Week 03 · Guarding the State

Mission Quest

Mission Quest: Design a device

// Design a class first, then hide its data behind methods

⏱ about 30 min

Saturday: Mission Quest: Design a Device

Comet sketches a fan in her notebook: three speed buttons and a switch. "Could this be a class?" she asks. "It knows things and it does things."

Wren adds a question underneath. "What does the evidence say?" he asks. "If anyone could set the speed to 99, what would happen to the fan?"

Nova draws a box around the fan with three small windows. "How can I help?" she asks. "Decide which values stay inside, and which buttons outside users get."

Comet tears out a fresh page for you. "Find a family member, lead programmer. Pick a device at home."

Hidden values, public buttons

Design a class before writing it, with its attributes and behaviors. Encapsulation keeps the details hidden from other classes.

A real device works the same way. You press its buttons, but you cannot reach in and set the motor to any speed.

Fan (class diagram)	Hidden or public?	Why
_speed (0 to 3)	Hidden	Only the buttons may change it
_is_on	Hidden	Changed by the switch
press_speed(n)	Public	A button anyone may press
get_speed()	Public	Lets others read the speed

◇ DESIGN A DEVICE

You need paper and a pencil. Choose one device at home together, such as a lamp, a radio, a washing machine or a thermostat.
Draw a class box: the class name, three or four attributes and three behaviors.
For each attribute, decide together: hidden or public? Mark hidden ones with an underscore, like _speed.
For each hidden attribute, decide which method outside users get instead, such as a getter or a button.
Swap roles: one of you is outside code and asks for a change. The other checks it is allowed by a public method.

For a grown-up

Today's idea is encapsulation: an object keeps its own values safe and offers a few public methods instead.
Talk about it: which controls on your device could cause trouble if anyone could set them to any value? How does the design prevent it?

CHECK YOUR DESIGN

Read the question.
Tap your answer.

Which should be hidden in a Rover?

Why give a device public methods instead of open values?

When should you draw the class diagram?

Copy your device's class box. Which attribute did you decide to hide, and why?Write the def line of one public method for your device, with self as its first parameter.

Strong design work. Next week Pip starts carrying samples, and objects begin working together.

Grown-up checked my answers

← Friday Back to weeks