Implementing State in Java

State: A pattern that changes an object’s behavior by delegating state-specific logic to explicit state representations.

A Java state design usually has:

• a context that owns the current state
• state objects or state-specific handlers
• transitions that change which state handles behavior

1public interface OrderState {
2    void pay(OrderContext context);
3    void ship(OrderContext context);
4}

 1public final class PendingState implements OrderState {
 2    @Override
 3    public void pay(OrderContext context) {
 4        context.setState(new PaidState());
 5    }
 6
 7    @Override
 8    public void ship(OrderContext context) {
 9        throw new IllegalStateException("Cannot ship before payment");
10    }
11}

Why It Helps

The pattern localizes state-specific rules instead of scattering them across:

• switch statements
• status flags
• duplicated guard logic

When To Avoid It

If the lifecycle is tiny and stable, a full state object model may be more ceremony than value. Not every enum plus switch needs to become State.