Some Random Notes

State

1 min read

Imagine an Order in an e-commerce microservice. The rules for “canceling” an order change depending on its state. If it is Pending, you just delete it. If it is Shipped, you have to trigger a reverse-logistics return process.

The rules of a method (behavior) changing on the state

  1. The State interface
public interface OrderState {
    void cancelOrder();
}
  1. The different lifecycle behaviors
public class PendingState implements OrderState {
    public void cancelOrder() { System.out.println("Order deleted from DB."); }
}

public class ShippedState implements OrderState {
    public void cancelOrder() { System.out.println("Too late to delete. Initiating return process."); }
}
  1. The Context (The object doing the delegating)
public class Order {
    private OrderState currentState;

    public Order() {
        this.currentState = new PendingState(); // Starts here
    }

    public void shipOrder() {
        this.currentState = new ShippedState(); // State transitions!
    }

    public void cancel() {
        // The result of this method completely changes based on the internal state
        currentState.cancelOrder(); 
    }
}

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