Difference Between ArrayList and Vector

ArrayList and Vector are both classes that implement the List interface using a resizable array. They keep insertion order, allow duplicates and null, and give fast access by index. The key difference is that Vector is synchronized and ArrayList is not. In this lesson, you will learn all the differences between them and which one to choose.

Differences Between ArrayList and Vector

Example: Same Operations, Different Classes

Let's see an example that runs the same code on both.

import java.util.ArrayList;
import java.util.List;
import java.util.Vector;
 
public class ListCompare {
    static void fill(List<String> list) {
        list.add("Navika");
        list.add("Srihitha");
        list.add("Advitha");
        list.remove(1);
        System.out.println(list.getClass().getSimpleName() + ": " + list);
    }
 
    public static void main(String[] args) {
        fill(new ArrayList<>());
        fill(new Vector<>());
    }
}

ArrayList: [Navika, Advitha]
Vector: [Navika, Advitha]

Explanation: Both classes implement List, so the same method works with either and gives the same result. The difference isn't in what they do but in how they do it: each Vector call takes a lock, and each ArrayList call doesn't.

Which One to Use

• One thread: use ArrayList. It is faster and is the standard choice.
• Many threads, mostly reading: use CopyOnWriteArrayList.
• Many threads, mixed reads and writes: use Collections.synchronizedList(new ArrayList<>()) with your own locking around compound actions, or a concurrent queue.
• Vector: only when maintaining old code that already uses it.

Common Mistakes

Believing Vector is the "thread-safe ArrayList" that solves every concurrency problem. Only single calls are protected.

Choosing Vector for speed. It is slower than ArrayList in single-threaded code.

Interview Corner

Practice

1. Easy: Add 100 elements to an ArrayList and a Vector, then explain why their capacities differ.
2. Harder: Start two threads that each add 10,000 elements to a shared ArrayList, and print the final size. Repeat with a Vector. Explain the results.

Key Takeaways