ArrayList 'squash' operation on its elements

Question

I have a problem that seems to be trivial but I'm looking for the best way of resolving that issue.

Let's say I have a class:

Product.class

public class Product {
    private int id;
    private String code;
    private String price;
    private String quantity;

    public Product() {}

    //getters and setters

    @Override
    public boolean equals(Object obj) {
        boolean result = false;
        if (obj instanceof ProductOnStockDto) {
            ProductOnStockDto product = (ProductOnStockDto) obj;
            result = (this.code.equals(product.getCode()) && this.price.equals(product.getPrice()));
        }
        return result;
    }
}

And let's say I have a list of this objects:

List<Product> products;

Which is filled with data like this(1 row is one list element):

And what I need is to iterate over this list and squash elements that have the same code and the same price. For example:

In the result list I would like to have two products for code 3:

code 3 | 1.22 | 11
code 3 | 2.20 | 6

So I would do sth like this:

for(Product product : products) {
    for(Product productToSquash : products) {
        if(product.getId() != productToSquash.getId() && product.equals(productToSquash)) {
            //Here I would like to squash these two rows
            //Data conversion is ommited
            product.setQuantity(product.getQuantity() + productToSquash.getQuantity());
            //after that I would like to remove productToSquash object from the collection
        }
    }
}

But as I know it is not allowed to modify collection that I'm iterating on. So what would be the best way to squash all product list according to the example?

Answer 1

First of all, your equals() method refers to ProductOnStockDto. That should be Product.

To remove elements during iteration, use the Iterator directly, i.e. use an "old style" for-loop, and use a Map<Product, Product> to keep track of products previously seen. This requires that you also implement hashCode():

@Override
public int hashCode()
{
    return this.code.hashCode() * 37 + this.price.hashCode();
}

Map<Product, Product> map = new HashMap<Product, Product>();
for (Iterator<Product> productIter = products.iterator(); productIter.hasNext(); ) {
    Product product = productIter.next();
    Product productToKeep = map.get(product);
    if (productToKeep == null)
        map.put(product, product);
    else {
        productToKeep.setQuantity(productToKeep.getQuantity() + product.getQuantity());
        productIter.remove();
    }
}

You shouldn't really be doing it this way, since the equals() method is returning true for objects that are not truly equal.

You should instead have a key class with just code and price, and the key class is one that needs to implement equals() and hashCode(), not Product.