Adapter Design Pattern Demystified

Why Adapter Pattern

Recently I bought a laptop and found it difficult to work without a mouse. So I borrowed a mouse from my friend and to my surprise I couldn't plug it into my laptop. It was a PS/2 mouse, but my laptop supports only USB mouse. Now what do you think, a PS/2 to USB adapter to the rescue?


Figure 1: Mouse Laptop USB

Often times when we do development we encounter a situation where we have an old legacy interface that needs to fit into a new interface. In this case you will code a separate class that will make both the interfaces compatible with each other. This wrapper class provides seamless communication with both the old and new system. What you are implementing is in fact an adapter design pattern that in turn is a structural design pattern from the Gang of Four (GoF) software design patterns book.


Figure 2: Adapter Pattern

Implementation


Figure 3: Implement

In our case let's say we have a client class that contains the method Operate Laptop that takes an IUSB input as the parameter (refer to the preceding diagram). The IUSB interface is the new interface that expects objects of classes that implement IUSB. But we have only an object of the class that implements an IPS2. The PS2Mouse class is hence incompatible with the Operate Laptop method, since both exposes two different behaviors. This is like passing a square peg into a round hole. Completely incompatible, right?


Figure 4: Square Peg into a round hole


Figure 5: Connect Pass PS2 Mouse Instance

The first concept to understand when implementing an adapter pattern is to identify the source interface and target interface. In our case the source interface is IPS2 and the target interface is IUSB, both of them expose the OpenPS2 () and OpenUSB() methods respectively.