Dependency inversion

Definition

The dependency inversion principle says high-level code should depend on an abstraction, not on a concrete class. The concrete class is passed in from outside, which makes it easy to swap and to test.

Example code

A service that gets its sender from outside

interface MessageSender {
    void send(String to, String text);
}

class OrderService {
    private final MessageSender sender;                // depends on the interface only

    OrderService(MessageSender sender) { this.sender = sender; }

    void placeOrder(String customer) { sender.send(customer, "Your order is confirmed"); }
}

class FakeSender implements MessageSender {            // in a test, pass a fake instead of a real sender
    String lastMessage;
    public void send(String to, String text) { lastMessage = to + ": " + text; }
}

class Demo {
    public static void main(String[] args) {
        FakeSender fake = new FakeSender();
        new OrderService(fake).placeOrder("asha@example.com");
        System.out.println(fake.lastMessage);
    }
}

// Output:
// asha@example.com: Your order is confirmed