Question

I have the following hierarchy of classes:

public interface Message
public interface V2Message extends Message
public interface V3Message extends Message

I defined another interface to validate a message.

public interface Validation {
  boolean validate(Message message);
}

Each class that implements Validation can deal with V2 and/or V3 messages, so in my implementation I have to distinguish the kind of message I want to validate because the code will be different.

public class MyValidation implements Validation {
  public boolean validate(Message message) {
    if(message instanceof V2Message) {
      return validateV2((V2Message)message);
    } else if (message instanceof V3Message) {
      return validateV3((V3Message)message);
    }
}

I was wondering if there is a way to remove the use of instanceof.

Answer 1

You could give Validate two methods rather than one:

public interface Validation {
  boolean validate(V2Message message);
  boolean validate(V3Message message);
}

That would also prevent passing in a simple Message being a runtime error. This is also a more accurate API, since you’ve said that Validation implementations can’t deal with Message, only V2Message and V3Message.