Thread: How to put other backpack to main backpack?

I tryed to put some items to the backpack.
I can add and remove some items, but I can't put another backpack in main backpack.

After running the program I can see:
backpack and some items inside

Backpack (0.1 kg, 0.5 l) with
1: Ball (1.0 kg, 3.0 l)
2: Torch (0.5 kg, 1.0 l)
3: Box1 (5.0 kg, 25.0 l)
Total: 6.6, 29.5

backpack with removed second item

Backpack (0.1 kg, 0.5 l) with
1: Ball (1.0 kg, 3.0 l)
2: Box1 (5.0 kg, 25.0 l)
Total: 6.1, 28.5

backpack with other backpack

Backpack (0.1 kg, 0.5 l) with
1: Ball (1.0 kg, 3.0 l)
2: Box1 (5.0 kg, 25.0 l)
3: Backpack (0.01 kg, 0.1 l)
Total: 6.109999999999999, 28.6

but I can't see other backpack with its items...
I ought to see:
Backpack (0.1 kg, 0.5 l) with
1: Ball (1 kg, 3 l)
2: Torch (0.5 kg, 1 l)
3: Box1 (5 kg, 25 l)
Total: 5.6 kg 29.5 l
Backpack (0.1 kg, 0.5 l) with
1: Ball (1 kg, 3 l)
2: Box1 (5 kg, 25 l)
Total: 5.1 kg 28.5 l

Backpack (0.1 kg, 0.5 l) with
1: Ball (1 kg, 3 l)
2: Box1 (5 kg, 25 l)
3: Backpack (0.01 kg, 0.1 l) with
1: Box2 (3 g, 8 l)
Total: 3.01 kg, 8.1 l
Total: 5.1 kg 28.5 l

Maybe someone has any suggestions?
My code is here:

import java.util.ArrayList;
 
class TableSafe {
    ArrayList<Thing> item = new ArrayList<>();
}
 
class Thing {
    protected double weight;
    protected double volume;
    protected String name;
 
    public Thing(String name, double weight, double volume) {
        if (name != null)
            this.name = name;
        else
            this.name = "";
        this.weight = Math.max(weight, 0.0);
        this.volume = Math.max(volume, 0.0);
    }
    public double getWeight() {
        return weight;
    }
    public double getVolume() {
        return volume;
    }
    public String getName() {
        return name;
    }
    @Override
    public String toString() {
        return name + " (" + weight + " kg, " + volume + " l)";
    }
}
class Backpack {
    double weight;
    double volume;
    double maxWeight;
    double maxVolume;
    String name;
    TableSafe element = new TableSafe();
 
    public Backpack(double weight, double volume, double maxWeight, double maxVolume) {
        this.weight = weight;
        this.volume = volume;
        this.maxWeight = maxWeight;
        this.maxVolume = maxVolume;
 
    }
    void add(Thing thing) {
        element.item.add(thing);
 
    }
    void add(Backpack small) {
        add(new Thing(Backpack.class.getSimpleName(), small.weight, small.volume));
    }
    public void remove(int index) {
        element.item.remove(index - 1);
    }
    public void printContent() {
        System.out.println(new Thing(Backpack.class.getSimpleName(), this.weight, this.volume) + " with");
        double totalWeight = 0.0;
        double totalVolume = 0.0;
        for (int i = 0; i < element.item.size(); i++) {
            System.out.print((i + 1) + ": ");
            Thing thing = element.item.get(i);
            System.out.println(thing);
            totalWeight += thing.weight;
            totalVolume += thing.volume;
 
