如何访问对象内的数组

Question

Write a fully-documented class named Menu which stores a list of items in an array and provides an interface to interact with this list. A Menu can hold up to 50 items at a time, so use the final variable MAX_ITEMS = 50.

I created the constructor and a separate class called MenuItem

    public class Menu implements Cloneable {

        final int MAX_ITEMS = 50;

        public Menu(){
            MenuItem[] menu =  new MenuItem[MAX_ITEMS];
        }
    }

I want to create a method that clones a Menu. How do I access the properties of each individual MenuItem within Menu?

Answer 1

I don't think you have to go through all that, assuming A is the menu...

public static Menu clone(){
Menu B = this;
return B;
}

Then simply call it using

Menu B = A.clone();

Answer 2

You're creating MenuItem[] menu as a local variable and never storing it after the constructor. So not only can you not clone it, you can't ever access it again.

Try using a field for the menu variable, like this:

public class Menu implements Cloneable {

    final int MAX_ITEMS = 50;
    private MenuItem[] menu;

    public Menu(){
        menu =  new MenuItem[MAX_ITEMS];
    }
}

Now any method within the Menu class can access the menu you set during construction.

As for cloning, it depends how deeply you want to clone. If you just want a new Menu object that refers to the same menu array in memory, see cloneOne. If you want a new Menu object with a new menu array containing the same objects as the old menu array, see cloneTwo. If you want to go further than that, you'll have to put up some details for the MenuItem class:

public class Menu implements Cloneable {

    final int MAX_ITEMS = 50;
    private MenuItem[] menu;

    public Menu(){
        menu =  new MenuItem[MAX_ITEMS];
    }

    public Menu cloneOne(){
        Menu a = new Menu();
        a.menu = menu;
        return a;
    }

    public Menu cloneTwo(){
        Menu a = new Menu();
        a.menu = new MenuItem[menu.length];
        for(int i = 0; i < menu.length; i++)
            a.menu[i] = menu[i];
        return a;
}