Can all elements end up in the same bucket immediately after hashCode calculation and key comparison?

In theory, if all objects have the same hashCode, they will end up in the same bucket in a hash table, such as HashMap. However, in practice, this is highly undesirable because it leads to performance degradation due to a large number of collisions.

After computing the hashCode, the bucket index is calculated (usually via a modulo operation or bitmask). If the hashCode is the same, the bucket index will also be the same. When adding elements with the same bucket index, they are stored in a linked list or a tree within that bucket.

Thus, all elements can end up in one bucket if they have matching hashCodes and the equals method returns false when comparing keys (meaning the keys are different, but hashCodes are the same). This is called a collision.

Example:

Map<Key, String> map = new HashMap<>();

class Key {
    private int id;
    public Key(int id) { this.id = id; }
    @Override
    public int hashCode() { return 42; } // intentionally the same hashCode
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Key)) return false;
        Key key = (Key) o;
        return id == key.id;
    }
}

map.put(new Key(1), "value1");
map.put(new Key(2), "value2");
// Both objects will end up in the same bucket but will be stored as different entries

In conclusion: yes, all elements can end up in one bucket if they have the same hashCode, but the keys are not equal according to equals.