JAVA ArrayList retrieve all objects from ArrayList method

Question

I have an interface test class that implements another super-class. In the test class I have a method that is supposed to retrieve all objects in the ArrayList. First it checks to see if it is empty. If not, it should return the whole list. Here is what I have attempted.

public class ProductDBImpl implements ProductDB {

// field declarations
ArrayList<Product> products = new ArrayList<Product>();
@Override
public List<Product> getAllProducts() 
{
    // TODO Auto-generated method stub
    // TODO Auto-generated method stub
    // create an iterator for the Arraylist

    if (products.isEmpty())
    {
        return new ArrayList<Product>() ;
    }
    return products;
}

Unfortunately, when I run my test script (tests the interface test class and super-class) the getAllProducts() method is not working the way it should and it is returning a productdb.util.AssertionFailedError: expected:<1> but was:<0> message. Here is the way the test script is testing the getAllProducts() method:

    // testing getAllProducts()
    List<Product> productList = productDB.getAllProducts();     
    int size = productList.size();      
    productList.remove(0);      
    productList = productDB.getAllProducts();       
    Assert.assertEquals(size, productList.size());

Please help me identify my error. Thanks.

Is the above actual implementation of getAllProducts() or blue rpint? — NIKET BHANDARY
– NIKET BHANDARY, Commented Aug 21, 2015 at 20:43
yes the above is the actual implementation of getAllProducts(). — CircAnalyzer
– CircAnalyzer, Commented Aug 21, 2015 at 20:44
overridden method is doing that much only ,checking for empty . — NIKET BHANDARY
– NIKET BHANDARY, Commented Aug 21, 2015 at 20:45
PS. in getAllProducts() you have a comment that says 'create an iterator', except the method signature requires returning a List, not Iterator. — dsh
– dsh, Commented Aug 21, 2015 at 20:45
You remove the single element of the list and then you expect to keep its size? What? You know what references are and how they work? ... — Tom
– Tom, Commented Aug 21, 2015 at 20:46

dsh · Accepted Answer · 2015-08-21 20:48:43Z

2

You return a reference to the ArrayList object. Then you modify the object. Then you get another reference to it and discover that the object was modified.

If you want to protect the data structure from external modification, then return a new List or an UnmodifiableList:

@Override
public List<Product> getAllProducts() 
{
    return new ArrayList<>(products);
}

Or, to avoid extra memory allocation and copies (note that this will cause the call to .remove() in your test code to fail; the important part is whether you must allow callers to modify the list they receive or not):

@Override
public List<Product> getAllProducts() 
{
    return Collections.unmodifiableList(products);
}

edited Aug 21, 2015 at 20:48

answered Aug 21, 2015 at 20:36

dsh

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4 Comments

CircAnalyzer Over a year ago

Should I implement the above code as is, i.e remove the if block?

dsh Over a year ago

@DeeTee let me ask you: what will be different with and without the if block? : )

CircAnalyzer Over a year ago

I guess either way it is going to return the ArrayList products as is; with or without objects in it...

dsh Over a year ago

Correct: the ArrayList constructor will not fail when the provided Collection is empty. So either way you make a new ArrayList that is empty, or has references to each of the elements of the original.

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