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copied!<p>Dependency injection is useful for creating <code>service</code> style objects. These have the following characteristics:-</p> <ul> <li>multiple implementations are likely,</li> <li>heavy on behaviour, </li> <li>internal state is limited to their dependencies and they are generally not mutable</li> <li>would map to an actor in the real world (e.g. cashier), rather than a thing</li> </ul> <p>Based on this, <code>Payment</code> is a service object. I would rename it <code>PaymentService</code> to distinguish it from the ledger entry you may store about a payment (which would be a value object).</p> <p>You example does not show much about what the <code>Order</code> class, does, but I assume it would hold information like some line items, delivery address and total amount. This is a <code>value</code> object. It represents a thing in the business domain.</p> <p>Value objects are heavy on state, and lighter on behaviour. Multiple implementations are possible, but you are less likely to want to replace one implementation with another.</p> <p>Value objects are not created by your dependency injection framework. They are created by your business logic code. In your example, you are creating all the objects using Guice. I expect in reality you would need to create the <code>Order</code> at run-time based on user input.</p> <p>Service objects can depend on value objects, but never the other way around. I think you should be looking to implement:</p> <p><code>checkoutService.payfor( order, method );</code></p> <p>and not <code>order.finishOrder( method )</code></p> <p>Within the <code>CheckoutService</code> class, you could choose an approriate <code>PaymentService</code> and pass the <code>order</code> to it. The <code>CheckoutService</code> would take as constructor arguments <code>PaymentCardPaymentService</code> (equivalent to your PaymentCardImpl) and <code>CashPaymentService</code> (equivalent to your PaymentCashImpl).</p>

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