Java: Device Limit

Your task is to fix the device limit tool so that every command updates the data correctly and prints the correct success message or error reason. It reads the state from data/plan.json and data/sessions.json, processes the login, logout and switch commands, and writes the updated state back.

Project Structure

├── data/
│   ├── plan.json               # Read-only plan (plan, device_limit)
│   └── sessions.json           # Persistent sessions state (active)
├── src/
│   ├── main/java/com/devices/
│   │   ├── Commands.java       # Command handlers
│   │   ├── FileIo.java         # State file load and save
│   │   ├── Main.java           # CLI entry point
│   │   ├── Models.java         # Data models
│   │   └── Parser.java         # JSON parsing
│   └── test/java/com/devices/
│       └── CommandsTest.java
├── devices                     # CLI launcher script
├── run.sh                      # Setup and run script
├── reset.sh                    # Restore data/sessions.json to its initial state
├── test.sh                     # Test script (writes junit.xml)
├── pom.xml                     # Maven build configuration
└── README.md

Commands

Install

mvn -B -ntp -DskipTests package

Run

bash run.sh

Test

bash test.sh

Reset

Restore data/sessions.json to its initial state.

bash reset.sh

Examples

Each example below runs independently against the initial data files: the Family plan with a limit of 5, and five devices signed in: phone, tv, laptop, kids tablet and game console.

Example 1: A sixth device is turned away while the plan is full

Command:

./devices login projector

Output:

Error: DEVICE_LIMIT

The five devices on the list already take every slot on the plan, so there is no room for projector. data/sessions.json is not modified.

Example 2: A device that is signed in cannot sign in again

Command:

./devices login phone

Output:

Error: ALREADY_LOGGED_IN

phone is on the list already, so it is not signed in a second time. data/sessions.json is not modified.

Example 3: Switching trades one device for another in a single step

Command:

./devices switch laptop projector

Output:

Switched laptop to projector (5/5)

Final data/sessions.json:

{"active":["phone","tv","projector","kids tablet","game console"]}

laptop is signed out and projector signed in by the one command. projector takes the position laptop had, and the other devices are untouched.

Example 4: Signing out frees a slot for the next device

Commands:

./devices logout tv
./devices login projector

Output:

tv signed out (4/5)
projector signed in (5/5)

Final data/sessions.json:

{"active":["phone","laptop","kids tablet","game console","projector"]}

With tv signed out there is room on the plan for projector, which joins the end of the list.

Example 5: Both ends of a switch are checked

Commands:

./devices switch watch tv
./devices switch phone tv

Output:

Error: NOT_LOGGED_IN
Error: ALREADY_LOGGED_IN

watch is not signed in, so it cannot be the device that leaves. tv is signed in already, so it cannot be the device that arrives. Neither command changes the file.