Why Does the HashSet Implementation in Java Utilize HashMap as Its Underlying Structure?

Why does the HashSet implementation in Sun Java use HashMap as its backing structure?

// Example of HashSet using HashMap
HashSet<String> set = new HashSet<>();
set.add("Java");
set.add("HashSet");

The Java HashSet is designed to utilize a HashMap for its underlying implementation because of the efficiencies and functionalities that HashMap provides. This relationship allows HashSet to inherit behaviors like key uniqueness and fast lookups, contributing significantly to HashSet's performance characteristics.

// Demonstrating how HashSet works internally with HashMap
HashMap<String, Object> backingMap = new HashMap<>();
backingMap.put("key1", PRESENT);
backingMap.put("key2", PRESENT);

// Where PRESENT is a constant object used as a marker for presence.

Causes

• Efficiency in operations: HashMap provides O(1) time complexity for operations like add, remove, and contains, which is essential for HashSet's performance.
• Simplicity and code reuse: HashSet can leverage the existing HashMap functionality, reducing the need to reinvent the wheel for key management.
• Uniqueness: HashMap's key-based storage allows HashSet to inherently manage unique elements, as any duplicate attempt to add an already existing key is automatically handled.

Solutions

• Using HashMap enables HashSet to maintain a stronger performance base without compromising on functionality.
• As HashSet and HashMap share similar characteristics, it allows for easier maintenance of the Java Collections Framework.

Mistake: Assuming HashSet's size includes the dummy object for each entry.

Solution: The size of the HashSet only accounts for the number of unique entries, not the overhead of the dummy object.

Mistake: Believing that HashSet does not handle duplicates correctly due to the use of HashMap.

Solution: HashSet indeed handles duplicates correctly, as it relies on HashMap's key checking to enforce uniqueness.