OOP concepts

Object-oriented programming organises a program around objects: bundles of data (fields) and the methods that operate on that data. Encapsulation hides an object's internal state behind its methods, so outside code cannot corrupt it directly. Worked example: a BankAccount class keeps its balance private and exposes deposit and withdraw methods that refuse negative amounts and overdrafts. The invariant — the balance never goes negative through a public method — is enforced in exactly one place, the class itself. That is the whole point of encapsulation: the class guards its own rules, and every other part of the program must go through the guard.