over a year ago

Updated with more explanation - hope this helps.

Matt Over a year ago

So if I created a class of, say, Kittens and I needed to fill an ArrayList with those objects would I declare ArrayList<Kittens>?

Elemental Over a year ago

Exactly - this is what Java generics are for - giving you a type-safe way to implement algorithms (such as linked lists/array lists) which are not specific to a particular data type.

Peter Keller Over a year ago

Better use interfaces and not the concrete implementation class for name, i.e. List<Rectangle> name = new ArrayList<Rectangle>();

Koitoer · Accepted Answer · 2014-01-31 07:59:23Z

2

ArrayList used need to be

ArrayList<Rectangle> name = new ArrayList<Rectangle>();

As you are using the arrayList to save Rectangles not Doubles.

edited Jan 31, 2014 at 7:59

answered Jan 31, 2014 at 7:57

Koitoer

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Comments

Toon Borgers · Accepted Answer · 2014-01-31 07:58:38Z

0

Your arraylist must be of type Rectangle to be able to do this.

ArrayList<Rectangle> name = new ArrayList<Rectangle>()

answered Jan 31, 2014 at 7:58

Toon Borgers

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Scary Wombat · Accepted Answer · 2014-01-31 07:59:35Z

0

Eclipse is right.

ArrayList<Rectangle> name = new ArrayList<Rectangle>();
name.add(new_rec);

answered Jan 31, 2014 at 7:59

Scary Wombat

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Comments

Kick · Accepted Answer · 2014-01-31 07:59:59Z

0

Also parameter and ArrayList name is same,change it as below :

   WhyWontThisWork(Double nameDouble, Double rec_name){
                Rectangle new_rec = new Rectangle(23.1,43.0);
                ArrayList<Rectangle> name = new ArrayList<Rectangle>();
                for(int i = 0; i < 10; i++){
                    name.add(new_rec);
                }
            }

answered Jan 31, 2014 at 7:59

Kick

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Comments

SebastianH · Accepted Answer · 2014-01-31 08:02:17Z

0

You are trying to add a Rectangle object to a ArrayList that holds Double objects. That is why you see the error. As pointed out in the other answers the likely best solution is to change the list to ArrayList<Rectangle>.

Maybe you should read a bit about generics in Java here or here.

answered Jan 31, 2014 at 8:02

SebastianH

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