JAVA EE: Adapter Pattern – Implementation [Class Adapter]

Thursday, 7 November 2013

Adapter Pattern – Implementation [Class Adapter]

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Introduction

The Adapter Design Pattern can be implemented in two ways: Object Adapter (using composition) and Class Adapter (using inheritance). In this tutorial, we focus on the Class Adapter approach using a real-world enterprise integration scenario: connecting an existing Human Resource (HR) System with a Third-Party Billing System.

The Incompatibility Problem

Client (HR System): Stores employee data as a two-dimensional String[][] array.
Adaptee (Third-Party Billing System): Contains a processSalary() method that only accepts a List<Employee>.

Neither system should be modified. To bridge this gap, we create a Class Adapter that implements the target interface required by the client and inherits (extends) the third-party billing class directly.

Class Adapter Role Mapping

Pattern Role	Component in Example	Role Description
Client	`HumanResourceSystem`	Supplies employee details in `String[][]` format
Target Interface	`ITarget`	Declares `processCompanySalary(String[][])`
Adaptee	`ThirdPartyBillingSystem`	Provides `processSalary(List<Employee>)`
Class Adapter	`EmployeeAdapter`	Extends `ThirdPartyBillingSystem` and implements `ITarget`

Step-by-Step Java Implementation

1. The Employee Model Class

public class Employee {
    private int empId;
    private String name;
    private String designation;

    public Employee(int empId, String name, String designation) {
        this.empId = empId;
        this.name = name;
        this.designation = designation;
    }

    public int getEmpId() { return empId; }
    public String getName() { return name; }
    public String getDesignation() { return designation; }
}

2. Adaptee: ThirdPartyBillingSystem

import java.util.List;

public class ThirdPartyBillingSystem {
    public void processSalary(List<Employee> employeeList) {
        for (Employee emp : employeeList) {
            int salary = 0;
            if ("Team Leader".equalsIgnoreCase(emp.getDesignation())) {
                salary = 70000;
            } else if ("Developer".equalsIgnoreCase(emp.getDesignation())) {
                salary = 40000;
            } else if ("Tester".equalsIgnoreCase(emp.getDesignation())) {
                salary = 30000;
            }
            System.out.println("Credited Rs. " + salary + " to " + emp.getName() + " (" + emp.getDesignation() + ")");
        }
    }
}

3. Target Interface: ITarget

public interface ITarget {
    void processCompanySalary(String[][] employeesArray);
}

4. Class Adapter: EmployeeAdapter

Notice how EmployeeAdapter extends ThirdPartyBillingSystem (inheritance) instead of holding an instance:

import java.util.ArrayList;
import java.util.List;

public class EmployeeAdapter extends ThirdPartyBillingSystem implements ITarget {

    @Override
    public void processCompanySalary(String[][] employeesArray) {
        List<Employee> employeeList = new ArrayList<>();

        for (String[] empData : employeesArray) {
            int id = Integer.parseInt(empData[0]);
            String name = empData[1];
            String designation = empData[2];
            employeeList.add(new Employee(id, name, designation));
        }

        // Direct call to inherited superclass method
        processSalary(employeeList);
    }
}

5. Client: HumanResourceSystem

public class HumanResourceSystem {

    public static String[][] getEmployees() {
        return new String[][] {
            {"101", "Dave", "Team Leader"},
            {"102", "Peter", "Developer"},
            {"103", "Raj", "Developer"},
            {"104", "Rahul", "Tester"}
        };
    }

    public static void main(String[] args) {
        ITarget target = new EmployeeAdapter();
        String[][] employeesArray = getEmployees();
        
        System.out.println("HR System passing employee array to adapter...\n");
        target.processCompanySalary(employeesArray);
    }
}

Console Output

HR System passing employee array to adapter...

