Understanding the Default Size of a HashSet in Java

In Java, the HashSet class implements the Set interface backed by a hash table which is actually a HashMap instance. This class offers constant time performance for the basic operations (add, remove, contains, and size), assuming the hash function disperses the elements properly among the buckets.

The key to understanding this question is knowing the default size of a HashSet when you instantiate it without specifying a size. The correct answer is 16.

HashSet<Integer> hashSet = new HashSet<>();

When you declare a HashSet like the one shown above, it's default initial capacity is 16. This means that it can hold up to 16 elements before its size needs to increase. The load factor, which determines when the HashSet should be resized, is 0.75 by default in HashSet. This means once the HashSet is filled up to 75% of its capacity, it will increase its size.

It's important to specify the initial size of the HashSet if you have an idea of the number of elements that will be stored. This is because increasing the size of the HashSet (known as re-hashing) can be an expensive operation in terms of processing time. If the number of elements is certain and larger than the default size, it would be more efficient to set the initial size at the time of creation.

HashSet<Integer> largeHashSet = new HashSet<>(50);

In the above example, the HashSet largeHashSet is initialized with a capacity of 50. This optimization reduces the need for expensive re-hashing operations.

Remember, while the concept of default initial size and resizing can seem trivial, it’s these little details that can help write more efficient code and make you a better programmer. Understanding how your data structures operate under the hood allows you to write code that makes full use of these structures and their properties.