What is the default initial capacity and load factor? Why those values?

The default initial capacity is 16 and the default load factor is 0.75. Together they mean the first resize happens at size 12 (16 * 0.75).

Why 16?

Capacity must be a power of two so the index calculation (n - 1) & hash works as a fast bit mask. Sixteen is small enough to be cheap for tiny maps but big enough that a handful of inserts don't immediately trigger a resize. The constant lives in HashMap as:

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 16
static final float DEFAULT_LOAD_FACTOR = 0.75f;

Why 0.75?

The load factor is a space/time tradeoff:

• Lower (e.g. 0.5) → fewer collisions, more wasted slots, more frequent resizes.
• Higher (e.g. 1.0) → denser table, longer chains, slower lookups.

Java's choice of 0.75 comes from analysis of bucket occupancy under random hashing. With a good hash function, bucket sizes follow a Poisson distribution. At load factor 0.75 the mean λ ≈ 0.5, and the source comments give the probability of a bucket containing exactly k entries:

0:    0.60653066
1:    0.30326533
2:    0.07581633
3:    0.01263606
4:    0.00157952
5:    0.00015795
6:    0.00001316
7:    0.00000094
8:    0.00000006

So a bucket of 8+ — the treeify threshold — occurs with probability under one in ten million. The map stays list-based almost always, and trees are reserved for genuinely bad keys (or attack inputs).

Tuning

If you know the final size up front, pre-size to avoid resizes:

// expecting ~1000 entries: capacity = 1000 / 0.75 = ~1334, rounds up to 2048
Map<String, User> users = new HashMap<>(1334);