Hi I am new to java, I am having trouble adding a new line for my invoice class from my other class. The underline is where the issue is. We are supposed to addLine a method that takes the following four input parameters: items number, item description, item price, and quantity. The method then adds a line to the invoice only if the invoice has less than four lines. If the invoice already has four lines, then this method should print an error message.

public class Invoice {
// Instance Variables
private String customerName;
private int numItems;
private InvoiceLine line1;
private InvoiceLine line2;
private InvoiceLine line3;
private InvoiceLine line4;

// Contructors
public Invoice(){}

public Invoice(String customerNam){
customerName = customerNam;
}
//Getters
public String getCustomerName(){
return customerName;
}
//Setters
private void setCustomerName(String customerNam){
customerName = customerNam;
}
public void addLine(int itemNum, String Description, double itemPrice, int quantity){
if (numItems <= 4){
numItems += line1;
if (numItems <= 4)
numItems += line2;
if (numItems <= 4)
numItems += line3;
if (numItems <= 4)
numItems += line4;
}
if (numItems == 4){
System.out.print("Error");
}
}
public double getInvoiceTotal(){
numItems *= numItems;
return numItems;
}
public String toString(){
String invoiceOutput = "";
invoiceOutput += (customerName + " brought "+ numItems);
return invoiceOutput;
}
}

My invoiceLine class:

public class InvoiceLine {
//Instance Variables
private Item itemSold;
private int quantity;
public double total = 0;

// Constructors
public InvoiceLine(){}

public InvoiceLine(Item itemSold, int quantity){
this.itemSold = itemSold;
this.quantity = quantity;
}

public InvoiceLine(int itemNum, String itemDescription, double itemPrice, int quantity){
this.itemSold = new Item(itemNum, itemDescription, itemPrice);
this.quantity = 0;
}
//Getters
public Item getItemSold(){
return itemSold;
}
public int getQuantity(){
return quantity;
}
//Setters
private void setItemSold(Item itemSold){
this.itemSold = itemSold;
}
private void setQuantity(int quantity){
this.quantity = quantity;
}
// Methods
public double getLineTotal(double total){
total = (quantity * itemSold.getItemPrice());
return total;
}
public String toString(){
String invoiceLine = "";
invoiceLine = (itemSold + " brought "+ quantity + ", which the total cost is "+ total);
return invoiceLine;
}

}