Question 1. Which data types cannot be used as keys in a HashMap? - Arrays - Atomics (AtomicBoolean, AtomicInteger, etc.) - Primitives - References

Any objects that correctly implement the hashCode() and equals() methods can be used as keys in a HashMap. Let's consider the listed options:

• Arrays — they can be used, but it is not recommended because arrays do not override hashCode() and equals(), inheriting them from Object, where comparison is by reference. This can lead to unexpected behavior when searching for keys.

• Atomics (AtomicBoolean, AtomicInteger, etc.) — they can be used as they correctly override hashCode() and equals().

• Primitives — cannot be used directly because HashMap works with objects. However, their wrapper classes (Integer, Boolean, etc.) can be used.

• Reference types — can be used, provided they correctly implement hashCode() and equals().

In summary, primitives cannot be used directly (wrappers are needed), and arrays are technically possible but considered bad practice due to the lack of proper hashCode() and equals() implementations.