如何搜索Java链接列表

Question

I am working on my last assignment for my intro Java class. We are doing a Linked List (we built ourselves) Inventory program. I am having trouble with my findItem method. How would I search through my Linked List by Item number?? The way I am doing it works, but I need to get rid of the break. My instructor said I "need to set a compound condition, the !found you have and the not null." I have tried several different variations and cannot get it. Please, any help, suggestions, or examples would be greatly appreciated, its finals week and everything is due tomorrow. My main issue is in findItem() in InventoryLL class, but I posted the whole code for context.

InventoryLL

import java.util.Scanner;
import java.io.PrintWriter;
import java.io. FileOutputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class InventoryLL
{
int count = 0;
private ItemNode head;
Scanner scannerObject = new Scanner(System.in);

public InventoryLL()
{
    head = null;
}

public void addItem()
{
    try{
        System.out.print("\nPlease enter name of item: ");
        String lname = scannerObject.nextLine();
        System.out.print("\nPlease enter a brief description of the item: ");
        String ldesc = scannerObject.nextLine();
        System.out.print("\nPlease enter the amount on hand: ");
        int lonHand = scannerObject.nextInt();
        System.out.print("\nPlease enter unit price of the item: $");
        Double lunitPrice = scannerObject.nextDouble();
        ItemNode temp = new ItemNode(count + 1, lname, ldesc, lonHand, lunitPrice);
        count++;
        temp.setNext(head);
        head = temp;
        System.out.println("\nThank you. The ID number for " + lname + " is " + count);
        scannerObject.nextLine();
    }catch(Exception e){    
            System.out.println("\nERROR! Please try again:\n");
            scannerObject.nextLine();
            }   
}

public ItemNode findItem(){
    int inputID;
    boolean found = false;
    ItemNode current = null;
        try{
        System.out.print("\nGreetings, please enter the ID number for item:\n");
        inputID = scannerObject.nextInt();
        scannerObject.nextLine();
            for (current = head; !found; current = current.getNext()){
                if (current.getID() == inputID){
                    found = true;
                    break; // I need to get rid of break
                    }
                }       
            }catch(Exception e)
            {
                System.out.println("\nERROR!");
            }
    return current;
}

public void modify()
{   
    ItemNode current = findItem();
        if (current == null){
        System.out.println("\nInvalid input! Please try again:");
            }else{
        try{
            System.out.print("\nPlease enter name of item: ");
            String lname = scannerObject.nextLine();
            current.setName(lname);
            System.out.print("\nPlease enter a brief description of the item: ");
            String ldesc = scannerObject.nextLine();
            current.setDesc(ldesc);
            System.out.print("\nPlease enter the amount on hand: ");
            int lonHand = scannerObject.nextInt();
            current.setOnHand(lonHand);
            System.out.print("\nPlease enter unit price of the item: $");
            double lunitPrice = scannerObject.nextDouble();
            current.setUnitPrice(lunitPrice);
            scannerObject.nextLine();
        }catch (Exception e)
                {
                System.out.println("\nInvalid command! Please try again: ");
                }
        }
}

public void displayAll()
{   System.out.println("_______________________________________________________________________________\n");    
    System.out.println("                                 Inventory                                     ");
    System.out.println("_______________________________________________________________________________\n");
    System.out.printf("\n%-6s%-20s%-24s%-12s%-6s\n", "ID:", "Name:", "Description:","On Hand:", "Unit Price:\n"); //Header
    System.out.println("_______________________________________________________________________________\n");
    ItemNode current = head;
        if(current == null){
            System.out.println("The list is empty.");
        }else{
            while(current != null){
                current.display();
                current = current.getNext();
            }
        }
    }


public void displayOne()
{
    ItemNode current = findItem();
    if (current == null){
        System.out.println("\nInvalid input! Please try again:");
          }else{
        System.out.println("_______________________________________________________________________________\n");    
        System.out.println("                                 Inventory                                     ");
        System.out.println("_______________________________________________________________________________\n");
        System.out.printf("\n%-6s%-20s%-24s%-12s%-6s\n", "ID:", "Name:", "Description:","On Hand:", "Unit Price:\n"); //Header
        System.out.println("_______________________________________________________________________________\n");
            current.display();
        }
    }



}

ItemNode

import java.text.NumberFormat;

public class ItemNode
{
private int ID;
private String name;
private String Desc;
private int onHand;
private double unitPrice;
private ItemNode next;

public ItemNode(int pID)
{
    ID = pID;
}

public ItemNode(int pID, String pName, String pDesc, int pOnHand, double pUnitPrice) {
    ID = pID;
    name = pName;
    Desc = pDesc;
    onHand = pOnHand;
    unitPrice = pUnitPrice;
}

public void display()
{ 
    NumberFormat dollars = NumberFormat.getCurrencyInstance();
    System.out.printf("%-6s%-20s%-24s%-12s%-6s\n", ID, name, Desc, onHand, dollars.format(unitPrice));
}

