Write a public String toString() method to do an in-order traversal of the tree

Question

Write a public String toString() method to do an in-order traversal of the tree to set up a string with the Items in increasing order; each Item should appear on a separate line. I already have the recursive method but I don't know how exactly to put it in the toString() method. (Using Java)

Item Class:

public class Item
{
   private String number;
   private String description;
   private int quantity;
   private double price;

   public Item()
   {
       this.number = "";
       this.description = "";
       this.quantity = 0;
       this.price = 0;
   }

   public Item (String number, String description, int quantity, double price)
   {
       this.number = number;
       this.description = description;
       this.quantity = quantity;
       this.price = price;
   }

   public Item (Item i)
   {
       this.number = i.number;
       this.description = i.description;
       this.quantity = i.quantity;
       this.price = i.price;
   }

   public String getNumber()
   {
       return number;
   }

   public String getDescription()
   {
       return description;
   }

   public int getQuantity()
   {
       return quantity;
   }

   public double getPrice()
   {
       return price;
   }

   public void setNumber(String number)
   {
       this.number = number;
   }

   public void setDescription(String description)
   {
       this.description = description;
   }

   public void setQuantity(int quantity)
   {
       this.quantity = quantity;
   }

   public void setPrice(double price)
   {
       this.price = price;
   }

   public void setItem(String number, String description, int quantity, double price)
   {
       this.number = number;
       this.description = description;
       this.quantity = quantity;
       this.price = price;
   }

   public String toString()
   {
       return String.format("%-5s %-20s %5d %7.2f", number, description, quantity, price);
   }

   public boolean equals (Item obj)
   {
       return this.number.equals(obj.number);

   }

   public int compareTo(Item obj)
   {
       return number.compareTo(obj.number);

}

}

InventoryTree Class:

public class InventoryTree
{
   private class TreeNode
   {
       Item data;
       TreeNode left;
       TreeNode right;

       public TreeNode()
       {
           data = null;
           left = null;
           right = null;
       }

       public TreeNode(Item value)
       {
           data = value;
           left = null;
           right = null;
       }

   }

   private TreeNode root;

   public InventoryTree()
   {
       root = null;
   }

   public InventoryTree (InventoryTree t)
   {
       if (t.root == null)
       root = null;
       else
       {
           root = new TreeNode();
           TreeNode temp = t.root;
           fillValues(root, temp);
       }
   }

   private void fillValues(TreeNode parent, TreeNode temp)
   {
       parent.data = new Item(temp.data);
       if (temp.left != null)
       {
           TreeNode current = new TreeNode();
           parent.left = current;
           fillValues (current, temp.left);
       }
       else
       {
           parent.left = null;
       }
       if (temp.right != null)
       {
           TreeNode current = new TreeNode();
           parent.right = current;
           fillValues (current, temp.right);
       }
       else
       {
           parent.right = null;
       }
   }

   public Item findItem(String value)
   {
       TreeNode parent = find(value);
       TreeNode current = getCurrent(parent, value);

       if (current != null && current.data.getNumber().equals(value))
           return current.data;
       else
           return null;
   }

   private TreeNode find(String value)
   {
       TreeNode current = root, parent = null;
       while (current != null && !current.data.getNumber().equals(value))
       {
           if (current.data.getNumber().compareTo(value) < 0) //search on right link
           {
               parent = current;
               current = current.right;
           }
           else //search on the left link
           {
               parent = current;
               current = current.left;
           }
       }
       return parent;
   }

   private TreeNode getCurrent(TreeNode parent, String value)
   {
       TreeNode current;
       if (parent == null)
           return root;
       else //determine which child of parent is current
       {
           if (parent.data.getNumber().compareTo(value) < 0)
               current = parent.right;
           else
               current = parent.left;

           return current;
       }
   }

public boolean addItem(Item add)
   {
       TreeNode addNode = new TreeNode(add);
       TreeNode parent = find(add.getNumber());

       if (parent != null && parent.data.equals(add)) //if already exists
           return false;
      
       else if (parent == null)   //if tree is empty
       {
           root = addNode;
           return true;
       }

       else if (parent.data.getNumber().compareTo(add.getNumber()) < 0) //go on right link
       {
           parent.right = addNode;
           addNode.left = null;
           addNode.right = null;
           return true;
       }

       else //go on left link
       {
           parent.left = addNode;
           addNode.left = null;
           addNode.right = null;
           return true;
       }

   }

   public boolean adjustQuantity(String value, int add)
   {
       TreeNode parent = find(value);
       TreeNode current = getCurrent(parent, value);

       if (current == null || !current.data.getNumber().equals(value))
           return false;

       else
       {
           int quantity = current.data.getQuantity() + add;
           if (quantity < 0)
               return false;
           else
           {
               current.data.setQuantity(quantity);
               return true;
           }

       }

   }

   public boolean changeUnitPrice (String value , double newPrice)
   {
       TreeNode parent = find(value);
       TreeNode current = getCurrent(parent, value);

       if (current == null || !current.data.getNumber().equals(value))
           return false;

       else
       {
           current.data.setPrice(newPrice);
           return true;
       }

   }

private void inOrder(TreeNode current)
{   
if (current != null)
{
inOrder(current.left);
System.out.println(current.data + " ");
inOrder(current.right);
}
}
  
   public void inOrder() //Print in-order transversal of all Item values
{
inOrder(root);
}
     

}

Explanation / Answer

Add these two methods in InventoryTree class

private String inOrderString(TreeNode current, String s)
{   
if (current.left != null)
s=inOrderString(current.left,s);

s=s+ current.data+" ";

if (current.right != null)
s=inOrderString(current.right,s);
  
return s.trim();
}

public String toString()
{
return inOrderString(root," ");
}

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