Still working on this inventory project, issue with inventory array

[Mike Osterhout]

Well, I got just about done with this project when I realized the array I had created would not allow me to insert, delete or update the inventory.


This is the inventory class that should allow for creating new inventory objects.

    
class Inventory{
	
	static String name;
	static double whole;
	static double retail;
	static int inv;

	public String getInvName(){
		return name;
	}
	
	public double getInvWhole(){
		return whole;	
	}
	
	public double getInvRetail(){
		return retail;	
	}
	
	public int getInvInv(){
		return inv;	
	}
	
	public void setInvName(String name){
		this.name = name;	
	}
	
	public void setInvWhole(double whole){
		this.whole = whole;	
	}
	
	public void setInvRetail(double retail){
		this.retail = retail;
	}
	
	public void setInvInv(int inv){
		this.inv = inv;
	}

This is the newinv class which I am attempting to hard code the initial inventory into, that is not going so well...

    
class Newinv{
	
	int count;
	public static Inventory newinv[] = new Inventory[200];
	
	public Inventory[] startInv1(){
		newinv[0] = new Inventory();
		newinv[0].setInvName("Stanley 16oz Framing Hammer");
		newinv[0].setInvWhole(10.50);
		newinv[0].setInvRetail(16.99);
		newinv[0].setInvInv(5);
		return newinv;
		
	}
	
	public Inventory[] startInv2(){
		newinv[1] = new Inventory();
		newinv[1].setInvName("Dewalt 3/8 12v Portable Drill");
		newinv[1].setInvWhole(103.53);
		newinv[1].setInvRetail(149.99);
		newinv[1].setInvInv(7);
		return newinv;	
	}
	
	public Inventory[] startInv3(){	
		newinv[2] = new Inventory();
		newinv[2].setInvName("Stanley #2 Phillips Screwdriver");
		newinv[2].setInvWhole(0.83);
		newinv[2].setInvRetail(1.49);
		newinv[2].setInvInv(28);
		return newinv;
		
	}

I am trying to access the inventory with a code like this

Newinv.newinv[0].getInvName()

The problem is I am getting a nullpointerexception. I think that if I can start to work with this data, I can figure out how to insert and delete, but this is really got me stumped.

Thanks for any pointers.

[Christophe Verré]

Do you call startInv1, startInv2 and startInv3 somewhere ?

[Mike Osterhout]

In the words of my grandfather "Wowy".

I should have known that... thanks.

[Mike Osterhout]

Ok here is a stumper, when I use the following code it just grabs the last array and adds that up three times. So it grabs the wholesale value of .83 and adds that three times. Why wouldent it grab the value from newinv[1], then newinv[2], etc.

The true goal is to use the .length method but that is returning a null pointer exception again, so if I can get the hard code to work that would be victory.

    
for (int i = 0; i < 3; i++){
				
double whole = Newinv.newinv[i].getInvWhole();
sum += whole;
}
System.out.println("the total wholesale is $" + sum);

[pete stein]

These three methods smell wrong to me:

	public Inventory[] startInv1(){
		newinv[0] = new Inventory();
		newinv[0].setInvName("Stanley 16oz Framing Hammer");
		newinv[0].setInvWhole(10.50);
		newinv[0].setInvRetail(16.99);
		newinv[0].setInvInv(5);
		return newinv;
		
	}

You're creating one Inventory object but you are returning an array of them, and that's not right. It's not the likely source of your current problem, but will set you up for future problems. Also, newinv array should not be static.

If I had a method that created an Inventory array, I'd return the array. If it just created a single Inventory object it would return just that object. Myself, I'd give the Inventory class a constructor that takes a name String, a wholesale double, a retail double(though I cringe at using floating point to represent currency), and a count int and would get rid of these hard-coded methods altogether.

[Mike Osterhout]

Is there a way that I could return just that one inventory object that was created rather than the array of them?

Also when I make newinv[] non static it gives me the nonstatic variable cannot be referenced from a static context.

[Mike Osterhout]

I changed my startinv methods to look like this

   
public static Inventory startInv1(){
		newinv[0] = new Inventory();
		newinv[0].setInvName("Stanley 16oz Framing Hammer");
		newinv[0].setInvWhole(10.50);
		newinv[0].setInvRetail(16.99);
		newinv[0].setInvInv(5);
		return newinv[0];
		
	}
 

This does not help the problem of the bad for loop, any ideas?

[pete stein]

Mike Osterhout wrote:Is there a way that I could return just that one inventory object that was created rather than the array of them?

yes, not trying to sound trite, but you just return one inventory object, simple as that. You of course change the signature of the method so that it returns Inventory not Inventory[]. The only thing that should be static here in this simple class / program is the main method; that's it.

Another thought, you could make the method return nothing, void, but have it update the array:

public void addInventory(int index, String invName, double invWhole, double invRetail, int invCount) {
   // here create your new Inventory object
   // then add it to the array at the index position
}

then have another public method to return the Inventory array if another object needs it, something like

public Inventory[] getInventoryArray() {
  // return the array
} 

Also when I make newinv[] non static it gives me the nonstatic variable cannot be referenced from a static context.

