Java List vs ArrayList: Key Differences Explained
Java List is an interface defining an ordered collection; ArrayList is a concrete class that implements List using a resizable array.
Developers often say “make it a List” when they really create an ArrayList, because the interface hides the messy internals and lets us swap implementations later without rewriting code.
Key Differences
List can’t be instantiated; it only declares methods like add, get, remove. ArrayList supplies the actual storage, grows automatically, and gives fast index access but slower inserts in the middle.
Which One Should You Choose?
Declare variables as List for flexibility, instantiate as ArrayList for everyday work. Switch to LinkedList only when frequent middle insertions outweigh indexed reads.
Examples and Daily Life
Think of List as a restaurant menu—what you can order—while ArrayList is the kitchen’s actual pantry that refills shelves as needed.
Can I write List list = new ArrayList<>();?
Yes, that’s the idiomatic way: program to the interface, not the implementation.
When does ArrayList resize?
When its internal array fills, it creates a new one at 1.5× size and copies elements.
Is ArrayList thread-safe?
No; use Collections.synchronizedList or CopyOnWriteArrayList for concurrent access.