when do we need Adapter pattern?

Question

When do we need to go for Adapter pattern? If possible give me a real world example that suits that pattern...

Answer 1

Existing Interface

interface Shape {
    public int calculateArea(int r);
}

Current Implementation for Shape interface

class Square implements Shape {
    @Override
    public int calculateArea(int r) {
        return r * r;
    }
}

Now Consider that you want Circle class to adapt to our existing interface which in no way we can modify (Written by third party).

class Circle {
    public double calculateCircularArea (int r) {
        return 3.14 * r * r;
    }
}

Now we have adapt Circle implementation to our Shape interface. So we need an adaptor as they are incompatible.

class CirCleAdaptor extends Circle implements Shape {
    @Override
    public int calculateArea(int r) {
        return (int) calculateCircularArea(r);
    }
}

CircleAdaptor - Is the Adaptor for Circle
Circle - Is the Adaptee
Shape - Is the Target Interface

public class AdapterPattern {
    public static void main(String[] args) {
        Shape circle = new CirCleAdaptor();
        System.out.println("Circle Area " + circle.calculateArea(5));
        Shape square = new Square();
        System.out.println("Square Area " + square.calculateArea(5));
    }
}

Hope this gives a better idea about When to use it.