ArrayList Methods Cheat Sheet

1. Creating an ArrayList

ArrayList<”Type”> list = new ArrayList<”Type”>();

2. Common Methods

Add Elements

list.add(value); → Appends a value to the end.
list.add(index, value); → Inserts value at a specific index.

Access Elements

list.get(index); → Returns the element at the specified index.

Update Elements

list.set(index, value); → Replaces the element at index with value.

Remove Elements

list.remove(index); → Deletes element at index.
list.remove(value); → Deletes the first occurrence of value.

Size of the List

list.size(); → Returns the number of elements.

Check for Elements

list.contains(value); → Returns true if value is in the list.
list.indexOf(value); → Returns the index of the first occurrence of value, or -1 if not found.

ArrayList Loops Cheat Sheet

1. Traversing an ArrayList

You can use for loops or for-each loops to iterate through an ArrayList.

// Basic for loop
for (int i = 0; i < list.size(); i++) {
    System.out.println(list.get(i)); // Access element at index i
}

2. Using a For-Each Loop

Simplified syntax, no index access.

for (Type item : list) {
    System.out.println(item); // Access each element directly
}

3. Modifying Elements

Use a for loop if you need to update values.

for (int i = 0; i < list.size(); i++) {
    list.set(i, list.get(i) + 1); // Example: Increment each element
}

4. Avoid ConcurrentModificationException

Cannot modify ArrayList (add/remove elements) during a for-each loop. Use an iterator or a for loop for such operations.

// Correct usage with a for loop
for (int i = 0; i < list.size(); i++) {
    if (list.get(i) < 0) {
        list.remove(i);
        i--; // Adjust index after removal
    }
}

5. Nested Loops with ArrayLists

Use nested loops to compare or process multiple elements.

for (int i = 0; i < list.size(); i++) {
    for (int j = i + 1; j < list.size(); j++) {
        if (list.get(i).equals(list.get(j))) {
            System.out.println("Duplicate: " + list.get(i));
        }
    }
}