What is the difference between ArrayList clear and ArrayList removeAll

Question

Assuming that arraylist is defined as ArrayList lt String gt  arraylist  is arraylist removeAll arraylist  equivalent to arraylist clear     If so  can I assume that the clear   method is more efficient for emptying the array list   Are there any caveats in using arraylist removeAll arraylist  instead of arraylist clear

User · Answer

The clear   method removes all the elements of a single ArrayList  It s a fast operation  as it just sets the array elements to null   The removeAll Collection  method  which is inherited from AbstractCollection  removes all the elements that are in the argument collection from the collection you call the method on  It s a relatively slow operation  as it has to search through one of the collections involved

User · Answer

Clear is faster because it does not loop over elements to delete  This method can assume that ALL elements can be deleted   Remove all does not necessarily mean delete all elements in the list  only those provided as parameters SHOULD be delete  Hence  more effort is required to keep those which should not be deleted   CLARIFICATION  By  loop   I mean it does not have to check whether the element should be kept or not  It can set the reference to null without searching through the provided lists of elements to delete   Clear IS faster than deleteall

User · Answer

The source code for clear     public void clear         modCount            Let gc do its work     for  int i   0  i  lt  size  i            elementData i    null       size   0      The source code for removeAll   As defined in AbstractCollection    public boolean removeAll Collection lt   gt  c        boolean modified   false      Iterator lt   gt  e   iterator        while  e hasNext              if  c contains e next                   e remove                modified   true                      return modified      clear   is much faster since it doesn t have to deal with all those extra method calls   And as Atrey points out  c contains     increases the time complexity of removeAll to O n2  as opposed to clear s O n

User · Answer

They serve different purposes  clear   clears an instance of the class  removeAll   removes all the given objects and returns the state of the operation

User · Answer

clear   will go through the underlying Array and set each entry to null   removeAll collection  will go through the ArrayList checking for collection and remove Object  it if it exists    I would imagine that clear   is way faster then removeAll because it s not comparing  etc

User · Answer

The time complexity of ArrayList clear   is O n  and of removeAll is O n 2    So yes  ArrayList clear is much faster

User · Answer

Unless there is a specific optimization that checks if the argument passed to removeAll   is the collection itself  and I highly doubt that such an optimization is there  it will be significantly slower than a simple  clear     Apart from that  and at least equally important   arraylist removeAll arraylist  is just obtuse  confusing code  It is a very backwards way of saying  clear this collection   What advantage would it have over the very understandable arraylist clear

User · Answer

clear   will be much more efficient  It will simply remove each and every item  Using removeAll arraylist  will take a lot more work because it will check every item in arraylist to see if it exists in arraylist before removing it

User · Answer

Array    once the space is allocated for an Array variable at the run time  the allocated space can not be extended or removed   ArrayList    This is not the case in arraylist  ArrayList can grow and shrink at the run time               The allocated space can be minimized or maximized at the run time

[java] What is the difference between ArrayList.clear() and ArrayList.removeAll()?

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