Implement the command pattern in Java 21 on a to-do list with undo, redo and macro commands, backed by 18 checks and the traps that quietly break undo.
Build an order desk in Java 21 with the chain of responsibility pattern: validators reject, approvers escalate, and a trail shows who decided. Gradle, tested.
Build an immutable room booking in Java 21 with a nested builder, validation in build(), toBuilder() copies and a step builder, all compiled and run.
Build a URL shortener in Java 21 with a static factory, a registry factory and an abstract factory, then swap memory for file storage with one config value.
Build a stock-alert system with the observer pattern in Java 21: a thread-safe subject, lambda listeners, safe unsubscribe, and 16 checks that prove it works.
Build a currency-rates client and bolt on logging, retries, caching and validation with the decorator pattern in Java 21. Full code, order traps, 25 checks.
Build a parcel-tracking state machine in Java 21 with enums and a transition table, add a guard for failed deliveries, and prove the rules with 19 checks.
Replace a growing if/else chain with the strategy pattern in Java 21. A library late-fee calculator, built step by step, with lambdas, an EnumMap and 15 checks.
Gopi Gorantala