Question

I can find plenty of Q&A on here about whether an arraylist can equal null, which was helpful in its own way, but I can’t find an answer for throwing errors if any fields in the arraylist are null. As I’m adding objects to the arraylist, I want to throw an exception if the user tries to pass in anything that is null. Here is the code:

    void addInvoiceItem(Item item, Integer quantity, double discount) throws Exception {    
for (InvoiceItem thing: invoiceList) {
    if (thing == null) {
        throw new IllegalArgumentException("Part of the invoice item is blank (null). Please review your invoice items and ensure you have specified values for the item.");
    }
    else {
        invoiceList.add(thing);
        thing.setItemQuantity(quantity);
        thing.setItemDiscount(discount);
        System.out.println(invoiceList);
    }
}
}

Here is the Item class:

final class Item {

String itemDescription;
double itemPrice;
Integer itemSKU;

Item (String description, double price, Integer sku) {
    this.itemDescription = description;
    this.itemPrice = price;
    this.itemSKU = sku;
}

}

Here are the test methods that are letting me know I’m definitely omitting something. One is to test for a valid InvoiceItem, the other for an invalid one (contains nulls):

public class InvoiceTest {
//create the static values to be used
//for InvoiceItem
String goodDescription = "wheel";
double goodPrice = 500.00;
Integer goodSku = 0002;
Item goodInvoiceItem = new Item(goodDescription, goodPrice, goodSku);

String emptyDescription = null;
double emptyPrice = 0;
Integer emptySku = 0;
Item badInvoiceItem = new Item(emptyDescription, emptyPrice, emptySku);

Integer itemQuantity = 0;
double itemDiscount = 0.05;

@Test
public void invalidItemAddTest() {
    Invoice badInvoice = new Invoice(null);
    try {
        badInvoice.addInvoiceItem(badInvoiceItem, itemQuantity, itemDiscount);
        System.out.println(badInvoice);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

@Test
public void validItemAddTest() {
    Invoice goodInvoice = new Invoice(null);
    try {
        goodInvoice.addInvoiceItem(goodInvoiceItem, itemQuantity, itemDiscount);
        System.out.println(goodInvoice);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Thanks in advance for all your help

Edit with additions:
So Mel’s answer was my starting point and I made some additions to get it working the way I needed to. My adding method now looks like this:

    void addInvoiceItem(Item item, Integer quantity, double discount) {
    if(item == null || quantity == 0 || discount == 0) {
        throw new IllegalArgumentException("Part of the invoice item is blank (null). Please review your invoice items and ensure you have specified values for the item.");
    } else {
    InvoiceItem invoice = new InvoiceItem(item, quantity, discount);
    invoiceList.add(invoice);
}
}

and my test methods look like this:

public class InvoiceTest {
//create the static values to be used
//for InvoiceItem
String goodDescription = "wheel";
double goodPrice = 500.00;
int goodSku = 0002;
Item goodInvoiceItem = new Item(goodDescription, goodPrice, goodSku);

String emptyDescription = null;
double emptyPrice = 0;
int emptySku = 0;
Item badInvoiceItem = new Item(emptyDescription, emptyPrice, emptySku);

int badItemQuantity = 0;
double badItemDiscount = 0;
int goodItemQuantity = 1;
double goodItemDiscount = 0.05;


/**
 * @Before - initialize what we need for the test
 * @throws Exception
 */
@Before
public void setUp() {
    //things needed for testInvalidItemAdd()


}

/**
 * @throws Exception 
 * @Test - confirm you cannot add an item that is null
 */
@Test
public void invalidItemAddTest() {
    boolean exceptionThrown = false;
    Invoice badInvoice = new Invoice(null);
    try {
        badInvoice.addInvoiceItem(badInvoiceItem, badItemQuantity, badItemDiscount);
        System.out.println(badInvoice);
    } catch (Exception e) {
        e.printStackTrace();
        exceptionThrown = true;
    }
    assertTrue(exceptionThrown);
}

@Test
public void validItemAddTest() {
    boolean exceptionThrown = false;
    Invoice goodInvoice = new Invoice(null);
    try {
        goodInvoice.addInvoiceItem(goodInvoiceItem, goodItemQuantity, goodItemDiscount);
        System.out.println(goodInvoice);
    } catch (Exception e) {
        e.printStackTrace();
        exceptionThrown = true;
    }
    assertFalse(exceptionThrown);
}

Answer 1

Your add routine is a little off. It tries to use an item that is already in the list rather than creating a new one. Try this:

void addInvoiceItem(Item item, Integer quantity, double discount) {
    if(
        item == null || 
        quantity == null || 
        item.sku == null ||
        item.description == null
    ) {
        throw new NullPointerException();
    }
    InvoiceItem invoice = new InvoiceItem(item, quantity, discount);
    invoiceList.add(invoice);
}

Also take a look a checkNotNull( ) from the google library to reduce typing a bit.

It may be useful to check for null in the InvoiceItem constructor rather than in the adder, unless you want to allow nulls elsewhere.