Arraylist not working properly

Question

I have this code in Android Studio:

           for (ArrayList<Bitmap> arr : imgs) {
                for (Bitmap img : arr) {
                    if (img != null) {
                        arrayList.add(img.getHeight());
                    }
                    else {
                        arrayList.add(null);
                    }
                }
                heights.add(arrayList);
                arrayList.clear();
            }

With imgs declared and filled earlier and heights declared as an

ArrayList<ArrayList<Integer>>

What I get is:

[[58]]
[[null], [null]]
[[75], [75], [75]]
[[null], [null], [null], [null]]
[[1200], [1200], [1200], [1200], [1200]]
[[960], [960], [960], [960], [960], [960]]
[[960], [960], [960], [960], [960], [960], [960]]
[[612], [612], [612], [612], [612], [612], [612], [612]]
[[1632], [1632], [1632], [1632], [1632], [1632], [1632], [1632]]

And so on. Every time it doesn't add the value to the array but it replaces all the values in it with the new one. What's wrong?

ArrayList is working fine. Start by assuming it's your code that's not working. — Andreas
– Andreas, Commented May 8, 2016 at 12:22

Eran · Accepted Answer · 2016-05-08 12:06:16Z

3

heights.add(arrayList);
arrayList.clear();

You always add the same arrayList instance to heights, and you clear it, so all the previous values are lost.

You should create a new ArrayList instance in each iteration of the outer loop.

       for (ArrayList<Bitmap> arr : imgs) {
            arrayList = new ArrayList<Integer> ();
            for (Bitmap img : arr) {
                if (img != null) {
                    arrayList.add(img.getHeight());
                }
                else {
                    arrayList.add(null);
                }
            }
            heights.add(arrayList);
       }

answered May 8, 2016 at 12:06

Eran

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2 Comments

Andrea BR Over a year ago

heights is the global Array<Array> and arrayList is the local Array. arrayList is updated, put into heights, and then cleared. Isn't it right?

Eran Over a year ago

@AndreaBR No, it's not. When you add arrayList to heights, you are not adding a copy of arrayList, you are just adding a reference to it. Therefore all the elements of heights refer to the same ArrayList.

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