CODEWITHSUNDEEP
javaperformancefor-looparraylist
chamilad
chamilad

Reputation: 1659

ArrayList or several for loops?

I know this might be simple, I have a situation where I need to decide between using four for-loops to (two to count and remove null elements, two to add elements) merge two String arrays or use two for-loops with an ArrayList and convert the ArrayList to array using ArrayList.toArray().

Performance wise are there any differences between these two approaches?

EDIT

I had to drop the ArrayList with generics approach because of compatibility issues. But here is the earlier code.

List<String> newList = new ArrayList<String>();

    for (String element : array1)
    {
      if (element != null)
      {
        newList.add(element);
      }
    }

    for (String element : array2)
    {
      if (element != null)
      {
        newList.add(element);
      }
    }

    return newList.toArray(new String[]{});

I wrote a new code with one loop, but I think I might be mentally killing the next one reading this code. 

String[] newArr = new String[array1.length + array2.length];
int n = 0;

for (int i = 0; i < newArr.length; i++)
{
  if (i < array1.length && array1[i] != null)
  {
    newArr[n] = array1[i];
    n++;
  }

  if (i >= array1.length)
  {
    int a = 0;
    if (array1.length < array2.length)
    {
      a = (i - array1.length) + (array2.length - array1.length);
    }
    else
    {
      a = i - array1.length;
    }

    if (array2[a] != null)
    {
      newArr[n] = array2[a];
      n++;
    }
  }
}

return newArr;

And finally got to know that null element check won't be needed so went ahead with this simple code.

String[] newArr = new String[array1.length + array2.length];

    System.arraycopy(array1, 0, newArr, 0, array1.length);
    System.arraycopy(array2, 0, newArr, array1.length, array2.length);

    return newArr;

I guess from the discussion below the second method is the better performing one.

Upvotes: 0

Views: 142

Answers (3)

Ankit Rustagi
Ankit Rustagi

Reputation: 5637

Assuming 1 for loop has O(n) time complexity, 4 for loops and 2 for loops would have the same time complexity

4*O(n) = O(n)
2*O(n) = O(n)

But using arrays instead of ArrayLists would take less memory. So go with the first alternative.

Upvotes: 3

Dark Knight
Dark Knight

Reputation: 8347

as with second option 2 iterations over collection will be saved, this will definitely improve performance if count of objects in list is high.

Upvotes: 0

Scientist
Scientist

Reputation: 1464

Try to avoid looping as much as you can because every time u loop with n elements the time and space complexity increases by n.

Please paste your code.

Upvotes: 0

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