List

tags: Collection, Data Structures

List Interface

• List interface is the child interface of Collection interface.
• It inhibits a list type data structure in which we can store the ordered collection of objects.
• It can have duplicate values.
• List interface is implemented by the classes ArrayList, LinkedList, Vector, and Stack.

ArrayList

• The ArrayList class implements the List interface.
• It uses a dynamic array to store the duplicate element of different data types.
• The ArrayList class maintains the insertion order and is non-synchronized.
• Capacity represents the total number of elements the array list can contain.
• Therefore, the capacity of an array list is always greater than or equal to the size of the array list.
• When we add an element to the array list, it checks whether the size of the array list has become equal to the capacity or not. If yes, then the capacity of the array list increases.

//Converting ArrayList to Array  
String[] array = fruitList.toArray(new String[fruitList.size()]);
String[] array = fruitList.toArray();

LinkedList

• LinkedList implements the Collection interface.
• It uses a doubly linked list internally to store the elements.
• It can store the duplicate elements. It maintains the insertion order and is not synchronized.
• In LinkedList, the manipulation is fast because no shifting is required.

//Traversing the list of elements in reverse order  
Iterator i=ll.descendingIterator();

Vector

• Vector uses a dynamic array to store the data elements.
• It is similar to ArrayList. However, It is synchronized and contains many methods that are not the part of Collection framework.

Stack

The stack is the subclass of Vector. It implements the last-in-first-out data structure, i.e., Stack. The stack contains all of the methods of Vector class and also provides its methods like boolean push(), boolean peek(), boolean push(object o), which defines its properties.

What is the List interface in Java?

List is a subtype of Collection that represents an ordered collection of elements.
It allows duplicate elements and access by index.

What are the key features of List?

• Maintains insertion order
• Allows duplicates
• Provides indexed access (get(int index))
• Can contain null elements (based on implementation)

Which classes implement the List interface?

• ArrayList
• LinkedList
• Vector
• Stack

What are the commonly used methods of List?

Method	Description
add(E e)	Adds element at end
add(int index, E)	Inserts at a specific position
get(int index)	Retrieves element at index
remove(int index)	Removes element at index
set(int index, E)	Replaces element
indexOf(Object)	Returns index of first occurrence
lastIndexOf(Object)	Returns index of last occurrence
subList(from, to)	Returns a view of a portion of the list

How is List different from Set?

• List: Ordered, allows duplicates
• Set: Unordered (except LinkedHashSet), no duplicates

How do you iterate over a List?

Using:

• Enhanced for-loop
• Iterator
• For loop using index
• Java Streams

for (int i = 0; i < list.size(); i++) {
    System.out.println(list.get(i));
}

What is the difference between ArrayList and LinkedList?

Feature	ArrayList	LinkedList
Backed by	Dynamic array	Doubly linked list
Access time	Fast (O(1) for get)	Slow (O(n) for get)
Insert/remove	Slow (shifting needed)	Fast (node manipulation)
Memory	Less (only data)	More (stores pointers)

Is List thread-safe?

No. Implementations like ArrayList and LinkedList are not thread-safe.
Use:

• Collections.synchronizedList(new ArrayList<>())
• CopyOnWriteArrayList for concurrent scenarios

What is the default capacity of an ArrayList?

Default capacity is 10, and it grows by 50% when full (in Java 8+).

Can a List contain null values?

Yes. Most implementations like ArrayList and LinkedList allow null elements.

How does ArrayList grow internally?

It creates a new array with a larger capacity, copies existing elements, and replaces the old array.

What happens when you access an invalid index in a List?

You get an IndexOutOfBoundsException.

How can you remove all elements from a List?

Using clear() method.

list.clear();

What is the time complexity of operations in List?

Operation	ArrayList	LinkedList
get()	O(1)	O(n)
add() (end)	O(1) (amortized)	O(1)
add() (index)	O(n)	O(n)
remove()	O(n)	O(n)

How to convert an array to a List?

List<String> list = Arrays.asList("A", "B", "C");

How to sort a List?

Collections.sort(list); // natural order

You can also use a custom comparator.

What is the difference between add() and set()?

• add() inserts a new element
• set() replaces an existing one at the specified index

What is List.of() in Java 9?

It creates an immutable list.

List<String> list = List.of("A", "B", "C");

Modifying it will throw UnsupportedOperationException.

What does subList() do?

It returns a view of a specific range of the list. Changes made will also reflect in the original list.

How to remove duplicates from a List?

List<String> unique = new ArrayList<>(new HashSet<>(list));