Wrong initialization of ArrayList in a loop

Question

While coding in Android, I needed an ArrayList of Points which I called wormPt. I initialized it through a loop.

ArrayList<Point> wormPt = new ArrayList<Point>();
Point pt = new Point();
.
.
.
private void initializeWorm() {
    // TODO Auto-generated method stub
    pt.x = 220;
    pt.y = 300;
    for (int i = 0; i <= 5; i++) {
        wormPt.add(pt);
        Log.d("wormdebug", wormPt.toString());

        pt.x -= 5;
    }
    Log.d("wormdebug", wormPt.toString());
}

My last log.d should report points (220,300) (215,300) (210,300) (205,300) (200,300) (195,300)

Instead, all my points are (190, 300)

Here is my log data

11-21 23:48:11.549: D/wormdebug(3273): [Point(220, 300)]
11-21 23:48:11.600: D/wormdebug(3273): [Point(215, 300), Point(215, 300)]
11-21 23:48:11.600: D/wormdebug(3273): [Point(210, 300), Point(210, 300), Point(210, 300)]
11-21 23:48:11.600: D/wormdebug(3273): [Point(205, 300), Point(205, 300), Point(205, 300), Point(205, 300)]
11-21 23:48:11.600: D/wormdebug(3273): [Point(200, 300), Point(200, 300), Point(200, 300), Point(200, 300), Point(200, 300)]
11-21 23:48:11.600: D/wormdebug(3273): [Point(195, 300), Point(195, 300), Point(195, 300), Point(195, 300), Point(195, 300), Point(195, 300)]
11-21 23:48:11.630: D/wormdebug(3273): [Point(190, 300), Point(190, 300), Point(190, 300), Point(190, 300), Point(190, 300), Point(190, 300)]
11-21 23:48:14.669: W/KeyCharacterMap(3273): No keyboard for id 0
11-21 23:48:14.679: W/KeyCharacterMap(3273): Using default keymap: /system/usr/keychars/qwerty.kcm.bin

I tried Can't add element to ArrayList in for loop and others, but they don't seem to be having the same problem. Any help will be appreciated. Thanks in advance.

You seem to be incorrectly assuming that wormPt.add() is cloning pt. — Sam Dufel
– Sam Dufel, Commented Dec 1, 2012 at 7:57

Jon Skeet · Accepted Answer · 2012-12-01 08:04:48Z

5

The problem is that your ArrayList contains several references to the same object. All you're doing in the loop is adding the same reference and mutating the object.

If you change the loop to create a new Point on each iteration, it will work:

int x = 220;
for (int i = 0; i <= 5; i++) {
    wormPt.add(new Point(x, 300));
    x -= 5;
}

It's very important to understand the difference between variables, objects and references. pt is a variable. Its value is a reference to a Point object. Unless you ask for a new object, Java won't create one for you. For example:

Point a = new Point(10, 20);
Point b = a; // Copies the *reference*
a.x = 100;
System.out.println(b.x); // 100

Note that this isn't associating the a and b variables with each other - it's just giving them the same value (the same reference). So you could later change a to be a reference to a different Point, and that wouldn't change b:

Point a = new Point(10, 20);
Point b = a; // Copies the *reference*
a.x = 100;
a = new Point(0, 0); // This doesn't affect b, or the object its value refers to
System.out.println(b.x); // 100

In this case, it's a bit like giving 10 different people a piece of paper with your home address on. If one of those people visits the address and paints the front door green, then another of them visits the address, they'll see a green front door.

edited Dec 1, 2012 at 8:04

answered Dec 1, 2012 at 7:58

Jon Skeet

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1 Comment

Ryan Over a year ago

Thank you, worked like a charm. I'm use to coding in C where we explicitly have to declare references, so I was thinking of the same thing in this case. Anyways, thanks again.

bellum · Accepted Answer · 2012-12-01 07:58:39Z

1

You must create new Point every time when adding to ArrayList. Instead there is only one Point with changing coordinates.

answered Dec 1, 2012 at 7:58

bellum

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Comments

Bharat Sinha · Accepted Answer · 2012-12-01 08:04:34Z

0

It's pretty simple. You are adding the same object to the list...

you should created a new object in each iteration of the loop and then add it to the list.

Try something like:

wormPt.add(new Point(x, 300));

answered Dec 1, 2012 at 8:04

Bharat Sinha

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Comments

Pratik · Accepted Answer · 2012-12-01 08:27:55Z

0

The Statement

wormPt.add(pt);

causing addition of same object into ArrayList.

You need to add different object of Point to arraylist.

for that you can use
new Point(x, y)
or
you can create an object like
Point p = new Point(x,y)

and then add it to the arraylist.

answered Dec 1, 2012 at 8:27

Pratik

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