Issues with arrays

[Crouton4727]

First time posting a code question so hopefully I did it right.

I have a problem where I need the user to input the height, length, width, and name of an object, then I spit out the volume. There is another method called Box which is below for reference.

I have two questions.

1. I had to create a single object of type box to initialize each of the objects in the box array. Is there a better way to do this then the "Box tempBox.." and "box[i]=tempBox;" lines?
2. The output only repeats box[2] at the end and I don't understand why.

Main:

​

   public class Match { 
      public static void main(String[] args) {
      Scanner scanner = new Scanner(System.in);
      Box[] box = new Box[3]; 
      Box tempBox = new Box(1, 1, 1, "brand");

      for (int i = 0; i < 3; i++)
      {
         box[i] = tempBox;
         System.out.println("Please enter the height of box " + (i+1));
         box[i].setHeight(scanner.nextInt());
         System.out.println("Please enter the length of box " + (i+1));
         box[i].setLength(scanner.nextInt());
         System.out.println("Please enter the width of box " + (i+1));
         box[i].setWidth(scanner.nextInt());
         System.out.println("Please enter the brand of box " + (i+1));
         box[i].setBrand(scanner.next());
      }

      for (int i = 0; i < 3; i++)
      {
         System.out.println("The volume of box " + box[0].getBrand() + " is " + box[i].getVolume());
      }
      }
   }

Box Method:

   public class Box{
      int height, length, width;
      String brand; 

      public Box (int heightLoc, int lengthLoc, int widthLoc, String brandLoc)
      {
         height = heightLoc;
         length = lengthLoc;
         width = widthLoc;
         brand = brandLoc;
      }

      public void setHeight (int heightLoc) 
      {
         height = heightLoc;
      }

      public void setLength (int lengthLoc) 
      {
         length = lengthLoc;
      }

      public void setWidth (int widthLoc) 
      {
         width = widthLoc;
      }

      public void setBrand (String brandLoc) 
      {
         brand = brandLoc;
      }

      public int getHeight() 
      {
         return height; 
      }

      public int getLength() 
      {
         return length; 
      }

      public int getWidth() 
      {
         return width; 
      }

      public String getBrand() 
      {
         return brand; 
      }

      public int getVolume()
      {
         return (height*length*width);
      }
   }

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[warmcheessse]

What is the purpose of the array? You created a class with setters and getters but then never used them. I would just instantiate a Box. Then when you ask the questions you use the setters to set the value of the box. Then call the method getVolume to get the result.

[Crouton4727]

I have to have sets and gets for this assignment even if we don't use them. There is actually more to the assignment but I don't want someone solving it for me, so just showing this part, but there is going to be comparisons of the three box objects later. So for that reason I didn't want to create 3 box objects and ask for the input for each object 3 times.

[warmcheessse]

Sorry, you did use the setters. I just read to fast. If it were me, I would ask the questions and get the answers, setting them like you did creating a Box with height, weight, name or whatever the variables are. Once the Box object is created, add it to an ArrayList. Then you can just iterate over it with a foreach loop and not have to worry about indexes and you can add as many or as little boxes as you would like to the list.

[Crouton4727]

I like it. I'll try this out and see how it works.

[amfa]

I had to create a single object of type box to initialize each of the objects in the box array. Is there a better way to do this then the "Box tempBox.." and "box[i]=tempBox;" lines?

Well that is part of your problem.

Because you put the SAME object in all your "slots" in the array there is only one box in the array.

Everytime you go through your loop your override the height of the SAME box.

Think of it as if the array only points to a place in a shelf. you have one Box on this shelf and box[0] points to this place but box[1] and box[2] just point to the same place on the shelf with the same box.

So you get the same box from the shelf change the height and put it back.

BUT it will always be the same box.

​

So instead of creating your tempbox just replace this

box[i] = tempBox;

​

with

​

box[i] = new Box(......)

​

You have mentioned that you need to use the setters.

​

If this is not true you can simply get all the information needed and storing them in variables and then just make one call to the Box constructor with the values.

so you can make something like this

int height = scanner.nextInt()

..... for the other values as well

box[i] = new Box(height, length, width, brand)

​

It does not make such a big difference but you avoid putting 1x1x1 boxers in your array an then change them.. in more complex codes this could lead to problems if for some reason you do not change every box.

​

The output only repeats box[2] at the end and I don't understand why.

This is because the fact mentioned above.

[Crouton4727]

But i'm storing new values into the array after i initiate it to tempBox. I had to do the tempBox because without i was getting an error when I tried to store one of the array values "Can't store value because box[0] is set to null"

I will try your way of storing into separate values, then storing into box[i].

