How to remove an element from an ArrayList in Java? (By Index & Object)
There are two ways to delete elements from ArrayList in Java: by index and by object: remove(int index) deletes by position and returns the element, remove(Object o) deletes the first matching item by value and returns a Boolean value; Iterator.remove() must be used to avoid ConcurrentModificationException when traversing; both have O(n) time complexity.
To remove an element from an ArrayList in Java, you can do it either by index or by object value — but the behavior and caveats differ. The key is knowing which method to use, and when.
Remove by index
Use remove(int index) to delete the element at a specific position. This shifts all subsequent elements left by one position and returns the removed element.
- Index must be valid:
0 , otherwise <code>IndexOutOfBoundsExceptionis thrown. - Returns the element that was removed (useful if you need its value).
- Works reliably even if the list contains duplicate values — it removes only the one at that position.
Example:
list.remove(2); // removes the third element (index 2)Remove by object
Use remove(Object o) to delete the first occurrence of a matching element — based on equals() . It returns true if an element was found and removed, false otherwise.
- Relies on correct
equals()implementation — especially important for custom objects. - Only removes the first match, even if duplicates exist.
- Returns
boolean, not the removed object — so you won't get the actual instance back unless you find it first.
Example:
list.remove("apple"); // removes first "apple", if presentAvoid ConcurrentModificationException when removing while iterating
You cannot safely call remove() directly on the list inside a standard for-each loop — it throws ConcurrentModificationException .
- Use
Iterator.remove()instead for safe removal during iteration. - Or collect indices/objects to remove first, then remove them afterward (eg, using
removeAll()or reverse-index loops).
Safe iterator example:
Iteratorwhile (it.hasNext()) {
if (it.next().startsWith("a")) it.remove();
}
Performance note
Both remove() methods are O(n) operations because elements after the removal point must be shifted.
- Removing from the end (
remove(size()-1)) is fastest — no shifting needed. - Removing from the beginning or middle requires copying array elements.
- If order doesn't matter, consider swapping with the last element and removing there for better performance.
Basically just pick the right remove() overload — index for position, object for value — and watch out for iteration pitfalls and performance trade-offs.
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