notes / design-patterns / structural / adapter.md · 2026-03 · 2 min

# Adapter Pattern

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### Convert the interface of a class into another interface that clients expect.

This means that an adapter allows two incompatible interfaces to work together without modifying their existing code.

## 🍽️ Imagine a restaurant:

### ❌ Bad Design

A new coffee machine arrives, but:

• It uses a different plug
• It doesn’t fit existing sockets

👉 You cannot use it with your current system

### ✅ Good Design with Adapter Pattern

You use an adapter plug:

• Converts the plug type
• Makes the machine compatible

##### You don’t change the machine!

##### You don’t change the socket!

##### You just add an adapter!

## 🧑‍💻 In software:

### ❌ Bad Design

class OldPrinter {
    void printText(String text) {
        System.out.println(text);
    }
}

class Client {
    void print(OldPrinter printer) {
        printer.printText("Hello");
    }
}

Now imagine a new printer with a different interface:

class NewPrinter {
    void printDocument(String doc) {
        System.out.println(doc);
    }
}

👉 Client cannot use NewPrinter without modification

### ✅ Good Design with Adapter Pattern

interface Printer {
    void print(String text);
}

class OldPrinterAdapter implements Printer {
    private OldPrinter oldPrinter;

    OldPrinterAdapter(OldPrinter oldPrinter) {
        this.oldPrinter = oldPrinter;
    }

    public void print(String text) {
        oldPrinter.printText(text);
    }
}

class NewPrinterAdapter implements Printer {
    private NewPrinter newPrinter;

    NewPrinterAdapter(NewPrinter newPrinter) {
        this.newPrinter = newPrinter;
    }

    public void print(String text) {
        newPrinter.printDocument(text);
    }
}

class Client {
    void print(Printer printer) {
        printer.print("Hello");
    }
}

Now both old and new printers can be used without changing the client code.

##### Existing code stays untouched!

##### New functionality is added through adapters!