Can I create a decorated object based on a list?

Question

I have a program that can print a pizza with decorators. I have an interface:

public interface PizzaPie{
    String top();
}

And an implementation of the interface

public class PizzaPieImplementation implements PizzaPie{

    @Override
    public String top() {
        return "Pie of pizza";
    }
}

And an abstract class that implements it with the same object.

public abstract class PizzaTopper implements PizzaPie{
    private PizzaPie pizza;
    
    @Override
    public String top() {
        return pizza.top();
    }
}

And I have several decorator classes, such as

public class Onions extends PizzaTopper{

    public Onions(PizzaPie pizza) {
        super(pizza);
    }
    
    public String top() {
        return super.top() + topWithOnions();
    }
    
    private String topWithOnions() {
        return " with onions";
    }

And similar classes for peppers, pepperoni, anchovies, pineapple, etc.

I have a list as follows:

List<String> toppings = {onions, pineapple};

Is there a way to take each topping from the toppings list, and use that to create a new pizza with those toppings, to return something like:

Pie of pizza with onions with pineapple

The method would look something like this:

    public PizzaPie CreatePizzaWithUserInput(List<String> toppings) {
            //code
    }
    

And ultimately it would create code that looks like this:

PizzaPie pizza1 = new Onion(new Pineapple(new PizzaPieImplementation()));

In theory this can be done with a lot of ugly if statements but I'm wondering if there's a quicker way of doing it.

Answer 1

You can use Java Reflection to achieve this.

Here we build our factory with the different possibilities of toppings, then we create a Pizza using createPizzaWithUserInput. In this example we built a pizza with 3 of the four possible toppings. Notice that to add a new Topping possibility you just extends PizzaTopper and add it on the toppingOptions on the PizzaFactory instantiation.

import java.util.List;
import java.util.LinkedList;

class Main {
  public static void main(String[] args) {
    // Possibilities
    PizzaFactory pizzaFactory = new PizzaFactory(Onions.class, Bacon.class, Olives.class, Tomatos.class);

    // User input
    List<String> toppings = new LinkedList<String>();
    toppings.add("onions");
    toppings.add("olives");
    toppings.add("bacon");
    try{
      PizzaPie pizza = pizzaFactory.createPizzaWithUserInput(toppings);
    System.out.println(pizza.top());
    } catch (Exception e) {
      e.printStackTrace();
    }
    
  }
}

Here we have the PizzaFactory. We use the class' name to do the matching with the topping option, but you can customize it if you want (to match also "onion"/"onions" or "tomato"/"tomatos" for instance).

import java.util.List;
import java.util.Arrays;
import java.lang.reflect.*;

public class PizzaFactory {

  List<Class<? extends PizzaTopper>> toppingOptions;

  public PizzaFactory(Class<? extends PizzaTopper>... toppingOptions) {
    this.toppingOptions = Arrays.asList(toppingOptions);
  }

  public PizzaPie createPizzaWithUserInput(List<String> toppings) throws NoSuchMethodException, InstantiationException, IllegalAccessException,InvocationTargetException {
    PizzaPie pizza = new PizzaPieImplementation();
    for(String toppingName : toppings) {
       for(Class<? extends PizzaTopper> top : toppingOptions) {
         if(top.getName().toLowerCase().equals(toppingName.toLowerCase())) {
           Constructor<? extends PizzaPie> constructor = top.getConstructor(PizzaPie.class);
           pizza = constructor.newInstance(pizza);
         }
       }       
    }
    return pizza;
  }
}

Here we have the interface PizzaPie:

public interface PizzaPie{
  
    String top();
}

And its implementation.

public class PizzaPieImplementation implements PizzaPie{

    @Override
    public String top() {
        return "Pie of pizza";
    }
}

The decorator class.

public abstract class PizzaTopper implements PizzaPie{
    private PizzaPie pizza;

    protected PizzaTopper(PizzaPie pizza) {
      this.pizza = pizza;
    }
    
    @Override
    public String top() {
        return pizza.top();
    }
}

Tomatos topping.

public class Tomatos extends PizzaTopper{

    public Tomatos(PizzaPie pizza) {
        super(pizza);
    }
    
    public String top() {
        return super.top() + topWithOnions();
    }
    
    private String topWithOnions() {
        return " with tomatos";
    }
}

Onions topping.

public class Onions extends PizzaTopper{

    public Onions(PizzaPie pizza) {
        super(pizza);
    }
    
    public String top() {
        return super.top() + topWithOnions();
    }
    
    private String topWithOnions() {
        return " with onions";
    }
}

Olives topping

public class Olives extends PizzaTopper{

    public Olives(PizzaPie pizza) {
        super(pizza);
    }
    
    public String top() {
        return super.top() + topWithOnions();
    }
    
    private String topWithOnions() {
        return " with olives";
    }
}

Bacon topping.

public class Bacon extends PizzaTopper{

    public Bacon(PizzaPie pizza) {
        super(pizza);
    }
    
    public String top() {
        return super.top() + topWithOnions();
    }
    
    private String topWithOnions() {
        return " with bacon";
    }
}