list.clear() vs list = new ArrayList<Integer>();

Which one of the 2 options is better and faster to clear an ArrayList, and why?

list.clear() 

or

list = new ArrayList<Integer>();

It happens that I have to, at random times, clear all entries from my ArrayList and I have no way to know how many new entries there will be in the future, there might be 0 or a 1000. Which method is faster and better, and why?

It's hard to know without a benchmark, but if you have lots of items in your ArrayList and the average size is lower, it might be faster to make a new ArrayList.

http://www.docjar.com/html/api/java/util/ArrayList.java.html

public void clear() {
    modCount++;
     
    // Let gc do its work
    for (int i = 0; i < size; i++)
        elementData[i] = null;
     
    size = 0;
}

List.clear would remove the elements without reducing the capacity of the list.

groovy:000> mylist = [1,2,3,4,5,6,7,8,9,10,11,12]
===> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
groovy:000> mylist.elementData.length
===> 12
groovy:000> mylist.elementData
===> [Ljava.lang.Object;@19d6af
groovy:000> mylist.clear()
===> null
groovy:000> mylist.elementData.length
===> 12
groovy:000> mylist.elementData
===> [Ljava.lang.Object;@19d6af
groovy:000> mylist = new ArrayList();
===> []
groovy:000> mylist.elementData
===> [Ljava.lang.Object;@2bfdff
groovy:000> mylist.elementData.length
===> 10

Here mylist got cleared, the references to the elements held by it got nulled out, but it keeps the same backing array. Then mylist was reinitialized and got a new backing array, the old one got GCed. So one way holds onto memory, the other one throws out its memory and gets reallocated from scratch (with the default capacity). Which is better depends on whether you want to reduce garbage-collection churn or minimize the current amount of unused memory. Whether the list sticks around long enough to be moved out of Eden might be a factor in deciding which is faster (because that might make garbage-collecting it more expensive).

I think that the answer is that it depends on a whole range of factors such as:

• whether the list size can be predicted beforehand (i.e. can you set the capacity accurately),
• whether the list size is variable (i.e. each time it is filled),
• how long the lifetime of the list will be in both versions, and
• your heap / GC parameters and CPU.

These make it hard to predict which will be better. But my intuition is that the difference will not be that great.

Two bits of advice on optimization:

• Don't waste time trying to optimize this ... unless the application is objectively too slow AND measurement using a profiler tells you that this is a performance hotspot. (The chances are that one of those preconditions won't be true.)

• If you do decide to optimize this, do it scientifically. Try both (all) of the alternatives and decide which is best by measuring the performance in your actual application on a realistic problem / workload / input set. (An artificial benchmark is liable to give you answers that do not predict real-world behavior, because of factors like those I listed previously.)

The first one .clear(); will keep the same list just clear the list.

The second one new ArrayList<Integer>(); creates a new ArrayList in memory.

Suggestion: First one because that's what is is designed to do.

Tried the below program , With both the approach.

1. With clearing the arraylist obj in for loop

2. creating new New Arraylist in for loop.

   List al= new ArrayList(); for(int i=0;i<100;i++) { //List al= new ArrayList();

    		for(int j=0;j<10;j++)
    		{
    			al.add(Integer.parseInt("" +j+i));
    			//System.out.println("Obj val " +al.get(j));
    		}
    		//System.out.println("Hashcode : " + al.hashCode());
    		al.clear();
    		
    	}
    
   

and to my surprise. the memory allocation didnt change much.

With New Arraylist approach.

Before loop total free memory: 64,909 ::

After loop total free memory: 64,775 ::

with Clear approach,

Before loop total free memory: 64,909 :: After loop total free memory: 64,765 ::

So this says there is not much difference in using arraylist.clear from memory utilization perspective.

If there is a good chance that the list will contain as much elements as it contains when clearing it, and if you're not in need for free memory, clearing the list is a better option. But my guess is that it probably doesn't matter. Don't try to optimize until you have detected a performance problem, and identified where it comes from.

list.clear() is going to keep the same ArrayList but the same memory allocation. list = new ArrayList<int>(); is going to allocate new memory for your ArrayList.

The big difference is that ArrayLists will expand dynamically as you need more space. Therefore, if you call list.clear() you will still, potentially, have a large amount of memory allocated for an ArrayList that might not be needed.

That said list.clear() will be faster but if memory maters you might want to allocate a new ArrayList.

I would suggest using list.clear() rather than allocating a new object. When you call the "new" keyword, you are creating more space in memory. In reality, it doesn't matter much. I suppose that if you know how large the list will be, it might be a good idea to create a new space but then specify how large the array will be.

The truth is, it's not going to matter unless you're doing scientific programming. In that case, you need to go learn C++.