1.6 KiB
State Design Pattern — Java Example
Pattern: Behavioral → State Article: https://ankurm.com/state-design-pattern-java/
What this example shows
A traffic light with zero if-else state machines. TrafficLightState declares what every state must do: handle entry (onEnter), handle the transition trigger (next), and report its own color. TrafficLight is the context — it holds a reference to the current state and delegates every call to it. RedState, GreenState, and YellowState each know only their own entry message and their own next state (Red→Green→Yellow→Red); none of them knows about the full cycle. Main creates the light, cycles through six transitions, and prints why this beats a single class full of if-else branches.
How to run
javac state/*.java -d out/state
java -cp out/state state.Main
Requires Java 25.
Post Section ↔ File Mapping
| Post Section | File(s) |
|---|---|
| With State: Traffic Light — the State interface | TrafficLightState.java |
| With State: Traffic Light — the Context | TrafficLight.java |
| With State: Traffic Light — RedState | RedState.java |
| With State: Traffic Light — GreenState | GreenState.java |
| With State: Traffic Light — YellowState | YellowState.java |
| With State: Traffic Light — wiring it together | Main.java |
Note: the "Without State: The if-else Machine" snippet (the fragile TrafficLight with a String state field) is illustrative only — it is not part of this repository's runnable example.
Article: https://ankurm.com/state-design-pattern-java/ All patterns: https://ankurm.com/design-patterns-java/