How does HashMap behave if all objects have the same hashCode, which is a constant?

If all objects in a HashMap have the same hashCode (for example, a constant), then they will all fall into the same bucket of the internal structure. As a result, the HashMap degrades in performance to a linked list (or a tree if the number of elements in the bucket exceeds a threshold), and insert, search, and delete operations will have a complexity of O(n) instead of the expected O(1).

Example:

class BadHash {
    int value;
    BadHash(int value) { this.value = value; }
    @Override
    public int hashCode() { return 42; } // constant hash
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof BadHash)) return false;
        return this.value == ((BadHash) o).value;
    }
}

Map<BadHash, String> map = new HashMap<>();
map.put(new BadHash(1), "one");
map.put(new BadHash(2), "two");
// All elements will be in one bucket, slowing down operations.