Index 0 out of bounds

package my.code.java;
 
class PizzaType {
    String ingredients[];
    private int price;
    
    void setPrice(int price) {
        this.price = price;
    }
    
    int getPrice() {
        return price;
    }
    
}
 
class Pizza {
    double price = 0;
    int num;
    double sizeMod;
    boolean GreensOn;
    boolean SauceOn;
    Pizza(PizzaType type, int num, String size, boolean greens,
          boolean sauce) {
        this.num = num;
        switch(size) {
            case("small"):
                this.sizeMod = 1;
                break;
            case("medium"):
                this.sizeMod = 1.5;
                break;
            case("big"):
                this.sizeMod = 2;
                break;
            default:
                this.sizeMod = 1;
                break;
        }
        this.price = type.getPrice();
        if(GreensOn) this.price += 10;
        if(SauceOn) this.price += 15;
        this.price *= sizeMod;
    }   
}
 
class Day {
    int p_num;
    Day(int p_num) {
        this.p_num = p_num;
    }
    Pizza order[] = new Pizza[p_num];
    int defineDayIncome() {
        int income = 0;
        for(int i = 0; i < order.length; ++i) {
            income += order[i].num * order[i].price;
        }
        return income;
    }
}
 
public class Main {
 
    public static void main(String args[]) {
        
        // Classic Pizza
        PizzaType classic = new PizzaType();
        classic.ingredients = new String[] {"cheese", "meet"};
        classic.setPrice(200);
        
        // Cheese Pizza
        PizzaType cheese = new PizzaType();
        cheese.ingredients = new String[] {"cheese", "onions"};
        cheese.setPrice(190);
        
        // Vegetable Pizza
        PizzaType vegetable = new PizzaType();
        vegetable.ingredients = new String[] {"tomatoes", "onions"};
        vegetable.setPrice(280);
        
        // Sunday
        Pizza PSun1 = new Pizza(classic, 16, "middle", true, true);
        Pizza PSun2 = new Pizza(classic, 3, "big", true, false);
        Pizza PSun3 = new Pizza(cheese, 20, "middle", false, true);
        Pizza PSun4 = new Pizza(cheese, 9, "big", false, false);
        Pizza PSun5 = new Pizza(vegetable, 13, "small", true, true);
        Pizza PSun6 = new Pizza(vegetable, 15, "small", false, true);
        
        Day Sun = new Day(6);
        Sun.order[0] = PSun1;
        Sun.order[1] = PSun2;
        Sun.order[2] = PSun3;
        Sun.order[3] = PSun4;
        Sun.order[4] = PSun5;
        Sun.order[5] = PSun6;
        System.out.println(Sun.defineDayIncome());
        
    }   
 
}

    Pizza order[] = new Pizza[p_num];

class Day {
    int p_num;
    Pizza order[];
    Day(int p_num) {
        this.p_num = p_num;
        order = new Pizza[p_num];
    }
...
}