Adaptor Pattern

What

Converts the interface of a class into one the client expects. Wraps an object and provides a different interface to it.

When

You want to use an existing class but its interface doesn’t match your needs. You need to make one interface work with another.

class LegacyPrinter {
    public void printOld(String s) {
        System.out.println("Printing: " + s);
    }
}
 
class Client {
    public void print(String s, LegacyPrinter printer) {
        // Client expected `print()` but only `printOld()` exists
        // You’d need to change either the client or the legacy class
        printer.printOld(s); // tightly coupled workaround
    }
}
 

How

• Create an adapter that implements the target interface.
• Adapter holds a reference to the adaptee and translates calls.

interface Target { void request(); }
 
class Adaptee { public void specificRequest() { System.out.println("Adaptee called"); } }
 
class Adapter implements Target {
    private Adaptee adaptee;
    public Adapter(Adaptee a) { this.adaptee = a; }
    public void request() { adaptee.specificRequest(); }
}
 

Examples

• Wrapping legacy APIs for use with new code.
• Adapting a third-party library to fit your application.

Why

Advantages

• Promotes reusability of existing code.
• Enables integration of incompatible interfaces.

Disadvantages

• Can add layers of complexity.
• May not work well if adaptee’s interface is complex or changes frequently.