Thread: Inheritence and problems with static methods.

If you're learning OOP principles, one of the first rules to learn is that you should avoid static anything except in very specific situations. Here your use of static methods are preventing inheritance from working at all. I suggest that *all* of your methods be instance or non-static methods with the only exception being the main method. To call non-static methods from the main method (or any static method), you need to first create an instance of your class, and then call the methods on that instance. In your code above, you're trying to call methods directly on the class, which again you shouldn't be doing. Create an Order object or a ShippedOrder object and call the *non-static* methods from the object.

You mean to instead of creating three different classes I should just have one class with an Order and ShippedOrder method in the one class? The assignment is to create three classes I can't really get around that part.

Unless I totally missed what you meant, which I probably did.

Originally Posted by Sylis

You mean to instead of creating three different classes I should just have one class with an Order and ShippedOrder method in the one class? The assignment is to create three classes I can't really get around that part.

Unless I totally missed what you meant, which I probably did.

Yep, you missed what I meant. Let me try again. You should have one class, Order, that has no static methods, no static fields, no static *anything*. You should have another class, ShippedOrder, that extends Order, that likewise contains *nothing* static. You should have a third class, UseOrder, that has a static main method, that creates an instance of ShippedOrder in main, and that calls methods on this instance.

Thank you, I figured it out by using:

import javax.swing.*;
 
public class UseOrder
{
	public static void main(String[] args)
	{
		Order anOrder = new Order();
		JOptionPane.showMessageDialog(null, "First, enter data for a pick up order.");
		anOrder.setCustNumber();
		anOrder.setCustomer();
		anOrder.setOrderQuant();
		anOrder.setUnitCost();
		anOrder.computePrice();
		anOrder.display();
 
		ShippedOrder aShippedOrder = new ShippedOrder();
		JOptionPane.showMessageDialog(null, "Now, enter data for a shipped order.");
		anOrder.setCustNumber();
		anOrder.setCustomer();
		anOrder.setOrderQuant();
		anOrder.setUnitCost();
		aShippedOrder.computePrice();
		anOrder.display();	
	}	
}

my only problem now is the overriding of the computePrice(); method, the second compute price should be calculating an extra $4.00 on top for shipping and handling, but the computePrice() method in aShippedOrder isn't overriding the total from Order.

Again, have you changed the code in Order and ShippedOrder as I've recommended?

Yes I have.

import javax.swing.*;
public class Order 
{
	private String customer, numb, quant, unitPrice;
	private int custNumber;
	private double orderQuant, unitCost;
	public double total;
 
	public void setCustomer()
	{
		customer = JOptionPane.showInputDialog(null,"Enter customer name:");		
	}
	public  String getCustomer()
	{
		return customer;
	}
	public  void setCustNumber()
	{
		numb = JOptionPane.showInputDialog(null, "Enter costumer number:");
		custNumber = Integer.parseInt(numb);
	}
	public int getCustNumber()
	{
		return custNumber;
	}
	public void setOrderQuant()
	{
		quant = JOptionPane.showInputDialog(null, "Enter quantity ordered:");
		orderQuant = Double.parseDouble(quant);
 
	}
	public double getOrderQuant()
	{
		return orderQuant;
	}
	public void setUnitCost()
	{
		unitPrice = JOptionPane.showInputDialog(null, "Enter quantity ordered:");
		unitCost = Double.parseDouble(unitPrice);
 
	}
	public double getUnitCost()
	{
		return unitCost;
	}
	public void computePrice()
	{
		total = (orderQuant * unitCost);
	}
	public void display()
	{
		JOptionPane.showMessageDialog(null, "Customer Number: " + custNumber + "\nCustomer Name: " + customer + "\nQuantity Ordered: " + Math.round(orderQuant) + 
				"\n Price Each: $" + unitCost + "\nThe cost of your order is: $" + total);
	}	
 
}

public class ShippedOrder extends Order
{
	private final double shipHandle = 4.00;
	public void computePrice()
	{
		total = ((getOrderQuant() * getUnitCost()) + shipHandle);
	}
	public double getTotal()
	{
		return total;
	}
}

I took out every static out of the program, except main.

Check your UserOrder's main method again. Which object are you using in the second half of this method, the Order object or the ShippedOrder object. Check again.

Shouldn't it be the ShippedOrder method? The first set of dialog boxes set up the information, and then the computeOrder() in Order computes only quantity * price, and the computeOrder uses quantity * price + 4.00 for shipping and handling.

Shouldn't it be the ShippedOrder method?

Yes it should be, but what objects are you using mostly here?:

                // second half of main method
		ShippedOrder aShippedOrder = new ShippedOrder();
		JOptionPane.showMessageDialog(null, "Now, enter data for a shipped order.");
		anOrder.setCustNumber();  // ** anOrder??? **
		anOrder.setCustomer();  // ** anOrder??? **
		anOrder.setOrderQuant();  // ** anOrder??? **
		anOrder.setUnitCost();  // ** anOrder??? **
		aShippedOrder.computePrice();
		anOrder.display();  // ** anOrder??? **

Also, note that your setXXX(...) methods are non-standard. Usually they accept a parameter with which you set the class field, and usually they don't themselves do any user interaction (i.e., they don't contain Scanner or JOptionPane code). Perhaps this is what your instructor wants, but again typically the setXXX(...) methods don't do this.

Well, I use the methods to set the customer number, customer name, order quantity, and unit cost from Order, and then computePrice from a shipped order class in order to calculate the four dollars into it total. should, ShippedOrder have it's own set methods for each variable?

Originally Posted by Sylis

Well, I use the methods to set the customer number, customer name, order quantity, and unit cost from Order, and then computePrice from a shipped order class in order to calculate the four dollars into it total. should, ShippedOrder have it's own set methods for each variable?

ShippedOrder already has all of the non-static methods of Order. That's what inheritance is all about. Again please make the indicated method calls on the ShippedOrder object, not the Order object.

You sir are a gentleman and a scholar.
I like that you gave me the answers I was looking for without just giving it to me. Made me think, and using my head, I figured it out, and instantly inheritance makes a world more sense to me than it originally did. Thank you very much.

You're very welcome!

Wow you guys are over-shooting the mark. The solution is far more simple:

1. Static fields and methods are not associated with objects. Thus - using them is out of the question, if you want to have sub-classes.
2. To use a method from another class, even non-static, you create a new instance of it. You don't have to make the methods static.

There, problem solved.

Originally Posted by Programming_Hobbyist

Wow you guys are over-shooting the mark. The solution is far more simple:

1. Static fields and methods are not associated with objects. Thus - using them is out of the question, if you want to have sub-classes.
2. To use a method from another class, even non-static, you create a new instance of it. You don't have to make the methods static.

There, problem solved.

Aren't you being a bit presumptuous here? Please explain how my advice was "over-shooting the mark". I told him basically the same thing in my first post if you'd take the time to read the whole thread.