Java ArrayList clear function

Question

Let s say I have a ArrayList lt String gt  data   I m adding some data into this Array  using data add   function  Let s say I ve added 10 Strings into this array  The size of the array is 10 now   How do i destroy all elements in the array  The data clear   function just set s all 10 elements to null  So when I try to add another element  it just get s on the 11 position  I want just to start it over from 0   I can use a for loop to replace all the data  but I really think there is a way just to empty the ArrayList  Is it

User · Answer

Source code of clear shows the reason why the newly added data gets the first position.

 public void clear() {
    modCount++;

    // Let gc do its work
    for (int i = 0; i < size; i++)
        elementData[i] = null;

    size = 0;
    }

clear() is faster than removeAll() by the way, first one is O(n) while the latter is O(n_2)

User · Answer

data removeAll data   will do the work  I think

User · Answer

After data clear   it will definitely start again from the zero index

User · Answer

it s not true  the clear   function  clear the Arraylist and start from index 0

User · Answer

Your assumptions don t seem to be right  After a clear    the newly added data start from index 0

User · Answer

ArrayList clear From Java Doc    Removes all of the elements from this list  The list will be empty after this call returns

User · Answer

If you in any doubt  have a look at JDK source code  ArrayList clear   source code   public void clear         modCount            Let gc do its work     for  int i   0  i  lt  size  i            elementData i    null       size   0      You will see that size is set to 0 so you start from 0 position   Please note that when adding elements to ArrayList  the backend array is extended  i e  array data is copied to bigger array if needed  in order to be able to add new items  When performing ArrayList clear   you only remove references to array elements and sets size to 0  however  capacity stays as it was

[java] Java ArrayList clear() function

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