CODEWITHSUNDEEP
javaif-statementdesign-patternspolymorphisminstanceof
privalou
privalou

Reputation: 60

How to implement pattern matching in java and avoid instanceof?

At the moment I don't know how to avoid code smells in my piece of code. I've tried several patterns (Strategy, Visitor) and they didn't provide a clean and maintainable solution. Here is an example of my code for strategy pattern:

public interface Strategy {
  <T> T foo(FirstParam firstParam, SecondParam secondParam);
}

public class StrategyOne implements Strategy {
  FirstReturnType foo(FirstParam firstParam, SecondParam secondParam);
}

public class StrategyTwo implements Strategy {
  SecondReturnType foo(FirstParam firstParam, SecondParam secondParam);
}

@Setter
public class Context {
    private Strategy strategy;
    public void execute(FirstParam firstParam, SecondParam secondParam) {
        if (strategy != null) {
            strategy.fo(firstParam, secondParam);
        }
    }
}

And there is a example of objects.

public abstract class Action {
 abstract void bar();
} 

public class ActionOne extends Action {
  void bar() {}
}

public class ActionTwo extends Action {
  void bar() {}
}

And I want to make this piece of code cleaner 

public class ActionExecutor {
   private Context context;
   private FirstParam firstParam;
   private SecondParam secondParam;
   public ActionExecutor(FirstParam firstParam, SecondParam secondParam) {
    this.context = new Context();
    this.firstParam = firstParam;
    this.secondParam = secondParam;
   }

  public void doSmth(Item item) {
    Action action = item.getAction();
    if(action instanceof ActionOne) {
     context.setStrategy(new StrategyOne());
    }
    if(action instanceof ActionTwo) {
     context.setStrategy(new StrategyTwo());
    }
    context.execute(firstParam, secondParam);
  }
}

The idea is to perform a specific action for a specific object type. But I don't know how to avoid the usage of instanceof in this situation.

Upvotes: 3

Views: 917

Answers (3)

Riaan Nel
Riaan Nel

Reputation: 2535

This seems like a textbook use case for the Factory Method pattern. You can use the same code you have now (or the Map example in another answer), but put it in a factory - then it's purpose-specific and decoupled from the code that uses it.

Something like this.

public class StrategyFactory {

    public static Stategy getStrategy(Action action) {
        if(action instanceof ActionOne) {
            return new StrategyOne();
        } else if(action instanceof ActionTwo) {
            return new StrategyTwo();
        }
    }
}

And then, something like this.

Action action = item.getAction();
action.setStrategy(StrategyFactory.getStrategy(action));

There's another example here: https://dzone.com/articles/design-patterns-the-strategy-and-factory-patterns

Upvotes: 0

Alex Chernyshev
Alex Chernyshev

Reputation: 1745

Could be something like this:

   public void doSmth(Item item) {

    Action action = item.getAction();

    Map<String,Strategy> strategies = new HashMap<>();
    strategies.put(ActionOne.getClass().getSimpleName(),new StrategyOne());
    strategies.put(ActionTwo.getClass().getSimpleName(),new StrategyTwo());
    ..
    strategies.put(ActionHundred.getClass().getSimpleName(),new StrategyHundred());

    if(strategies.containsKey(action.getClass().getSimpleName())) {
     context.setStrategy(strategies.get(action.getClass().getSimpleName()));
    }
    context.execute(firstParam, secondParam);  }

Upvotes: 2

Athanasios Kataras
Athanasios Kataras

Reputation: 26342

Two ways top of my head. 

public abstract class Action {
 public Strategy strategy;
 abstract void bar();
} 

public class ActionOne extends Action {
  void bar() {}
   // set strategy here,  possibly
}

public class ActionTwo extends Action {
  void bar() {}
}

public void doSmth(Item item) {
    Action action = item.getAction();
    action.strategy.execute(firstParam, secondParam);
  }

Second way, have an enum in all your actions and force it by declaring it as a parameter in your abstract class constructor. Then just use switch in instead of instanceof

Upvotes: 2

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