How would you test a coffee machine?

Clarify first

• What types of drinks does the machine make, and what are the size options?
• Does it use a water reservoir or mains connection; pods, capsules, or ground beans?
• Is payment required, or is this an office/free-use machine?
• Are there cleaning or descaling cycles, and does the machine enforce them?

Functional

• Select each drink type → correct drink dispensed at the correct quantity
• Size selection (small/medium/large) produces the correct volume
• Water level indicator accurately reflects the current level and updates after each use
• Temperature of the dispensed drink is within the safe and palatable range
• Cup sensor detects the presence of a cup before dispensing begins
• Cleaning cycle completes successfully; machine returns to ready state

Negative / error handling

• No water in the reservoir → machine refuses to start; clear "Fill Water" message displayed (not "Error 42")
• No pod or beans → machine prompts to insert consumable; no partial dispense
• Cup not present → machine does not dispense; safety alert shown
• Descaling overdue → machine blocks operation until descaling is performed, or shows a warning (depending on the design)
• Power interrupted mid-brew → machine drains safely; no hot liquid retained under pressure

Edge & boundary

• Water level just sufficient for the smallest drink but not the largest → machine dispenses the small, warns or refuses for the large
• Cup that is too small for the selected quantity → overflow detection (if sensor exists) or drink stops at cup capacity
• Two users attempting to use the machine simultaneously → second user is queued or shown "busy" state
• Descaling mode runs during normal use hours → other functions blocked during the cycle

Non-functional

• Noise level within specification during normal operation
• Heat-up time from cold start within the documented tolerance
• Brew time within the documented range for each drink type
• Machine surface temperature safe to touch (excluding steam nozzle area)

Usability

• Controls intuitive without the manual (can a new user make a coffee without instructions?)
• Error messages are actionable ("Fill Water" not a generic error code)

Close: hardware behavior (temperature, brew time, noise, surface heat) requires physical measurement against specifications. Software state logic (error states, resource checks, sequence of operations) can be covered by unit tests of the controller firmware or simulation.