Question

I’m trying to remove a 2d arraylist from another 3d arraylist.
I mean, delete all the ar1[][] from ar2[][][]
I can’t understand why my code is not deleting all of them:

    public void removeSame(ArrayList<ArrayList<String>> ar1,ArrayList<ArrayList<ArrayList<String>>> ar3)
    {
        for(int k=0;k<ar3.size();k++)
            for(int j=0;j<ar3.size();j++)
                if(isSame(ar1,ar3.get(k)))
                {
                    ar3.remove(k);
                }
    }
    public boolean isSame(ArrayList<ArrayList<String>> ar1,ArrayList<ArrayList<String>> ar2)
    {
        for(int k=0;k<ar2.size();k++)
            for(int j=0;j<ar2.size();j++)
                if(!ar1.get(k).get(j).equals(ar2.get(k).get(j)))
                    return false;
        return true;
    }

Answer 1

I can see one issue when there are more than one occurrences of ar1 in ar3. After first removal of ar1 from ar3 using ar3.remove(k);, the index and size of ar3 ArrayList is changed ie. both size and indexes are reduced by one. Now when you continue your loop, it will skip one element in between as you continue using the original index. Also your inner for loop seems to be unnecessary.

There are two ways to resolve the issue.

Option1: User List#iterator i.e.

Iterator<ArrayList<ArrayList<String>>> iter = ar3.iterator();
while(iter.hasNext()){
    if(isSame(ar1, iter.next())){
        iter.remove();
    }
}

Option2: After removal, reduce the index i.e. k value by one before continuing e.g.

    for(int k=0;k<ar3.size();k++){
       if(isSame(ar1,ar3.get(k))){
                ar3.remove(k--);
       }
    }