list clear vs list new ArrayList Integer

Question

Which one of the 2 options is better and faster to clear an ArrayList  and why   list clear      or  list   new ArrayList lt Integer gt       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

User · Answer

The first one  clear    will keep the same list just clear the list    The second one new ArrayList lt Integer gt     creates a new ArrayList in memory   Suggestion  First one because that s what is is designed to do

User · Answer

list clear   is going to keep the same ArrayList but the same memory allocation  list   new ArrayList lt int gt     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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt 100 i                            List al  new ArrayList                 for int j 0 j lt 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

User · Answer

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  lt  size  i            elementData i    null       size   0

User · Answer

List clear would remove the elements without reducing the capacity of the list   groovy 000 gt  mylist    1 2 3 4 5 6 7 8 9 10 11 12      gt   1  2  3  4  5  6  7  8  9  10  11  12  groovy 000 gt  mylist elementData length     gt  12 groovy 000 gt  mylist elementData     gt   Ljava lang Object  19d6af groovy 000 gt  mylist clear       gt  null groovy 000 gt  mylist elementData length     gt  12 groovy 000 gt  mylist elementData     gt   Ljava lang Object  19d6af groovy 000 gt  mylist   new ArrayList        gt     groovy 000 gt  mylist elementData     gt   Ljava lang Object  2bfdff groovy 000 gt  mylist elementData length     gt  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

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

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