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In all my previous posts I wrote about Creational Design Patterns: Singleton, Abstract Factory, Factory method, Prototype and Builder. In this posts I will introduce you with: Adapter - Structural Pattern.
Definition of “Adapter” says:
Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces, so it converts the interface of a class into another interface clients expect. Here is UML diagram

Here is the Java code which demonstrates using of Adapter pattern. This is abstract class Shape. I use it to define some very simple interface.

abstract class Shape {
	public abstract double GetVolume();
}

and now I create 2 clases which implements this interface

public class Square extends Shape {

	private int radius;

	public Square(int r)
	{
		radius = r;
	}

	public double GetVolume()
	{
		return radius * radius;
	}
}

public class Triangle extends Shape {

	private int a;

	public Triangle(int x)
	{
		a = x;
	}

	public double GetVolume()
	{
		return (a * a * Math.sqrt(3)) /4;
	}
}

and what if we have one more class but with the wrong interface (functionality is ok, but interface not)

public class XCircle {

	private int radius;

	public XCircle(int r)
	{
		radius = r;
	}

	public double XGetVolume()
	{
		return radius * radius * Math.PI;
	}
}

but we need Cirlce class. Here is what we do than:

public class Circle extends Shape {

	private int radius;
	private XCircle circle;
	public Circle(int r)
	{
		radius = r;
		circle = new XCircle(radius);
	}

	public double GetVolume()
	{
		return circle.XGetVolume();
	}
}

at the end here is simple client application to demonstrate how it works:

public class Circle extends Shape {

	private int radius;
	private XCircle circle;
	public Circle(int r)
	{
		radius = r;
		circle = new XCircle(radius);
	}

	public double GetVolume()
	{
		return circle.XGetVolume();
	}
}

that is all