Difference Between List and Set in Java

In Java, both List and Set are interfaces that represent collections of elements. However, there are some key differences between them in terms of characteristics and usage:

Duplicates: List allows duplicate elements, meaning you can have multiple occurrences of the same element in a list. On the other hand, Set does not allow duplicate elements. Each element in a set must be unique.
Ordering: List preserves the order of elements, meaning the elements are stored in a specific sequence and can be accessed by their index. Set does not have a defined order for its elements. The elements in a set are typically unordered, although some implementations (such as LinkedHashSet) maintain insertion order.
Implementation Classes: Java provides several implementation classes for List, such as ArrayList, LinkedList, and Vector. These classes offer different performance characteristics and usage patterns. For Set, common implementations include HashSet, TreeSet, and LinkedHashSet. Each implementation has its own characteristics and trade-offs.
Performance: Since List allows duplicates and maintains order, it can be more efficient when accessing elements by their index or performing operations like adding or removing elements at specific positions. Set, on the other hand, focuses on uniqueness and may provide faster lookup operations for membership testing.
Usage: List is typically used when the order of elements or duplicates are important, such as maintaining a sequence, allowing multiple occurrences of elements, or retrieving elements by their index. Set is used when uniqueness is required or when performing operations like membership testing, eliminating duplicates, or ensuring distinct elements.

Here’s an example to illustrate the differences:

List<String> myList = new ArrayList<>();
myList.add("apple");
myList.add("banana");
myList.add("apple"); // Duplicate element

Set<String> mySet = new HashSet<>();
mySet.add("apple");
mySet.add("banana");
mySet.add("apple"); // Duplicate element, not added

System.out.println(myList); // Output: [apple, banana, apple]
System.out.println(mySet); // Output: [apple, banana]

List<String> myList = new ArrayList<>();

myList.add("apple");

myList.add("banana");

myList.add("apple"); // Duplicate element

Set<String> mySet = new HashSet<>();

mySet.add("apple");

mySet.add("banana");

mySet.add("apple"); // Duplicate element, not added

System.out.println(myList); // Output: [apple, banana, apple]

System.out.println(mySet); // Output: [apple, banana]

In the example, the List allows duplicate elements, so both occurrences of “apple” are stored. However, the Set only contains unique elements, so the duplicate “apple” is not added to the set.

Overall, the choice between List and Set depends on your specific requirements, such as the need for ordering, duplicates, and the desired performance characteristics for your application.