Can an instance of a user-defined class (class MyType) be a key in a dictionary?

In programming languages that have dictionaries (hash tables), keys are usually limited to types that support comparison and hashing.

If we talk about a language with classes (for example, Python, Java, C#):

• In Python, instances of user-defined classes can be keys in a dictionary if they correctly implement the __hash__ and __eq__ methods.
• In Java, objects can be keys in a HashMap if they implement the hashCode() and equals() methods.

If the class does not override these methods, then keys will be compared by default (for example, by reference), which may be undesirable.

Example in Python:

class MyType:
    def __init__(self, value):
        self.value = value

    def __hash__(self):
        return hash(self.value)

    def __eq__(self, other):
        return isinstance(other, MyType) and self.value == other.value

my_dict = {}
obj = MyType(10)
my_dict[obj] = "data"

Thus, an instance of a user-defined class can be a key in a dictionary if it supports the necessary comparison and hashing operations.