List Interface

#Introduction

The List interface is one of the most frequently used collection types in Java. Almost every Java application uses ArrayList or LinkedList, so interviewers expect you to know the List interface thoroughly.

#What is List?

A List is an ordered collection that:

• Allows duplicate elements
• Allows null elements (implementation dependent)
• Supports positional (index-based) access
• Preserves insertion order

Package:

java.util.List

Declaration:

public interface List<E> extends Collection<E>

#Characteristics of List

1. Ordered Collection

Elements are stored in insertion order.

List<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
list.add("C++");
System.out.println(list);

[Java, Python, C++]

The order remains exactly the same.

2. Allows Duplicates

Unlike Set, List allows duplicate values.

List<Integer> list = new ArrayList<>();
list.add(10);
list.add(10);
list.add(20);
System.out.println(list);

[10, 10, 20]

3. Allows Null Values

Most List implementations allow multiple null values.

List<String> list = new ArrayList<>();
list.add(null);
list.add(null);
System.out.println(list);

[null, null]

4. Index-Based Access

Every element has an index.

list.get(1);

B

#Why List?

Suppose you're developing an online shopping application. Cart items: 1. Laptop, 2. Mouse, 3. Keyboard.

• Order matters.
• Duplicates are allowed.
• You need index-based access.

A perfect use case for List.

#List Interface Methods

Apart from the Collection methods, List introduces many new methods.

add(int index, E element)

Adds an element at a specific position.

List<String> list = new ArrayList<>();
list.add("A");
list.add("C");
list.add(1, "B");
System.out.println(list);

[A, B, C]

get()

Returns the element at an index.

String value = list.get(2);

C

Time complexity (interview favourite): ArrayList → O(1), LinkedList → O(n).

set()

Replaces an element and returns the old value.

list.set(1, "Java");
System.out.println(list);

[A, Java, C]

remove(int index)

Removes the element at an index.

list.remove(1);

[A, C]

indexOf()

Returns the first occurrence.

List<Integer> list = Arrays.asList(10, 20, 30, 20);
System.out.println(list.indexOf(20));

1

lastIndexOf()

Returns the last occurrence.

System.out.println(list.lastIndexOf(20));

3

subList()

Returns a view of the original list.

List<Integer> list = Arrays.asList(10, 20, 30, 40, 50);
System.out.println(list.subList(1, 4));

[20, 30, 40]

listIterator()

Returns a ListIterator<E>. Unlike Iterator, a ListIterator can:

• Move forward
• Move backward
• Update elements
• Add elements
• Remove elements

ListIterator<String> itr = list.listIterator();

while (itr.hasNext()) {
    System.out.println(itr.next());
}

#Iterator vs ListIterator

#Array vs List

#List Implementations

Java provides four major implementations, each with different characteristics. We'll study each in separate chapters.

#Equality in List

List<Integer> l1 = Arrays.asList(1, 2, 3);
List<Integer> l2 = Arrays.asList(1, 2, 3);
System.out.println(l1.equals(l2));

true

Lists compare order and elements. If the order changes ([1, 2, 3] vs [2, 1, 3]), the result is false. (Interview favourite.)

hashCode() in List

Contract: equal lists → equal hash codes. The hash code depends on order and elements.

#Immutable List

Java 9 introduced List.of():

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

UnsupportedOperationException

#Arrays.asList() vs List.of()

Interview favourite.

Arrays.asList()

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

• Fixed-size
• Allows set()
• Backed by the original array
• Allows null values

List.of()

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

• Completely immutable
• Does not allow add(), remove() or set()
• Does not allow null values

Arrays.asList() Example

List<String> list = Arrays.asList("A", "B");
list.set(0, "Java");
System.out.println(list);

[Java, B]

Works. But list.add("C"); throws an exception.

List.of() Example

List<String> list = List.of("A", "B");
list.set(0, "Java");

UnsupportedOperationException

#Time Complexity

*Amortized O(1)

#Real-world Use Cases