Adapter design pattern
The Adapter design pattern is a structural
pattern that allows the interface of an existing class to be used as
another interface. It acts as a bridge between two incompatible interfaces,
making them work together. This pattern involves a single class, known as the
adapter, which is responsible for joining functionalities of independent or
incompatible interfaces.
Let's understand this concept using a simple example:
Let's say you have two friends, one who speaks only
English and another who speaks only French. You want them to communicate, but
there's a language barrier.
- You
act as an adapter, translating messages between them. Your role allows the
English speaker to convey messages to you, and you convert those messages
into French for the other person.
- In
this way, despite the language difference, your adaptation enables smooth
communication between your friends.
- This
role you play is similar to the Adapter design pattern, bridging the gap
between incompatible interfaces.
Components of Adapter Design Pattern in Java
1. Target Interface
- Description: Defines
the interface expected by the client. It represents the set of operations
that the client code can use.
- Role: It's
the common interface that the client code interacts with.
2. Adaptee
- Description: The
existing class or system with an incompatible interface that needs to be
integrated into the new system.
- Role: It's
the class or system that the client code cannot directly use due to
interface mismatches.
3. Adapter
- Description: A
class that implements the target interface and internally uses an instance
of the adaptee to make it compatible with the target interface.
- Role: It
acts as a bridge, adapting the interface of the adaptee to match the
target interface.
4. Client
- Description: The
code that uses the target interface to interact with objects. It remains
unaware of the specific implementation details of the adaptee and the
adapter.
- Role: It's
the code that benefits from the integration of the adaptee into the system
through the adapter.
How Adapter Design Pattern works?
- Client
Request: The client initiates a request by calling a method on
the adapter using the target interface.
- Adapter
Translation: The adapter translates or maps the client's request
into a form that the adaptee understands, using the adaptee's interface.
- Adaptee
Execution: The adaptee performs the actual work based on the
translated request from the adapter.
- Result
to Client: The client receives the results of the call, remaining
unaware of the adapter's presence or the specific details of the adaptee.
Example :
1. Target Interface
package adapterdesignpattern;
public interface PaymentGateway {
void pay(double amount);
}
2. Adaptee
package adapterdesignpattern;
public class PayPal {
void
send(double amount) {
System.out.println("Paid
$" + amount + " via PayPal.");
}
}
package adapterdesignpattern;
public class Stripe {
void
makePayment(int amount) {
System.out.println("Paid
$" + amount + " via Stripe.");
}
}
package adapterdesignpattern;
public class RozarPay {
void
doTransaction(String amount) {
System.out.println("Paid
$" + amount + " via RozarPay.");
}
}
3. Adapter
package adapterdesignpattern;
public class PayPalAdapter implements
PaymentGateway {
private
PayPal payPal=new PayPal();
@Override
public
void pay(double amount) {
payPal.send(amount);
}
}
package adapterdesignpattern;
public class StripeAdapter implements
PaymentGateway {
private
Stripe stripe = new Stripe();
@Override
public
void pay(double amount) {
stripe.makePayment((int)
amount);
}
}
package adapterdesignpattern;
public class RozarPayAdapter implements
PaymentGateway {
private
RozarPay rozarPay=new RozarPay();
@Override
public
void pay(double amount) {
rozarPay.doTransaction(String.format("%.2f",
amount));
}
}
4. Client
package adapterdesignpattern;
public class PaymentGatewayDemo {
public
static void main(String[] args) {
PaymentGateway
payPalAdapter = new PayPalAdapter();
payPalAdapter.pay(500.0);
PaymentGateway
stripeAdapter = new StripeAdapter();
stripeAdapter.pay(800.0);
PaymentGateway
rozarPayAdapter = new RozarPayAdapter();
rozarPayAdapter.pay(900.0);
}
}
O/p:
Paid $500.0 via PayPal.
Paid $800 via Stripe.
Paid $900.00 via RozarPay.
Where Adapter Pattern is Used in Java
1. Java I/O (Input/Output) API
- Example:
InputStreamReader
- Use:
Adapts a ByteStream (InputStream) into a CharacterStream (Reader).
- Pattern:
Adapter
Reader reader = new InputStreamReader(new FileInputStream("file.txt"));
- How
it works: InputStreamReader adapts the InputStream to be usable as a Reader.
2. Java Collections Framework
- Example:
Arrays.asList(T... a)
- Use:
Converts (adapts) an array to a List.
List<String> list = Arrays.asList("a", "b",
"c");
- How
it works: Wraps the array and exposes it as a List.
3. Java AWT/Swing GUI Event Listeners
- Example:
MouseAdapter, KeyAdapter, etc.
- Use:
Adapter classes provide default (empty) implementations so developers can
override only needed methods.
frame.addMouseListener(new MouseAdapter() {
public void mouseClicked(MouseEvent
e) {
System.out.println("Mouse clicked");
}
});
- How
it works: MouseAdapter adapts the complex MouseListener interface by
providing empty methods.
4. JDBC (Java Database Connectivity)
- Example:
Wrapping custom ResultSet or Connection implementations
- Use:
JDBC drivers adapt vendor-specific DB operations to the standard java.sql.Connection,
ResultSet, etc.
- How
it works: Adapters ensure all drivers follow the standard JDBC
interface, despite underlying DB differences.
5. Third-party Library Integration
- Use:
Adapting a third-party library's class/interface to your application’s
expected interface.
- Example:
Suppose a library gives you a LoggerX class, but your code expects a Logger
interface — you can create an adapter.
6. Legacy Code Integration
- Use:
When working with older systems whose classes can’t be modified, you write
an adapter to make them compatible with your new system.
