private Fields and Encapsulation

You wrote the class, so you decide what counts as a valid value for its fields. Mark a field private and the compiler enforces that decision: only code inside that same class can read or write the field directly. Any outside code that tries gets a compile-time error, not a runtime surprise. This is encapsulation — bundling data with the code that's allowed to touch it, so the object controls its own state instead of trusting every piece of code that uses it.

Accessors (getters) and mutators (setters) are the public doorway you build into that locked room. A getter like getBalance() just hands back a copy of the value. A setter like setBalance(double amount) can check the value first — reject a negative balance, clamp a number to a range, log a change — before it ever gets stored in the field. That check is the whole point: it's a checkpoint that a bare public field can never have, because public code can assign anything with no one standing guard.

This sets up constructors and methods, which are the other public doorways into an object: a constructor sets the initial state once when the object is built, and methods like deposit or withdraw are mutators with a purpose, changing balance only in ways the class considers legal. Private fields are the reason those methods matter — without the lock, there's nothing for them to protect.