Structural Patterns - Adapter Pattern

The Adapter pattern is used so that two unrelated interfaces can work together. The joining between them is called an Adapter. This is something like we convert interface of one class into interface expected by the client. We do that using an Adapter.

Let’s try and understand this with the help of an example. Again, I will like to take a general example. We all have electric sockets in our houses of different sizes and shapes. I will take an example of a socket of 15 Ampere. This is a bigger socket and the other one which is smaller is of 5 Ampere. A 15 Amp plug cannot fit into a 5 Amp socket. Here, we will use an Adapter. The adapter can be called a connector here. The connector connects both of these and gives output to the client plug which is of 5 Amp.

The Adapter is something like this. It will be having the plug of suitable for 15 Amp and a socket suitable for a 5 Amp plug. So, that the 5 Amp plug which here is the client can fit in and also the server which here is the 15 Amp socket can give the output.

Let’s try and convert the same example into a software program. How do we do this? Let’s try and understand the problem once more. We have a 5 Amp plug and want a 5 Amp socket so that it can work. We DO NOT have a 5 Amp socket, what we have is a 15 Amp socket in which the 5 Amp plug cannot fit. The problem is how to cater to the client without changing the plug or socket.

The Adapter Pattern can be implemented in two ways, by Inheritance and by Composition.

Here is the example of Adapter by Inheritance:

Let’s say there is a socket interface.

Socket.java

package structural.adapter.inheritance; /** * The socket class has a specs for 15 AMP. */ public interface Socket {
	/** * This method is used to match the input to be * given to the Plug * * @return Output of the Plug (Client) */ public String getOutput();
}// End of interface

And there is a class Plug which wants the input of 5 AMP. This is the client.

Plug.java

package structural.adapter.inheritance; /** * The input for the plug is 5 AMP. which is a * mismatch for a 15 AMP socket. * * The Plug is the client. We need to cater to * the requirements of the Plug. */ public class Plug {
	private String specification = "5 AMP"; public String getInput() { return specification; }
}// End of class

Finally, there will be an adapter class. This will inherit the socket and give output for Plug.

ConnectorAdapter.java

package structural.adapter.inheritance; /** * ConnectorAdapter has is the connector between * the socket and plug so as to make the interface * of one system to suit the client. */ public class ConnectorAdapter implements Socket {
	/** * Method coming from the interface * Socket which we have to make to * fit the client plug * * @return Desired output of 5 AMP */ public String getOutput() { Plug plug = new Plug(); String output = plug.getInput(); return output; }
}// End of class

This class implements the getOutput() method of Socket and sets it to fit the client output.

Similarly, let’s consider the Association and Composition of objects by which Adapter can be implemented.

The class Socket gives the 15 AMP output.

Socket.java

package structural.adapter.composition; /** * Class socket giving the 15 AMP output. */ public class Socket {
	/** * Output of 15AMP returned. * * @return Value of output from socket */ public String getOutput() { return "15 AMP"; }
}// End of class

There is an interface Plug.java which has a method getInput(). This is the client and we need to adapt the output for this input which is 5 AMP.

Plug.java

package structural.adapter.composition; /** * The input for the plug is 5 AMP. which is a * mismatch for a 15 AMP socket. * * The Plug is the client. We need to cater to * the requirements of the Plug. */ public interface Plug {
	public String getInput();
}// End of class

Plug5AMP is the implementation of Plug which requires 5 AMP of input.

Plug5AMP.java

package structural.adapter.composition; public class Plug5AMP implements Plug {
	/** * Get the input of client i.e. Plug * * @return 5 AMP */ public String getInput() { return "5 AMP"; }
}// End of class

The Adapter here takes output from the Socket. If the output is what is needed, it gives it to the Plug else, it overrides the value and returns the adapter output.

ConnectorAdapter.java

package structural.adapter.composition; /** * Using composition */ public class ConnectorAdapter {
	Plug5AMP plug5; public ConnectorAdapter(Plug5AMP plug) { this.plug5 = plug; } public static void main(String[] args) { // Taking output from the Socket Socket socket = new Socket(); String outputFromSocket = socket.getOutput(); // Giving away input to the Plug ConnectorAdapter adapter = new ConnectorAdapter(new Plug5AMP()); String inputToPlug = adapter.getAdapterOutput(outputFromSocket); System.out.println("New output by adapter is: "+inputToPlug); } public String getAdapterOutput(String outputFromScoket) { /* * if output is same, return */ if (outputFromScoket.equals(plug5.getInput())) { return outputFromScoket; } /* * Else, override the value by adapterOutput */ else { String adapterOutput = plug5.getInput(); return adapterOutput; }
}// End of class

This is how the Adapter pattern works. When one interface cannot be changed and has to be suited to the again cannot-be-changed client, an adapter is used so that both the interfaces can work together.