CODEWITHSUNDEEP
javadesign-patterns
sidgate
sidgate

Reputation: 15234

Command pattern with too many classes

Our legacy code has a long code of if else blocks that depend on events and object type

if(event == A && objectType == O1){
.....
}
else if (event == A && objectType == O2){
 ....
}
else if (....)
....
....

With more and more conditions introducing, I was thinking of replacing this logic with Command pattern for each condition. But the number of classes required would be (no. of events) * (no. of object types). Is there any simpler way to refactor this code?

Upvotes: 5

Views: 787

Answers (2)

Bob Dalgleish
Bob Dalgleish

Reputation: 8227

The pattern you may be looking for is often called double dispatching or sometimes Visitor pattern. http://en.wikipedia.org/wiki/Visitor_pattern

Create a set of classes for events and a set for object types. Create an interface 

public interface VisitEvent {
    public void visit(EventA eventA);
    public void visit(EventB eventB);
    // for each event class
}

Within the event class, you have to invoke the visit pattern on the object type class.

public class EventA {
    public void visit(ObjectTypeParent otp) {
        otp.visit(this);
    }
}

Presuming that the object type classes inherit from a common class

public abstract class ObjectTypeParent implements VisitEvent {
    public void visit(EventA eventA) {
        // default code here
    }
    // same for each event visit from VisitEvent
}

then

public class ObjectType01 extends ObjectTypeParent {
    public void visit(EventA eventA) {
       // stuff you want done for this combination
    }
    // don't implement the ones that have common behavior
}

Upvotes: 3

fge
fge

Reputation: 121702

Create a class enclosing event and objectType, make it implement .equals() and .hashCode(). Create a generic class for each execution block too.

Then you'll be able to use a Map and a simple lookup will return what is needed to execute.

Upvotes: 6

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