Java: How do i create objects in a static method a

I am doing this Java assignment for hours and stuck with this tester class for very almost 5 hours.

In this assignment, I have created a Product class, a Money class, a LineItem class and an Inventory class. Now i need to create a test class to test the program by putting new lineitems into the inventory array.

In the tester class, I am trying to create a static method public static void addTestItems(Inventory theInventory) which suppose to add 4 items. For each item I will need to create a product object followed by a LineItem object to contain the newly created product. next i need to use a method from the inventory class to add the items into the array in the inventory class.

What i have tried too so far:

private static void addTestItems(Inventory theInventory)
{
    Inventory[] _items;
    Product product1 = new Product("Book","Objects first with Java"," An excellent introductory Java textbook");
    Product product2 = new Product("CD","The dark side of the moon","The all-time classic Pink Floyd album");
    Product product3 = new Product("DVD", "Transformers","Robots in disguise");
    Product product4 = new Product("Laptop","Lenovo T42","A good yet affordabble laptop");
    Money unitPrice1 = new Money(29,99);
    Money unitPrice2 = new Money(4,99);
    Money unitPrice3 = new Money(9,99);
    Money unitPrice4 = new Money(450,0);
    _items[0] = new LineItem(product1,5,unitPrice1);
    _items[1] = new LineItem(product2,8,unitPrice2);
    _items[2] = new LineItem(product3,200,unitPrice3);
    _items[3] = new LineItem(product4,9,unitPrice4); 
}

The current error is incompatible types- found LineItem but expected Inventory so i tried changing Inventory[] _items; to LineItem[] _items;. But the error was variable _items may not be initialise.

Sorry guys I am a real noob in Java, I tried searching on-line for ages but I do not quite understand most results. The only one i understand was http://forums.devshed.com/java-help-9/bluej-compiler-error-cannot-find-symbol-variable-object-688573.html but i tired putting into my context but failed. I also found lot of results but they had constructors and instance variables in them which my teacher specifically mentioned that I will not need them.

Wonder if expert could guide me along like let me know my mistakes. Thanks thanks.

The inventory class:

/**
* In the Inventory class, it is merely to create a list / array of product which allows    the information from the linitem to be put with an index.
* For example, for the first product, we can use the inventory class to input it into the index 1. and he next product into index 2 and so on.
 * It is suse to create an array and inputing the lineitem information into it.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
 public class Inventory
{
// instance variables - replace the example below with your own
private LineItem[] _items;
private int _numItems;


/**
 * Constructor for objects of class Inventory
 */
public Inventory()
{
    // initialise instance variables
    _items = new LineItem[1000];
    _numItems = 0;
}

/**
 * An example of a method - replace this comment with your own
 * 
 * @param  y   a sample parameter for a method
 * @return     the sum of x and y 
 */
public void addItem(LineItem item)
{
   _items[_numItems]= item;
   _numItems++;
}

public String toString()
{
    String result="";
    int i=0;
    while (i < _numItems)
    {
        result = result + _items[i] + "/n";
        i++;
    }
    return result;
}

public void print()
{
    String myResult=this.toString();
    System.out.println(myResult);
}

public Money getTotalValue()
{
    int i=0;
    Money total= new Money(0);
    while (i<_items.length)
    {
        total = total.add(Money.NO_MONEY);
        i++;
    }
    return total;
}

public LineItem getItem(String productName)
{
    int i = 0;
    LineItem itemDetails = null;
    while (i<_items.length)
    {
        if (_items[i].equals(productName))
        {
            itemDetails= _items[i];
        }
        else
        {
            //do nothing
        }
        i++;
    }
    return itemDetails;
   }
}

I have yet to comment on the methods yet but will do so once i understand it.

回答1:

Your array is of type Inventory[] - but you're trying to assign references of type LineItem. You're also not initializing it. Change this:

Inventory[] _items;

to this:

LineItem[] _items = new LineItem[5];

And all should be well - although you're not using index 0 (which is why you need it to be size 5) and you're not doing anything with the array afterwards either...

Another alternative to using an array is to use a List:

List<LineItem> items = new ArrayList<LineItem>();
items.add(new LineItem(product1, 5, unitPrice1));
items.add(new LineItem(product2, 8, unitPrice2));
items.add(new LineItem(product3, 200, unitPrice3));
items.add(new LineItem(product4, 9, unitPrice4));

... next think about what you actually want to do with the items variable.

回答2:

LineItem[] _items = new LineItem[4];

then the index starts from 0 not from 1,

_items[4]

will return indexoutofbounds error

回答3:

A few things:

incompatible types- found LineItem but expected Inventory

is caused by the fact that your array is supposed to contain Inventory objects but you're assigning LineItems to it instead

variable _items may not be initialise

means that you have your _items object but you haven't initialized it to anything. You want to do

LineItem[] _items = new LineItem[4];

PS: If you want dynamically sized arrays, don't know how many line items you'll potentially load, etc etc use a vector or a collection or something along those lines.

Also,

_items[1] = new LineItem(product1,5,unitPrice1);
_items[2] = new LineItem(product2,8,unitPrice2);
_items[3] = new LineItem(product3,200,unitPrice3);
_items[4] = new LineItem(product4,9,unitPrice4);

In Java, array elements start with index 0 and not 1