shallow copy of the array but not a copy of the reference

Question

I stumbled upon a problem that I have to create a method which takes no input and return a shallow copy of the array(Not a copy of the reference) of MarketProduct of this Basket.

I know the solution is the following

public class Basket {
    private MarketProduct[] marketproducts;

    public Basket() {
        this.marketproducts = new MarketProduct[0];
    }

    public MarketProduct[] getProducts() {
        return this.marketproducts.clone();

what I don't understand is 1. if it's shallow copy then it's a copy of the reference right? 2. can I use clone method to any object in java? does it copy the reference or the acutal object? if so then the following can be simplified right(instead of using for loop i can use clone?)?

public void add(MarketProduct input) {
        MarketProduct[] list = new MarketProduct[marketproducts.length];
        for(int i = 0; i < marketproducts.length; i++) {
            list[i] = marketproducts[i];
        }
        list[list.length-1] = input;
        this.marketproducts = list;
    }

No, you cannot just "clone()" any Java object. Yes, Java arrays implement interface [Cloneable](https://docs.oracle.com/javase/8/docs/api/java/lang/Cloneable.html), so you *can* clone them. Your `Basket` has a reference to array `margetProducts`. Each element in marketProducts has a reference to a `MarketProduct` object. When you clone marketProducts, you *keep* the original references to the elements. "clone" or a "for loop", both copy the *reference*. Look [here](https://howtodoinjava.com/array/java-array-clone-shallow-copy/) and [here](https://stackoverflow.com/questions/1175620/) — paulsm4, Jul 06 '19 at 17:03

score 1 · Accepted Answer · answered Jul 06 '19 at 16:53

if it's shallow copy then it's a copy of the reference right?

No. It's a new array containing references to the same objects as the original array

can I use clone method to any object in java?

No. Read the javadoc of Object.clone() and Cloneable to know the rules.

if so then the following can be simplified right(instead of using for loop i can use clone?)?

No, since clone() will give you a new array of the same length, and thus you won't be able to add a new element at the end of the array.

score 1 · Answer 2 · answered Jul 06 '19 at 16:55

if it's shallow copy then it's a copy of the reference right?

No, it's a new reference to a new object with copies of that object's properties. The details depend on the implementation of clone.

can I use clone method to any object in java? does it copy the reference or the acutal object?

First, clone isn't the right tool for a shallow copy of an array. You might use System.arraycopy instead. In your case, you'd allocate an array with room at the end for your new entry, then arraycopy the original into it, then add your entry.

public void add(MarketProduct input) {
    MarketProduct[] list = new MarketProduct[marketproducts.length + 1];
    System.arraycopy(marketproducts, 0, list, 0, marketproducts.length);
    list[list.length-1] = input;
    this.marketproducts = list;
}

Obviously this is a bit expensive, you might look at using one of the List classes (perhaps ArrayList) so you don't grow the array each and every time.

Re clone, see the JavaDoc:

Creates and returns a copy of this object. The precise meaning of "copy" may depend on the class of the object. The general intent is that, for any object x, the expression:
 x.clone() != x
will be true, and that the expression:
 x.clone().getClass() == x.getClass()
will be true, but these are not absolute requirements. While it is typically the case that:
 x.clone().equals(x)
will be true, this is not an absolute requirement.
By convention, the returned object should be obtained by calling super.clone. If a class and all of its superclasses (except Object) obey this convention, it will be the case that x.clone().getClass() == x.getClass().
By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by super.clone before returning it. Typically, this means copying any mutable objects that comprise the internal "deep structure" of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by super.clone need to be modified.
The method clone for class Object performs a specific cloning operation. First, if the class of this object does not implement the interface Cloneable, then a CloneNotSupportedException is thrown. Note that all arrays are considered to implement the interface Cloneable and that the return type of the clone method of an array type T[] is T[] where T is any reference or primitive type. Otherwise, this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.
The class Object does not itself implement the interface Cloneable, so calling the clone method on an object whose class is Object will result in throwing an exception at run time.

So you can't use it on objets that don't implement Cloneable, but you can on objects that do.

shallow copy of the array but not a copy of the reference

2 Answers2