CODEWITHSUNDEEP
javavisitor-pattern
Gianni Spear
Gianni Spear

Reputation: 7936

Implementing Visitor Pattern in Java whit two different return types

I am trying to implement the Visitor Pattern in java (exercise for home, sorry) for an object structure which has methods with different return types (int and void).

A concreteVisitor (i.e., CostAss) returns int and a second ConcreteVisitor (i.e., drawCosts) returns void (i.e., a print of the cost).

enter image description here

I have the problem to understand how to implement this problem. I am not allowed to create two accept methods (one int e one void) in the interface Employee

enter image description here

Upvotes: 3

Views: 860

Answers (2)

vincenzopalazzo
vincenzopalazzo

Reputation: 1645

You can create a method accept with the void type return and the two concrete visitors have a state, an example

public interface IVisitor {

    void visit(Employee employee);

    int getStatus();
}

public class VisitPrint implement IVisitor{

    public void visit(Employee employee){
      //todo
    }    

    public int getStatus(){
        throw new IllegalArgumentException("Not supported operation");
    }
}

public class VisitSum implement IVisitor{

    int status;

    public void visit(Employee employee){
      //todo
    }

    public int getStatus(){
        return status;
    }
}

public interface Employee {
  void accept(Visitor visitor);
}

This solution resolves the problem and respect the teacher mentality.

Upvotes: 0

Andrew
Andrew

Reputation: 49626

From a Java perspective, these methods are the same because the return type is not a part of the method signature.

The diagram wasn't intended for Java. However, it's possible to work around it with generics.

interface Employee<T> {
  Optional<T> accept(Visitor visitor);
}

class Assistant implements Employee<Integer> {
  @Override
  public Optional<Integer> accept(Visitor visitor) {
    return Optional.of(100);
  }
}

class Manager implements Employee<Void> {
  @Override
  public Optional<Void> accept(Visitor visitor) {
    return Optional.empty();
  }
}

I am not a huge supporter of this idea, I just wanted to share the way it can be done.

Upvotes: 6

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