ArrayList in Java

ArrayList is Java's go-to resizable list. Internally it stores elements in an array, but when that array fills up, it automatically creates a bigger one and copies everything over. From the outside, it just grows as you add elements.

Use ArrayList when you need an ordered list, allow duplicates, and mostly access elements by index. It is the default choice for list data in Java.

Creating an ArrayList

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Core Methods

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Line by line:

• nums.add(1, 15) — inserts 15 at index 1. Elements from index 1 onwards shift right.
• nums.remove(Integer.valueOf(15)) — removes by value. Without valueOf, nums.remove(15) would be treated as remove(int index) — a different method!
• nums.remove(0) — removes element at index 0.
• Both remove forms exist: one takes an int (index), one takes an Object (value).

Iterating an ArrayList

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Sorting

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Sorting Custom Objects

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Converting ArrayList to Array and Back

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Note: Arrays.asList(array) returns a fixed-size list. Wrap it in new ArrayList<>(...) if you need to add or remove elements.

How ArrayList Grows Internally

ArrayList starts with a default internal array of 10 slots. When it fills up, Java creates a new array 1.5x the size and copies all elements.

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If you know you will store ~1000 elements, create new ArrayList<>(1000). This avoids multiple resize operations.

Common Mistake 1: remove(int) vs remove(Object)

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When the list holds Integer, list.remove(1) removes by index (not value 1). To remove by value, use remove(Integer.valueOf(value)).

Common Mistake 2: Modifying During For-Each

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Common Mistake 3: Storing Primitives Directly

Practice Task

Task: Write a program that manages a todo list using ArrayList<String>. Implement:

1. Add 5 tasks
2. Mark a task as done by removing it by value
3. List all remaining tasks with index numbers (1-based)
4. Sort the remaining tasks alphabetically
5. Check if a specific task exists

Hint: Use a for loop with i + 1 for 1-based indexing. Use Collections.sort() for alphabetical order. Use contains() to check.

Solution:

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What to observe: remove("Call dentist") removes by value. Index-based printing with i + 1 gives a natural 1-based list. After sorting, the order changes — notice the list is truly reordered, not just displayed differently.