Getters and Setters returning null

Question

I'm trying to write a very simple program that produces an invoice for an item purchased. I have a driver class with my setters and getters, and my object class which is supposed to call them. However, when I print out the invoice, it just produces null responses for everything.

Here is my driver class:

public class Invoice {

private String partNumber;  //part number of item
private String description; //description of item
private int quantity;       //quantity being purchased
private double itemPrice;   //price of item

//constructor
public Invoice(String partNumber, String description, int quantity, double itemPrice) {
    partNumber = partNumber;
    description = description;
    if (quantity < 0) {
        quantity = 0;
    }
    if (itemPrice < 0.0) {
        itemPrice = 0.0;
    }
}

public void setPartNumber(String number) {
    partNumber = number; //store the partNumber
}

public void setDescription(String description) {
    description = description;//store the description
}

public void setQuantity(int quantity) {
    quantity = quantity;//store the quantity
}

public void setItemPrice(double price) {
    itemPrice = price;//store the itemPrice
}

public String getPartNumber() {
    return partNumber;//retrieve the partNumber
}

public String getDescription() {
    return description;//retrieve the description
}

public int getQuantity() {
    return quantity;//retrieve the quantity
}

public double getItemPrice() {
    return itemPrice;//retrieve the itemPrice
}

//get price for item purchased.
public double getInvoiceAmount(double amount) {
    amount = itemPrice * quantity;
    if (amount < 0) {
        amount = 0.0;
    }
    return amount;
}
}

And here is my object class:

public class InvoiceTest {

public static void main(String[] args) {
    Invoice invoice1 = new Invoice ("0001", "Hammer: used to hit nails.", 1, 10.0);

    System.out.println("Item purchased: " + invoice1.getPartNumber() +
            "\n Description: " + invoice1.getDescription() +
            "\n Amount:" + invoice1.getQuantity() +
            "\n price: " + invoice1.getQuantity());
    System.out.println();
    System.out.println("Total price: " + invoice1.getInvoiceAmount(amount));
}

}

Answer 1

You forgot to assign the variables to the fields in constructor. Use this keyword to refer to the class fields:

public Invoice(String partNumber, String description, int quantity, double itemPrice) {
    //here's an example
    //partNumber = partNumber
    this.partNumber = partNumber;
    //description = description;
    this.description = description;
    //similar for other assignments inside the constructor...
}

You must also do this in your setters as well:

public void setPartNumber(String partNumber) {
    this.partNumber = partNumber; //store the partNumber
}

Answer 2

Your class's member fields are not being initialized as you intended, because of this:

public Invoice(String partNumber, String description, int quantity, double itemPrice)
{
    partNumber = partNumber;

Here, partNumber refers to the function argument, not the member field. To set the member field, use this.partNumber = partNumber.

Answer 3

Your setters are not setting the values into the object instance, you need to make changes like below

public Invoice(String partNumber, String description, int quantity, 
        double itemPrice) {
  this.partNumber = partNumber;
  this.description = description;
  this.quantity = quantity;
  if (this.quantity < 0) {
    this.quantity = 0;
  }
  this.itemPrice = itemPrice;
  if (this.itemPrice < 0.0) {
    this.itemPrice = 0.0;
  }
}

public void setPartNumber(String number) {
  this.partNumber = number; //store the partNumber
}

public void setDescription(String description) {
  this.description = description;//store the description
}

public void setQuantity(int quantity) {
  this.quantity = quantity;//store the quantity
}

public void setItemPrice(double price) {
  this.itemPrice = price;//store the itemPrice
}