Wednesday, 19 September 2012

Adapter design pattern in java

Real life example:

A very simple example is using phone charger. Suppose your mobile phone needs 9 Volts of supply to get charged but main supply is 220 V which is not what you require but it can be a source and you can have something that can get 9 V out of this 220 V and supply to your phone for charging.
Now phone charger is acting as an adapter for you.  So Basically it is making a relationship between two unrelated interfaces. This  is how Adpater pattern works.

Intent:

As described by Gof:
"Convert the interface of class into another interface clients expect.Adapter lets class work together that couldn't otherwise because of incompatible interfaces".

Also known as:

Wrapper

UML diagram of generic adapter design pattern:

Elements:

  • Target
    • defines domains-specific interface client uses.
  • Client
    • collaborates with objects conforming to target interface.
  • Adaptee
    • defines existing interface that needs adapting
  • Adapter 
    • adapts the interface of adaptee to target interface. 

WorkFlow:

Clients call operations on the adapter instance.In turn adapter calls adaptee operations that carry out the request.

When to use it:

  • You want to use existing class and its interface does not match the one you need.
  • You want to create a resuable class that cooperates with unrelated or unforeseen classes that is, class that don't necessarily have compatible interfaces.

Example:

UML Diagram for example:
Comparing to above generic description of adapter pattern.
My example includes following elements:
  • PrintableList(Target)
  • PrintString(Adaptee)
  • PrintableListAdapter(Adapter)

Java code for all above classes:

Consider that we have a third party library that provides print string functionality through PrintString class.
This is our Adaptee. I know this is silly assumpsion but lets go with it for now.

PrintString.java(Adaptee) :
package org.arpit.javapostsforlearning.designpatterns;

public class PrintString {

 public void print(String s)
 {
  System.out.println(s);
 }
}

Client deals with ArrayList<String> but not with string.We have provided a PrintableList interface that expects the client input.This is our target.

PrintableList.java(Target):
package org.arpit.javapostsforlearning.designpatterns;

import java.util.ArrayList;

public interface PrintableList {
 void printList(ArrayList<String> list);
}
Lets assume we can not change it now.
Finally we have PrintableListAdapter class which will implement PrintableList interface and will deal with our adaptee class.
PrintableListAdapter.java(Adapter):
package org.arpit.javapostsforlearning.designpatterns;

import java.util.ArrayList;

public class PrintableListAdapter implements PrintableList{

 public void printList(ArrayList<String> list) {
 
  //Converting ArrayList<String> to String so that we can pass String to
  // adaptee class
  String listString = "";

  for (String s : list)
  {
      listString += s + "\t";
  }
  
  // instantiating adaptee class
  PrintString printString=new PrintString();
  ps.print(listString);
 }
}

AdapterDesignPatternMain.java:
package org.arpit.javapostsforlearning.designpatterns;

import java.util.ArrayList;
public class AdapterDesignPatternMain {
 
 public static void main(String[] args)
 {
  ArrayList<String> list=new  ArrayList<String>();
  list.add("one");
  list.add("two");
  list.add("three");
  PrintableList pl=new PrintableListAdapter();
  pl.printList(list);
  
 }
}

Output:

one two three
 

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