I want to remove the last object from an ArrayList quickly.
I know that remove(Object O) takes O(n) in an ArrayList, but I wonder if it is possible to do this in constant time since I just want to remove the last object?
2 Answers
See the documentation for ArrayList#remove(int), as in the following syntax:
list.remove(list.size() - 1)
Here is how it's implemented. elementData does a lookup on the backing array (so it can cut it loose from the array), which should be constant time (since the JVM knows the size of an object reference and the number of entries it can calculate the offset), and numMoved is 0 for this case:
public E remove(int index) {
rangeCheck(index); // throws an exception if out of bounds
modCount++; // each time a structural change happens
// used for ConcurrentModificationExceptions
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // Let gc do its work
return oldValue;
}
1 Comment
mjs
O1 for this? arraycopy entire up to, or just cutting the array loose from end ?
Since Java 21, simply using List.removeLast, for example:
List<Integer> list = new ArrayList<>(List.of(1, 2, 3));
System.out.println(list.removeLast()); // 3 - removes and returns the last element
Note: if the list is not empty, the implementation of List.removeLast returns the result of calling remove(size() - 1). Otherwise, it throws NoSuchElementException.
The time complexity of removing the last element from ArrayList is O(1) - it is just decrementing the size of the list by 1 under the hood.
1 Comment
Vic Stalkeur
Wow, it took them 30 years to add something other languages had from day 1. Thanks
list.remove(list.size()-1)!!!