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Remove Operation

Learn how to remove elements from an ArrayList

The Remove Operation

The remove operation deletes an element from an ArrayList. This requires shifting remaining elements to fill the gap left by the removed element.

When removing an element, the ArrayList performs:

Finding the element to be removed
Shifting all elements after it one position to the left
Decreasing the size counter
Optionally shrinking capacity if size becomes too small

Removing by Index

Remove an element at a specific index. Time complexity depends on position: - Remove at end: O(1) - Remove at middle: O(n) - Remove at beginning: O(n)

Array Visualization

Size: 5 | Capacity: 10

[0]

[1]

[2]

[3]

[4]

[5]

—

[6]

—

[7]

—

[8]

—

[9]

—

Occupied

Available Capacity

Remove by Index (Java)

ArrayList<Integer> list = new ArrayList<>();
list.add(10);   // [10]
list.add(20);   // [10, 20]
list.add(30);   // [10, 20, 30]
list.add(40);   // [10, 20, 30, 40]
 
// Remove element at index 1 (value 20)
list.remove(1); // [10, 30, 40]
// Elements after index 1 shift left: 30 moves to index 1, 40 moves to index 2

Removing by Value

Remove the first occurrence of a specific value. The ArrayList searches for the value and removes it if found. Time complexity: O(n) for searching and shifting.

Remove by Value (Java)

ArrayList<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
list.add("Cherry");
list.add("Banana");
 
// Remove first occurrence of "Banana"
list.remove("Banana"); // [Apple, Cherry, Banana]
 
// Only the FIRST occurrence is removed
// The second "Banana" remains in the list

Removing Multiple Elements

When removing multiple elements, be careful with indices as they change after each removal.

Removing Multiple Elements (Java)

ArrayList<Integer> list = new ArrayList<>();
for (int i = 1; i <= 5; i++) {
    list.add(i * 10);
}
// [10, 20, 30, 40, 50]
 
// WRONG: Don't use forward iteration
for (int i = 0; i < list.size(); i++) {
    if (list.get(i) > 20) {
        list.remove(i);  // BUG: Skips elements!
    }
}
 
// CORRECT: Use backward iteration
for (int i = list.size() - 1; i >= 0; i--) {
    if (list.get(i) > 20) {
        list.remove(i);  // Removes 30, 40, 50
    }
}
// Result: [10, 20]

Tip: When removing multiple elements, iterate backwards to avoid skipping elements or use an iterator with the remove() method.

Clear All Elements

Remove all elements from the ArrayList. Time complexity: O(n) to clear all references.

Clear ArrayList (Java)

ArrayList<Integer> list = new ArrayList<>();
list.add(10);
list.add(20);
list.add(30);
// [10, 20, 30], size: 3, capacity: varies
 
// Clear all elements
list.clear();
// [], size: 0, capacity: unchanged
 
// The internal array capacity may remain the same to avoid reallocation

Practice Exercise

Key Takeaways

Remove by index: O(1) at end, O(n) at beginning/middle
Remove by value: O(n) to find and shift elements
Multiple removals: Iterate backwards to avoid index issues
Clear: O(n) time to remove all elements