extractinterface

Today we look at an often overlooked refactoring. Extract Interface. When you notice more than one class using a similar subset of methods on a class, it is useful to break the dependency and introduce an interface that the consumers to use. It’s easy to implement and has benefits from loose coupling.

public class ClassRegistration {
	public void create(){
		//creation code
	}
	public void transfer(){
		//transfer code
	}
	private int total;
	public int getTotal() {
		return total;
	}
	public void setTotal(int total) {
		this.total = total;
	}
}
public class RegistrationProcessor {
	public int processRegistration(ClassRegistration registration){
		registration.create();
		return registration.getTotal();
	}
}

In the after example, you can see we extracted the methods that both consumers use and placed them in an interface. Now the consumers don’t care/know about the class that is implementing these methods. We have decoupled our consumer from the actual implementation and depend only on the contract that we have created.

public interface ClassRegistration {
	public void create();
	public int getTotal();

}
public class ClassRegistrationImpl implements ClassRegistration {

	@Override
	public void create() {
		//creation code
	}

	private int total;
	@Override
	public int getTotal() {
		return total;
	}
}

public class RegistrationProcessor {
	public int processRegistration(ClassRegistration registration){
		registration.create();
		return registration.getTotal();
	}

}

This refactoring was first published by Martin Fowler and can be found in his list of refactorings here.

Original C# code from the book

public class ClassRegistration
{
    public void Create()
    {
        // create registration code
    }

    public void Transfer()
    {
        // class transfer code
    }

    public decimal Total { get; private set; }
}

public class RegistrationProcessor
{
    public decimal ProcessRegistration(ClassRegistration registration)
    {
        registration.Create();
        return registration.Total;
    }
}

public interface IClassRegistration
{
    void Create();
    decimal Total { get; }
}

public class ClassRegistration : IClassRegistration
{
    public void Create()
    {
        // create registration code
    }

    public void Transfer()
    {
        // class transfer code
    }

    public decimal Total { get; private set; }
}

public class RegistrationProcessor
{
    public decimal ProcessRegistration(IClassRegistration registration)
    {
        registration.Create();
        return registration.Total;
    }
}