Then you're trying to access it in a static way somewhere, and that's where the problem is. So solve the problem there, but don't try to solve this by making it static though.

[pete stein]

Mike Osterhout wrote:I changed my startinv methods to look like this

   
public static Inventory startInv1(){
		newinv[0] = new Inventory();
		newinv[0].setInvName("Stanley 16oz Framing Hammer");
		newinv[0].setInvWhole(10.50);
		newinv[0].setInvRetail(16.99);
		newinv[0].setInvInv(5);
		return newinv[0];
		
	}
 

This does not help the problem of the bad for loop, any ideas?

Again, statics really do not belong here

[Mike Osterhout]

Ok when I take your advice and make newinv[] and startinv1() non static I am unable to reference any of there stuff.

    
if (selection == 1){
			double sum = 0;
			Scanner scanner = new Scanner(System.in);
//cannont reference this in a nonstatic context
			Newinv.startInv1();
			
			for (int i = 0; i < 3; i++){
//cannont find this symbol
				double whole = newinv[i].getInvWhole();
				sum += whole;

I think that this is a key concept to understand. Can you provide some insight?

[pete stein]

Mike Osterhout wrote:Ok when I take your advice and make newinv[] and startinv1() non static I am unable to reference any of there stuff.

because you're still trying to access it in static way. Create an Newinv object, and use this to call your methods of the class.

I think that this is a key concept to understand. Can you provide some insight?

Not as good as the tutorials can explain this. Please read up on creating instances or objects in the tutorials. It will explain this better than I can.

[Mike Osterhout]

Well that was a good mini lesson on calling your methods statically... However unfortunately I still get that crazy output.


This is the code for my for loop. It simply grabs the startinv3 array 3 times, NO IDEA WHY.

  
Newinv newinv = new Newinv();
			newinv.startInv1();
			newinv.startInv2();
			newinv.startInv3();
			
			for (int i = 0; i < 3; i++){
				double whole = newinv.newinv[i].getInvWhole();
				sum += whole;
				System.out.println(newinv.newinv[i].getInvWhole());
			}
			System.out.println("the total wholesale is $"+ sum);

This is the inventory code which is used to create new inventory objects.

class Inventory{
	
	static String name;
	static double whole;
	static double retail;
	static int inv;

	public String getInvName(){
		return name;
	}
	
	public double getInvWhole(){
		return whole;	
	}
	
	public double getInvRetail(){
		return retail;	
	}
	
	public int getInvInv(){
		return inv;	
	}
	
	public void setInvName(String name){
		this.name = name;	
	}
	
	public void setInvWhole(double whole){
		this.whole = whole;	
	}
	
	public void setInvRetail(double retail){
		this.retail = retail;
	}
	
	public void setInvInv(int inv){
		this.inv = inv;
	}

And this is the modified newinv class which is no longer static and is currently being hardcoded to create inventory elements insinde the inventory array, eventually I will hopefully use this to allow the user to add inventory.

class Newinv{
	
	int count;
	public Inventory newinv[] = new Inventory[200];
	
	public Inventory startInv1(){
		newinv[0] = new Inventory();
		newinv[0].setInvName("Stanley 16oz Framing Hammer");
		newinv[0].setInvWhole(10.50);
		newinv[0].setInvRetail(16.99);
		newinv[0].setInvInv(5);
		return newinv[0];
		
	}
	
	public Inventory startInv2(){
		newinv[1] = new Inventory();
		newinv[1].setInvName("Dewalt 3/8 12v Portable Drill");
		newinv[1].setInvWhole(103.53);
		newinv[1].setInvRetail(149.99);
		newinv[1].setInvInv(7);
		return newinv[1];	
	}
	
	public Inventory startInv3(){
		newinv[2] = new Inventory();
		newinv[2].setInvName("Stanley #2 Phillips Screwdriver");
		newinv[2].setInvWhole(0.83);
		newinv[2].setInvRetail(1.49);
		newinv[2].setInvInv(28);
		return newinv[2];
		
	}

For now I am simply lost as to why that for loop produces command line output like this:

0.83
0.83
0.83
the total wholesale is $2.4899999999999998
press the enter key to continue

notice it grabs the last array 3 times...

[Christophe Verré]

Why are your the fields of your Inventory class all static ? If you do that, all instances will share the same value, which explain why you have the same output for each instance you have created.

[Paul Yule]

Mike Osterhout wrote:
notice it grabs the last array 3 times...

It doesn't. Every time you set the value of any one of those Inventory objects the value is share among all objects of that class.

Do you get a warning when you do something like:
this.retail = retail;

there is no "this.retail" all Inventory objects have the same retail value, it isn't specific to each instance... or 'this' instance.

Create an attribute that is not static on your inventory object and you can see it happening, I think it'll be a good teaching tool for you. Keep the rest of your code the way it is. You'll see the non-static variable be remembered for each instance of the object.

[Mike Osterhout]

Yeah I realized that, I made another post about this issue here

https://coderanch.com/t/453727/Java-General-beginner/java/Loop-Produces-Weird-Output

Making the variables static made it so when a new object was created its variables overwrote the previous objects. Thus making all objects have the same info.

Now the trick is allowing the user to create and delete inventory.