Adapters

Problem

What if a class exists that provides the desired functionality but doesn't implement the desired interface?

For example, in our cart example, suppose a legacy Print class existed:

class Print {
private double price;
private Artist artist;
public double getPrice() {
if (artist.isFamous()) {
return 10 * price;
} else {
return price;
}
}
}

Assume further that we cannot modify this code either because the source code is unavailable or because the class is being shared.

Solution: The Adapter Design Pattern

We can wrap the legacy class inside of an adapter that implements the desired interface:

class PrintAdapter implements Item {
private Print print;
public PrintAdapter(Print p) {
print = p;
}
public double getValue() {
return print.getPrice();
}
}

Alternatively, we can use inheritance:

class PrintAdapter extends Print implements Item {
public double getValue() {
return getPrice();
}
}