Concept of ArrayBag

Question

I am having trouble understanding a part of class slide that says:

Storing Items in an ArrayBag :
We store the items in an array of type Object.

public class ArrayBag implements Bag {
    private Object[] items; 
    private int numItems; 
....
}

This allows us to store any type of object in the items array, thanks to the power of polymorphism:

ArrayBag bag = new ArrayBag(); 
bag.add("hello"); 
bag.add(new Double(3.1416));

Is ArrayBag a specific type of object or is it just a Obj variable name?
Why do we need to cast 3.1416 as a Double and add a new?
(I know the code could be just be bag.add(3.1416) and Java will autobox it for you, but I'm having trouble understanding the meaning behind bag.add(new Double(3.1416)).

Are you following this book Data Structures and Other Objects using JAVA?? — ganeshvjy, Jul 18 '15 at 19:46
This is rather dated; a `Bag` interface should use generics and there's been no need to call `new Double()` since Java 1.5. I would not rely on this presentation for modern best practices. — dimo414, Jul 18 '15 at 20:03

AnkeyNigam · Accepted Answer · 2015-07-19T08:20:46.797

Is ArrayBag a specific type of object or is it just a Obj variable name?

ArrayBag is neither the specific type of object not its a Obj variable, its actually a Class.

Why do we need to cast 3.1416 as a Double and add a new?

No you need not to explicitly cast the double as AutoBoxing comes into picture (Java 1.5 and above) and convert double to Double ie from primitive to Object.

I'm having trouble understanding the meaning behind bag.add(new Double(3.1416)

bag is actually an instance of ArrayBag, and add() is a method defined in this class, which takes a parameter of type Object and might be adding it in items array, which is nothing but an array of Object.

Concept of ArrayBag

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