Why initialize HashSet<>(0) to zero?

Question

I love a HashSet<>() and use this eagerly while initializing this with the default constructor:

Set<Users> users = new HashSet<>();

Now, my automatic bean creator (JBoss tools) initializes this as:

Set<Users> users = new HashSet<>(0);

Why the zero? The API tells me that this is the initial capacity, but what is the advantage of putting this to zero? Is this advised?

Answer 1

The default initial capacity is 16, so by passing in 0 you may save a few bytes of memory if you end up not putting anything in the set.

Other than that there is no real advantage; when you pass 0 the set is created with a capacity of 1 and as soon as you add something it will have to be resized.

Answer 2

HashSet use HashMap store data:

public HashSet(int initialCapacity) {
map = new HashMap<E,Object>(initialCapacity);
}

while the initialCapacity = 0,

public HashMap(int initialCapacity, float loadFactor) {
    ....
    // Find a power of 2 >= initialCapacity
    int capacity = 1;
    while (capacity < initialCapacity)
        capacity <<= 1;
}

the HashMap capacity is 1.

but if use default constructor:

public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
    table = new Entry[DEFAULT_INITIAL_CAPACITY];
    init();
}

the HashMap capacity is 16*0.75.

So, new HashSet<>(0) save some memroy when init.