What is meant by the immutability of a class's state in object-oriented programming?

Immutability of class state in object-oriented programming means that once an object is created, its internal state cannot be changed. All fields of the object become read-only, and any operations that could modify the state instead return a new object with updated values.

This helps avoid errors related to data modification in different parts of the program, simplifies debugging, and makes the code more predictable.

An example in Python using @dataclass(frozen=True):

from dataclasses import dataclass

@dataclass(frozen=True)
class Point:
    x: int
    y: int

p1 = Point(1, 2)
# p1.x = 3  # This will cause an error because the object is immutable

# To "modify" the point, create a new object
p2 = Point(p1.x + 1, p1.y)