LinkedList 头尾问题

Question

I'm trying to figure out why some of the loops inside of InventoryList.java don't work, such as all the remove methods. By don't work, I mean that the loop I implemented doesn't work and I'm looking for solutions on how to get it fixed. Also another issue that isn't too important is the toString() in InventoryList.java isn't set up properly, but i'd rather know why my loops aren't working. For more information, here are the documentations I'm using to implement these classes: http://cs300-www.cs.wisc.edu/wp/wp-content/uploads/2020/12/p7/doc/package-summary.html

Thank you for reading

/**
 * This class models a Box used for inventory
 * 
 * @author mouna
 *
 */
public class Box {
  private static int nextInventoryNumber = 1; // generator of unique inventory numbers
  private Color color; // color of this box
  private final int INVENTORY_NUMBER; // unique inventory number of this box

  /**
   * Creates a new Box and initializes its instance fields color and unique inventory number
   * 
   * @param color color to be assigned of this box. It can be any of the constant color values
   *              defined in the enum Color: Color.BLUE, Color.BROWN, or Color.YELLOW
   */
  public Box(Color color) {
    this.color = color;
    this.INVENTORY_NUMBER = nextInventoryNumber++;
  }

  /**
   * Returns the color of this box
   * 
   * @return the color of this box
   */
  public Color getColor() {
    return color;
  }

  /**
   * returns the inventory number of this box
   * 
   * @return the unique inventory number of this box
   */
  public int getInventoryNumber() {
    return this.INVENTORY_NUMBER;
  }


  /**
   * Returns a String representation of this box in the format "color INVENTORY_NUMBER"
   *
   * @return a String representation of this box
   */
  @Override
  public String toString() {
    return this.color + " " + this.INVENTORY_NUMBER;
  }

  /**
   * This method sets the nextInventoryNumber to 1. This method must be used in your tester methods
   * only.
   */
  public static void restartNextInventoryNumber() {
    nextInventoryNumber = 1;
  }
}

/**
 * This Enumeration groups the name of constants representing the three colors BLUE, BROWN, and YELLOW
 *
 */
public enum Color{
  BLUE, BROWN, YELLOW;
}

public class LinkedBox {
    private Box box;
    private LinkedBox next;
    
  //constructors
    public LinkedBox(Box box) {
    this.box = box;
    }
    
    public LinkedBox(Box box, LinkedBox next) {
    this.box = box;
    this.next = next;
    }
    
    //getters
    public Box getBox() {
        return box;
    }
    
    public LinkedBox getNext() {
        return next;
    }
    
    //setters
    public void setNext(LinkedBox next) {
        this.next = next;
    }
    
    public String toString() {
        if (next == null) return box.toString() + " -> END"; //one arg
        return box.toString() + " -> " + this.next; //two+ args
    }
}

public class InventoryList {
  private LinkedBox head;
  private LinkedBox tail;
  private int size;
  private int yellowCount;
  private int blueCount;
  private int brownCount;

  //TODO remove these 2 methods later
  public LinkedBox getHead() {
    return this.head;
  }
  
  public LinkedBox getTail() {
    return this.tail;
  }

  //getters
  public int getBlueCount() {
    return this.blueCount;
  }

  public int getBrownCount() {
    return this.brownCount;
  }

  public int getYellowCount() {
    return this.yellowCount;
  }

  public Box get(int index) {
    int count = 0;
    Box box = new Box(Color.BLUE);
    while (head != null) {
      if (count == index) box = this.head.getBox();
      head = head.getNext();
      count++;
    }
    return box;
  }

  public int size() {
    return this.size;
  }

  //setters
  public void addBlue(Box blueBox) {
    if (head.getBox().getColor() != Color.YELLOW || tail.getBox().getColor() == Color.BROWN) head = new LinkedBox(blueBox, head); this.blueCount++; this.size++;
  }

  public void addBrown(Box brownBox) {
    tail = new LinkedBox(brownBox, tail);
    this.brownCount++; this.size++;
  }

  public void addYellow(Box yellowBox) {
    head = new LinkedBox(yellowBox, head);
    this.yellowCount++; this.size++;
  }

  //removers
  public Box removeBox(int inventoryNumber) {
    Box box = new Box(Color.BLUE);
    int index = 0;
    while (head != null) {
      if (inventoryNumber == index) {
        box = head.getBox();
        head = null;
      }
      index++;
      head = head.getNext();
    }
    return box;
  }

  public Box removeBrown() {
    Box box;
    if (tail.getBox().getColor() == Color.BROWN) {
      box = tail.getBox();
      tail = null;
      return box;
    }
    return new Box(Color.BROWN);
  }

  public Box removeYellow() {
    Box box;
    if (head.getBox().getColor() == Color.YELLOW) {
      box = head.getBox();
      head = null;
      return box;
    }
    return new Box(Color.YELLOW);
  }

  public void clear() {
    while (head != null) {
      head = null;
      head = head.getNext();
    }
  }

  //miscellaneous
  public boolean isEmpty() {
    if (head == null && tail == null) return true;
    return false;
  }

  public String toString() {
    if (head == null || tail == null) return "";
    return head+" "+tail+" <-TO STRING";
  }
}

Answer 1

If you are not using any integrated development environment (IDE) then use one, like Eclipse . To understand how code is working it's good to learn how to debug .

Your code has multiple issues. I'll give you a few hints you do the rest. Fix the toString() methods first so that you'll know what's going on.
In LinkedBox#toString() method, the code is going in recursion and printing entire linked list. Because this.next.toString() is getting called recursively. Change the last line to this:

return box.toString() + " -> " + this.next.getBox(); //

Then implement InventoryList#toString() like this:

public String toString() {
    if (head == null || tail == null) return "";
        
    LinkedBox temporaryLinkedBox = head; //<-- use temporary variable
    StringBuilder result = new StringBuilder();
    while(temporaryLinkedBox!=null) {
        result.append(temporaryLinkedBox.toString()).append(" | ");
        temporaryLinkedBox = temporaryLinkedBox.getNext();
   }
   result.append(" <-TO STRING");
   return result.toString();
}

Notice how we are not using head to iterate over the list. We are using temporary variable temporaryLinedBox to iterate.

Same is the case with InventoryList#get and InventoryList#removeBox and get methods don't use head for iterating the list. Use temporary variable:

  public Box removeBox(int inventoryNumber) {
    Box box = null;
    int index = 0;
    LinkedBox temporaryLinkedBox = head;
    while (temporaryLinkedBox != null) {
      if (inventoryNumber == index) {
        box = temporaryLinkedBox.getBox();
        break;     //<-- use 'break' to exit loop
      }
      index++;
      temporaryLinkedBox = temporaryLinkedBox.getNext();
    }
    return box;
  }

Note: this is just a sample code you'll have to debug and fix errors if there are any.