// GETTERS AND SETTERS
public void setNext(ItemNode pNext)
{
    next = pNext;
}

public ItemNode getNext()
{
    return next;
}

public int getID()
{
    return ID;
}

public void setName(String pName)
{
    name = pName;
}

public String getName()
{
    return name;
}

public void setDesc(String pDesc)
{
    Desc = pDesc;
}

public String getDesc()
{
    return Desc;
}

    public void setOnHand(int pOnHand)
{
    onHand = pOnHand;
}

public int getOnHand()
{
    return onHand;
}
public void setUnitPrice(double pUnitPrice)
{
    unitPrice = pUnitPrice;
}

public double getUnitPrice()
{
    return unitPrice;
}

}

inventUserLL

import java.util.Scanner;

public class inventUserLL 
{

public static void main(String[] args) 
{

    InventoryLL myInvent = new InventoryLL();
    Scanner scannerObject = new Scanner(System.in);
    int Choice = 0;



        do{

        dispMenu();

        Choice = getChoice(scannerObject);

        proChoice(Choice, myInvent);

        }while (Choice !=0);


}

public static void dispMenu()
{
    System.out.println("\n|=============================================|");
    System.out.println("|                                             |");
    System.out.println("|******************Welcome********************|");
    System.out.println("|_____________________________________________|");
    System.out.println("|                                             |");
    System.out.println("|       Press [1] To Add An Item              |");
    System.out.println("|                                             |");
    System.out.println("|       Press [2] To Display One Item         |");
    System.out.println("|                                             |");
    System.out.println("|       Press [3] To Display All Items        |");
    System.out.println("|                                             |");
    System.out.println("|       Press [4] To Modify An Item           |");
    System.out.println("|                                             |");
    System.out.println("|       Press [0] To Exit                     |");
    System.out.println("|_____________________________________________|");
    System.out.println("|=============================================|");
    System.out.println("|         Please Make Selection Now...        |");
    System.out.println("|=============================================|");
    System.out.println("|_____________________________________________|\n");
    }

    public static int getChoice(Scanner scannerObject)
    {
        boolean x = false;
        int pChoice = 0;
            do{
             try{
                pChoice = scannerObject.nextInt();
                x = true;
            }catch (Exception e){
                scannerObject.next();
                System.out.println("\nInvalid command! Please try again:\n");
                }
            }while (x == false);
        return pChoice;
    }

    public static void proChoice(int Choice, InventoryLL myInvent)
    {

        switch(Choice){
            case 1: myInvent.addItem();
            break;
            case 2: myInvent.displayOne();
            break;
            case 3: myInvent.displayAll();
            break;
            case 4: myInvent.modify();
            break;
            case 0: System.out.println("\nHave a nice day!");
            break;
            }
    }
}

UPDATE

public ItemNode findItem(){
    int inputID;
    boolean found = false;
    ItemNode current = head;
        try{
        System.out.print("\nGreetings, please enter the ID number for item:\n");
        inputID = scannerObject.nextInt();
        scannerObject.nextLine();
                if (head != null){
                    while ((current.getNext() != null) && (!found)) {
                        if (current.getID() == inputID){
                        found = true;
                        } 
                        if (!found){
                            current = current.getNext();
                            }   
                        }
                    if (!found){
                        current = null;
                        }   
                }                   
            }catch(Exception e)
            {
                System.out.println("\nERROR!");
            }
    return current;
}

Answer 1

If i'm not wrong here, a "compound condition" is as follows:

if (head != null) {
    while ((current != null) && (!found)) {
        // check to see if you have the right value, set `found` if you do
        // otherwise, continue
        if (!found) { // do this otherwise you'll return the next value everytime
            current = current.getNext();
        }
    }
    if (!found) { // if you're at the end of the list and you didn't find the value
        // set current = to something you want to return if the value was not found
    }

}
return current;

Answer 2

Assuming root holds the first Node value.

ItemNode root = ...;

public ItemNode findNode(int value){
   if(root !=null){
       ItemNode temp = root;
       while(temp!=null){
           if(temp.getID() == value) return temp;
           temp = temp.next;
       }
   }
   return null;
}

Answer 3

May I understand that you want to remove the 'break;' in your method and get the function still work?

Did u try that replace 'break;' with 'return current'?

public ItemNode findItem(){
    int inputID;
    boolean found = false;
    ItemNode current = null;
        try{
        System.out.print("\nGreetings, please enter the ID number for item:\n");
        inputID = scannerObject.nextInt();
        scannerObject.nextLine();
            for (current = head; !found; current = current.getNext()){
                if (current.getID() == inputID){                    
                    return current; // return the result when you find the node
                    }
                }       
            }catch(Exception e)
            {
                System.out.println("\nERROR!");
            }
    return current;
}

Answer 4

You can remove the break without loss of semantics. Now the definition of head seems to be missing.

What your instructor means: Your loop would cause a NullPointerException if the searched object is not in the list. So the condition when to break the loop must also contain a check on an existing next element, eg like this:

for (current = head; !found && current.hasNext(); current = current.getNext()) {

And you should check not to enter this loop with head == null .