如何删除两个不同的ArrayLists中的两个相同的元素？

Question

Just started to learn Java and have a question on how I can remove the common elements from two different Arraylists?

My thought process went with:

 for(int i =0; i<playerOneInputArray.size(); i++) {
       if(playerOneInputArray.contains(playerTwoInputArray.get(i))){
           playerOneInputArray.remove(playerTwoInputArray.get(i));

       }
   }

however, when I check the out put i get:

Player One! Choose your word!
hello
Player Two! Choose your word!
hellow
[h, e, l, l, o]
[h, e, l, l, o, w]
[e, l]

which I expected to only get a [w] in my new playerOneInputArray

what's going on?? :C

Answer 1

The problem is that you are concurrently modifying a data structure. Specifically, you are editing an ArrayList that you are iterating over.

Consider the simplified problem of removing any even numbers from an arrayList. You may consider using this approach:

public void removeEvens(ArrayList<Integer> arr){
    for(int i = 0; i < arr.size(); i++){
        if(arr.get(i) % 2 == 0) arr.remove(i);
    }
}

But there is an issue with this approach. Consider the sample input:

1, 3, 4, 6, 7, 8

When we hit i = 2 , we correctly remove 4 . However, at the next iteration ( i = 3 ), the arrayList now looks like:

1, 3, 6, 7, 8

So we are checking 7 , not 6 . By removing an element and moving i forward, we effectively skipped checking an element.

Your code is doing a similar operation, and suffers from a similar problem. You can both simplify your code and correct the issue by using built-in functions, such as removeAll :

playerOneInputArray.removeAll(playerTwoInputArray);

This will remove the duplicates from playerOneInputArray , but if you want to do this to playerTwoInputArray you'll have to change it a little bit, as once you've removed the duplicates from the first arrayList you can't remember which duplicates you removed. If that is the case, consider:

ArrayList<Character> duplicates = new ArrayList<Character>(playerOneInputArray);
duplicates.retainAll(playerTwoInputArray);
playerOneInputArray.removeAll(duplicates);
playerTwoInputArray.removeAll(duplicates);

Answer 2

Your problem is that you modify your playerOneInputArray in loop. Your code doesnt work as you expect because when you do remove() , then playerOneInputArray.size() changes. So, i recommend to use removeAll(playerTwoInputArray) instead of remove in loop.

Answer 3

An observation about your code:

You will encounter index out of bounds exception when the first array is longer than the second.

Also you will not check all elements between the arrays when the two arrays have different sizes because you are only using the size of the first array for the loop. Doing that creates the potential for the first issue I mentioned as well.

You need to handle these cases in some way as opposed to skipping some data or having an error.