I think of a nested class system like building a real, functional car. The main class — let’s say Car — represents the entire vehicle. Within this class, we can define major components as nested classes, such as Engine, FuelTank, and Piston, each with its own responsibilities and internal logic.

For instance, the Engine class might include a method isRunning() that checks whether its subcomponents — like Piston, SparkPlug, etc. — are functioning correctly. These subcomponents can also be implemented as nested classes inside Engine, creating a clear hierarchy and encapsulation of responsibilities.

If components like FuelTank and Engine are in the same enclosing scope (e.g. as nested classes inside Car), they can easily communicate with each other. For example, the Engine could call fuelTank.isFull() before attempting to start, mimicking how real car systems coordinate.

This design mirrors the modular and interconnected nature of a real car. It also improves encapsulation: the inner classes hide implementation details, while the outer Car class exposes only necessary, high-level methods like start() or drive().

In summary, nested classes allow related components to be grouped together logically, promote encapsulation, and facilitate clean interaction between parts — much like how subsystems in a real car work together under the hood.