 Solution

Encapsulation exposes operations with contracts while hiding representation details and controlling changes to state. In a text editor, a document object can privately store text and cursor position and expose insert, delete and moveCursor. Clients cannot directly splice internal storage or set a cursor outside the document. The document's methods enforce its representation invariant, and the implementation can later replace a character array by a different text representation without changing the public editing interface. Merely declaring a field private is insufficient if an accessor returns the mutable representation itself.