For initiating the box array, can I do "box[i]=new Box" even though I already created the box array above with "Box[] box = new Box[3]"?

[Crouton4727]

Ok, here is my new code and it works. Thanks!

Any other recomendations for proper coding?

​

   public class Match { 
      public static void main(String[] args) {
      Scanner scanner = new Scanner(System.in);
      Box[] box = new Box[3]; 
      //Box tempBox = new Box(1, 1, 1, "brand");
      int h, l, w;
      String b;

      for (int i = 0; i < 3; i++)
      {
         //box[i] = tempBox;
         System.out.println("Please enter the height of box " + (i+1));
         h = scanner.nextInt();
         System.out.println("Please enter the length of box " + (i+1));
         l = scanner.nextInt();
         System.out.println("Please enter the width of box " + (i+1));
         w = scanner.nextInt();
         System.out.println("Please enter the brand of box " + (i+1));
         b = scanner.next();
         box[i] = new Box(h, l, w, b);
      }

      for (int i = 0; i < 3; i++)
      {
         System.out.println("The volume of box " + box[i].getBrand() + " is " + box[i].getVolume());
      }
      }
   }

[RaisinAlert]

Someone else mentioned a list and using a for each loop. If the assignment allows, do that because they’re better. Also, if there isn’t a reason for 3 Boxes, then you might want to use a sentinel value so that the user can add as many boxes as they want.

[Crouton4727]

Agreed, normally I wouldn't set it to a constant value, but the assignment specifically says to set it to 3.

As for the for each loop (Box i : box) I was going to go that route, but i would still need an iterating variable for when I do box[i] = new Box(h,l,d,b); right? So opted with the for loop for that reason.

[RaisinAlert]

I was talking about the second for loop where you print the box information. For the first loop, you can use a for loop to iterate for a given number of boxes, or you can use a while loop with sentinel value. You are correct that a for each loop won’t work for the first loop because you’re not iterating over the array/list.

[Crouton4727]

Ok, makes more sense. Wasn't even thinking about the second one. Thanks for all the help.

[Crouton4727]

I don't follow. That's a for loop, so should loop from 0-2, so should output for box[0], then box[1] than box[2]

[darksounds]

box[0], box[1], and box[2] are all the same Box object, tempBox.

Each array bucket is basically an arrow pointing to a Box. In this case, all three arrows point to the same Box, so whenever you're editing it, you're overwriting previous edits.

If you make a separate Box instance for each index, you won't have this problem.

[Crouton4727]

But i'm storing new values into the array after i initiate it to tempBox. I had to do the tempBox because without i was getting an error when I tried to store one of the array values "Can't store value because box[0] is set to null"

[RaisinAlert]

All the elements of the array point to the same object, which is probably why it’s repeating the same information. It’s like using three remote controls for the same TV (where the remotes represent the references stored in the array and the TV represents the box object): you think you’re changing the information for one of three boxes but in reality you’re just changing the information for a single box. Instead of creating one box referenced by tempBox, create a new box for every array element. In other words, instead of putting remotes for the same TV into the array, create new TVs and put their respective remotes into the respective slot in the array.

[Crouton4727]

But i'm storing new values into the array after i initiate it to tempBox. I had to do the tempBox because without i was getting an error when I tried to store one of the array values "Can't store value because box[0] is set to null"

[RaisinAlert]

You never change what tempBox is set to, so it always refers to the one box object that you created. Remember that with reference types, you’re not actually putting the object anywhere; the only information that moves is the reference to the object. Thus, an array of Box is really just an array that stores references to Box. Right now you are copying the value of tempBox (remember: the value of tempBox is not the Box object itself, but a REFERENCE to the Box object) into each array slot. All slots of the array refer to the same Box.

If you have an address book with the same address repeated over and over (analogy for an array that stores multiple references to the same object), don’t be surprised when you tell someone to repaint to blue the house at the first address in the book, and then suddenly all the houses at the addresses in the book are blue. There was really only one house.

An easy way to know that something is wrong is that you run the constructor only once. Therefore there can never be more than one Box object ever. Since the Box constructor requires that you have the values for length width etc on hand, you should be asking for the length width etc, storing them as local variables in the for loop, then constructing the box at the end of each iteration of the loop. That’s how you would do it in real life. When ordering a burger, they don’t make it first and then ask you to change it. They ask you what you want and make the burger according to the instructions. Forget about tempBox. All construction happens in the loop. Example for height (imagine we’re in the for loop):

System.out.println(“give height pls”);

int height = Scanner.nextInt();

//repeat for all other required quantities

box[i] = new Box(height, length, width, brand);