ArrayList string loop mess up strings

Question

I've got an ArrayList called PhotoArrayList. It contains strings like "picture1.jpg, picture2.png, picture3.gif etc." There is like 50 or 60 strings in this ArrayList. I need to add path of their folder in the beginning like "mnt/sdcard0/Pictures/picture1.jpg etc." So, I'm using following code

Integer PhotoFileAmount = PhotoArray.length; //PhotoArray and PhotoArrayList are same
for(int i=0; i < PhotoFileAmount; i++){
           String PhotoFileAndPath = (PhotoFolder + '/' + PhotoArrayList.get(i));
           PhotoArrayList.remove(PhotoArrayList.get(i));
           PhotoArrayList.add(PhotoFileAndPath);
       }

But I'm getting a strange result. The beginning of PhotoArrayList is unchanged while it's middle part is okay and last part gets the path twitce. Like "picture1.jpg, mnt/sdcard0/Pictures/picture2.png, mnt/sdcard0/Pictures/mnt/sdcard0/Pictures/picture3.gif

fabian · Accepted Answer · 2015-08-16 01:13:11Z

2

If you want to change elements of a ArrayList, use the set method to assign a new value to the element at a given position:

int PhotoFileAmount = PhotoArray.length; // use int here to avoid unnecessary boxing / unboxing

for(int i=0; i < PhotoFileAmount; i++){
    String PhotoFileAndPath = (PhotoFolder + '/' + PhotoArrayList.get(i));
    PhotoArrayList.set(i, PhotoFileAndPath);
}

If you use remove and add you not only change the indices of the elements in the list; even if you make it work it's very inefficient, since all remaining elements in the list have to be moved every time remove is called.

answered Aug 16, 2015 at 1:13

fabian

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Sound Conception · Accepted Answer · 2015-08-16 01:14:09Z

1

ArrayList.add(E Object) adds the Object at the at the end of the list.
You are currently removing an item from where ever it is in the list, then adding a new version at the end of the list. As i approaches PhotoFileAmount your .get(i) statement is going to start retrieving the revised objects that you have added at the end.

answered Aug 16, 2015 at 1:14

Sound Conception

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