        }
 
        System.out.println("Total: " + (totalWeight + this.weight) + ", " + (totalVolume + this.volume));
        System.out.println();
    }
    public double getWeight() {
        double total = 0.0;
        for (int i = 0; i < element.item.size(); i++) {
            total += element.item.get(i).getWeight();
        }
        return total + this.weight;
    }
    public double getVolume() {
        double total = 0.0;
        for (int i = 0; i < element.item.size(); i++) {
            total += element.item.get(i).getVolume();
        }
        return total + this.volume;
    }
    public String getName() {
        for (int i = 0; i < element.item.size(); i++) {
            String name = Backpack.class.getSimpleName();
        }
        return name;
    }
}
 
class Box extends Thing {
    protected double height;
    protected double width;
    protected double depth;
    protected double volume;
 
    public Box(String name, double weight,
               double height, double width, double depth) {
        super(name, weight, height * width * depth * 1000);
        this.height = height;
        this.width = width;
        this.depth = depth;
        volume = height * width * depth * 1000;
    }
    public double getHeight() {
        return height;
    }
 
    public double getWidht() {
        return width;
    }
 
    public double getDepth() {
        return depth;
    }
    @Override
    public String getName() {
        return super.getName();
    }
}
public class Safe {
    public static void main(String[] args) {
        Backpack b = new Backpack(0.100, 0.5, 40, 50);
        b.add(new Thing("Ball", 1, 3));
        b.add(new Thing("Torch", 0.5, 1));
        b.add(new Box("Box1", 5, 0.1, 0.5, 0.5));
        b.printContent();
        b.remove(2);
        b.printContent();
        System.out.println(b.getWeight());
        System.out.println(b.getVolume());
        System.out.println();
 
        Backpack small = new Backpack(0.01, 0.1, 5, 13);
       // b = small;
        small.add(new Box("Box2", 3, 0.2, 0.2, 0.2));
        small.add(new Box("Box3", 1, 0.1, 0.2, 0.2));
        b.add(small);
 
        b.printContent();
 
    }
}

Thank's for every suggestions!

Please post the code here on the forum (wrapped in code tags) that show what you are talking about. No links.

this returns null.

The add method is defined as void which means it does not return anything.

Where is the null value you talk about?

the program returns the following response:

Ok. Try debugging the code to see where that null value comes from. Add some print statements that print out the values of variables as the code executes to show what the computer sees when the code is executed.

I do not get a null when I execute the posted code. Is the code you are executing different from the code that you posted? Please post the current code.

Did you try using some print statements to find where the null value comes from?

3: null (0.0 kg, 0.0 l) //Here is my problem

What is the expected output? What should be printed instead of null?

I asked:

What should be printed instead of null?

That is a lot more than the one line with the null value.

What does the code need to differently to get that output?
Where are the extra lines (instead of null) supposed to come from?

Did you try using some print statements to find where the null value comes from?

Do you want to place the backpack in another backpack or just transfer the items from one backpack to the other?

Regards,
Jim

how to put second backpack to the first backpack.

Can you describe what that means?
After adding, Should the original backpack now contain a backpack or should the contents be removed from the second backpack and merged with the contents of the original backpack?
What if a second backpack is added?
Can a backpack that contains a backpack be added to another backpack?

Can a backpack can 3 different contents: some stuff or a backpack or both?

But the backpack that is in another backpack can not contain a third backpack. Can only nest one deep.

have only nest one deep.

Does that mean that the add method for adding a backpack needs to check if the backpack that is being added contains a backpack?
Also does the add method need to check if a backpack has already been added (to allow only 1 backpack to be added)?

In other words, should the add method throw an exception when either of the above happens?

how to add one backpack to another

What is the problem with the current code?

What happens when a backpack is added now? Where is the reference to it saved?

My testing shows that the backpack is added successfully.

        small.name = "a Small BP";                                        //<<<<<<< give backpack a name
       System.out.println(b.element.item);    // [Ball (1.0 kg, 3.0 l), Box1 (5.0 kg, 25.0 l), Backpack >a Small BP (0.01 kg, 0.1 l) ]

My test code was to show that the values were added to the list. It was not supposed to format the output the way you show.
The code that makes the print out needs to be changed to create the desired output.