In Java, the List interface is one of the most widely used components of the Java Collections Framework (JCF). It represents an ordered collection (also known as a sequence), and it allows duplicate elements. Whether you're building a user-facing application or working on backend logic, List offers both flexibility and performance benefits.
In this tutorial, we’ll explore everything you need to know about the List interface—its purpose, internal workings, and real-world applications.
What is the Java List Interface?
Core Definition
Listis an ordered, index-based collection that can contain duplicate elements.- It extends the
Collectioninterface and adds positional access.
Syntax and Class Structure
List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
Popular Implementations
ArrayList: Backed by a dynamically resizing array.LinkedList: Doubly-linked list, better for insert/delete.Vector: Synchronized version of ArrayList (legacy).
Internal Working and Memory Model
ArrayList
- Backed by an array.
- Resizes internally (usually 1.5x or 2x) when capacity is exceeded.
LinkedList
- Each element (Node) holds reference to previous and next.
- Better for insertion/removal from the middle.
Big-O Performance
| Operation | ArrayList | LinkedList |
|---|---|---|
| get(index) | O(1) | O(n) |
| add(element) | O(1)* | O(1) |
| remove(index) | O(n) | O(n) |
| insert(index) | O(n) | O(1) |
* Amortized cost
Real-World Use Cases
- Maintaining User History: Order matters and duplicates are allowed.
- UI Elements: Lists of dropdowns, tabs, history stack.
- Database Results: List of fetched records.
Java 8+ Functional Programming Support
List<String> filtered = names.stream()
.filter(n -> n.startsWith("A"))
.collect(Collectors.toList());
Also supports removeIf, forEach, replaceAll methods.
Java Version Differences
📌 What's New in Java Versions?
Java 8
- Streams API
- Lambdas with
forEach,filter, etc.
Java 9
List.of(...)factory methods
List<String> immutableList = List.of("A", "B", "C");
Java 10+
varkeyword for type inference.
Java 21
- Improved performance and pattern matching in stream operations.
Best Practices and Misuse
✅ Do
- Use
ArrayListfor frequent access by index. - Use
LinkedListif insert/remove from middle is common. - Use
List.copyOf()for immutable copies.
❌ Avoid
- Calling
remove()insidefor-eachloop without iterator. - Using
LinkedListfor random access.
// Wrong: throws ConcurrentModificationException
for (String name : names) {
names.remove(name);
}
// Correct:
Iterator<String> itr = names.iterator();
while (itr.hasNext()) {
if (condition) itr.remove();
}
Refactoring Legacy Code
Before:
List names = new ArrayList();
names.add("John");
After:
List<String> names = new ArrayList<>();
names.add("John");
Use generics, forEach loops, and functional APIs to modernize.
Conclusion and Key Takeaways
Listis ideal when ordering matters and duplicates are needed.- Choose
ArrayListvsLinkedListbased on access vs insert frequency. - Prefer
List.of()for immutable lists. - Use Java 8+ APIs for clean and efficient code.
FAQ
1. Is List ordered in Java?
Yes, it maintains insertion order.
2. Can List contain null values?
Yes, unless it is created using List.of() which throws NPE.
3. How is ArrayList resized internally?
It increases the internal array capacity by 50% (Java 8).
4. When should I prefer LinkedList?
When frequent insertions/deletions from middle are required.
5. What’s the default capacity of ArrayList?
10 in most Java versions.
6. Is ArrayList thread-safe?
No, use Collections.synchronizedList() or CopyOnWriteArrayList.
7. How to remove duplicate entries from List?
Convert to Set and back, or use streams with distinct().
8. Does List allow primitive types?
No. Use wrapper classes like Integer, Double, etc.
9. How does removeIf() work?
It uses a predicate to remove matching elements in a single pass.
10. What happens on List.of(null)?
It throws NullPointerException.