Credited Rs. 70000 to Dave (Team Leader)
Credited Rs. 40000 to Peter (Developer)
Credited Rs. 40000 to Raj (Developer)
Credited Rs. 30000 to Rahul (Tester)

Employee.java

public class Employee
{
private int empId;
private String name;
private String designation;

public Employee( int empId, String name, String designation )
{
super();
this.empId = empId;
this.name = name;
this.designation = designation;
}

public int getEmpId()
{
return empId;
}

public void setEmpId( int empId )
{
this.empId = empId;
}

public String getName()
{
return name;
}

public void setName( String name )
{
this.name = name;
}

public String getDesignation()
{
return designation;
}

public void setDesignation( String designation )
{
this.designation = designation;
}

@Override
public String toString()
{
return "Employee [empId=" + empId + ", name=" + name + ", designation=" + designation + "]";
}

}

ThirdPartyBillingSystem.java

import java.util.ArrayList;

/*
* ThirdPartyBillingSystem accepts employees information as a List to process each employee salary
*/
public class ThirdPartyBillingSystem
{
public void processSalary( ArrayList<Employee> employeeList )
{
for( Employee employee : employeeList )
{
System.out.print("\n"+employee.toString()+" : ");
if( employee.getDesignation().equalsIgnoreCase("Team Leader") )
{
System.out.println("70000Rs Salary credited to " + employee.getName() + " Account\n");
}
else if( employee.getDesignation().equalsIgnoreCase("Developer") )
{
System.out.println("40000Rs Salary credited to " + employee.getName() + " Account\n");
}
else if( employee.getDesignation().equalsIgnoreCase("Tester") )
{
System.out.println("30000Rs Salary credited to " + employee.getName() + " Account\n");
}
}
}

}

ITarget.java

public interface ITarget
{
public void processCompanySalary(String[][] employeeInfo);
}

EmployeeAdapter.java

import java.util.ArrayList;

public class EmployeeAdapter extends ThirdPartyBillingSystem implements ITarget
{

/*
* This Adapter method converts String Array employee information to
* ArrayList of Employees
*/

@Override
public void processCompanySalary( String[][] employeeInfo )
{
String empId = null;
String name = null;
String designation = null;
ArrayList<Employee> employeeList = new ArrayList<Employee>();
for( int i = 0; i < employeeInfo.length; i++ )
{

for( int j = 0; j < employeeInfo[i].length; j++ )
{
if( j == 0 )
{
empId = employeeInfo[i][j];
}
else if( j == 1 )
{
name = employeeInfo[i][j];
}
else
{
designation = employeeInfo[i][j];
}
}
employeeList.add(new Employee(Integer.parseInt(empId), name, designation));

}

System.out.println("Adapter converted Array of Employee to ArrayList of Employee : \n"+employeeList+"\n"+
"then call the processSalary method inherited from the ThirdPartyBillingSystem for processing the employee salary");
processSalary(employeeList);

}

}

HumanResourceSystem.java

/*
* Client System which has employee information as String Array
*
*/

public class HumanResourceSystem
{

public static String[][] getEmployees()
{
String[][] employees = new String[4][];

employees[0] = new String[] { "100", "Dave", "Team Leader" };
employees[1] = new String[] { "101", "Ram", "Developer" };
employees[2] = new String[] { "102", "Raj", "Developer" };
employees[3] = new String[] { "103", "Rahul", "Tester" };

return employees;
}

public static void main( String args[] )
{

ITarget target = new EmployeeAdapter();
System.out.println("HR system passes employee string array to Adapter\n");
target.processCompanySalary(getEmployees());

}
}

Output

HR system passes employee string array to Adapter

Adapter converted Array of Employee to ArrayList of Employee :
[Employee [empId=100, name=Dave, designation=Team Leader], Employee [empId=101, name=Ram, designation=Developer], Employee [empId=102, name=Raj, designation=Developer], Employee [empId=103, name=Rahul, designation=Tester]]
then call the processSalary method inherited from the ThirdPartyBillingSystem for processing the employee salary

Employee [empId=100, name=Dave, designation=Team Leader] : 70000Rs Salary credited to Dave Account

Employee [empId=101, name=Ram, designation=Developer] : 40000Rs Salary credited to Ram Account

Employee [empId=102, name=Raj, designation=Developer] : 40000Rs Salary credited to Raj Account

Employee [empId=103, name=Rahul, designation=Tester] : 30000Rs Salary credited to Rahul Account