java - 循环队列可能尚未初始化

Question

    import linearstructures.*;
    import non_lineardatastructures.*;
    import workers.*;
    import binarynodes.*;
    import linearnodes.*;
    import dataobjects.*;
    import java.util.*;
    public class Main{
        public static Scanner input= new Scanner(System.in);

        public static BinarySearchTree tree;
        public static String surname;
        public static String surname2;
        public static int seqNum;
        public static double pay;
        public static Employee name;
        Node front;
        Node rear;

        //public static String surname3;

        public static void main(String args[])
        {
            int menu;

            do{ 
                System.out.println("\f\f");
                System.out.println("1. Construct an Empty Queue and Binary Tree");
                System.out.println("2. Populate Queue with Employees and Part-Timers");
                System.out.println("3. List all objects in Queue");
                System.out.println("4. Edit an Object in Queue");
                System.out.println("5. Populate BST with Queue objects");
                System.out.println("6. Search BST by surname");
                System.out.println("7. List BST In-Order");
                System.out.println("8. Exit menu");
                System.out.println("Enter your choice: ");

                menu=input.nextInt();
                switch(menu){
                    case 1:CQueue queue = new CQueue(); BinarySearchTree tree = new BinarySearchTree();System.out.println("A new Queue and BST have been constructed!");break;
                    case 2:System.out.println("Do you want to create an Employee or a Part-Timer?");
                    System.out.println("Enter e or p");
                    String choice=input.next();
                    if(choice.equals("e")){
                        queue=null;
                        Employee name=new Employee();
                        System.out.println("Enter surname:");
                        surname=input.next();
                        name.setSurname(surname);

                        System.out.println("Enter seqNum:");
                        seqNum=input.nextInt();
                        name.setSeq(seqNum);
                        System.out.println("Enter pay:");
                        pay=input.nextDouble();
                        name.setPay(pay);
                        queue.put(name);
                        System.out.println("A new Employee has been created!");
                    }
                    else if(choice.equals("p")){
                        queue=null;
                        PartTimer pt=new  PartTimer();
                        System.out.println("Enter surname:");
                        surname2=input.next();
                        pt.setSurname(surname);

                        System.out.println("Enter seqNum:");
                        int seqNum=input.nextInt();
                        pt.setSeq(seqNum);
                        System.out.println("Enter pay:");
                        double pay=input.nextDouble();
                        pt.setPay(pay);
                        System.out.println("Enter hours worked:");
                        int hours=input.nextInt();
                        pt.setHours(hours);
                        queue.put(pt);
                        System.out.println("A new Part-Timer has been created!");
                    }
                    else System.out.println("Enter a valid choice!");;break;
                    case 3:queue=null;queue.listAll();break;
                    case 4:System.out.println("Enter surname:");
                    String surname3=input.next();
                    System.out.println(surname3);

                    if(queue!=null){
                        System.out.print("not empty!");
                        if((queue.searchKey(surname3))!=null){
                            queue.editObject(surname2);
                            queue.listAll();
                        }
                    }else System.out.print("empty");break;
                    case 5: tree.populateFromQueue(queue);break;
                    case 6:System.out.println("Enter surname:");
                    surname3=input.next();
                    tree.search(surname3);break;
                    case 7: tree.inorderBST();break;
                    case 8:break;
                    default: System.out.print("Enter a valid choice!");
                }
                System.out.println("Press any key to continue..");
                String cont=input.next();
            }while(menu!=8);
        }   




    }
    package linearstructures;
    import dataobjects.*;
    import linearnodes.*;
    import java.util.*;
    import workers.*;

package linearstructures;
import dataobjects.*;
import linearnodes.*;
import java.util.*;
import workers.*;

public class CQueue
{
    public static  Node front, rear,temp;
    public static int maxNodes; // maximum number of nodes allowed
    public static int currNodes;// current number of nodes in the queue
    public static boolean full = false;
    String key;

    Scanner input= new Scanner(System.in);
    // Constructor. Limits size of th queue to the number of nodes.
    public CQueue()
    {
        front = null;
        rear = null;
        maxNodes = 20;
        currNodes = 0;
    }

    // Appends the given newObj to the queue if able. Returns true if successful, otherwise false
    public static boolean  put(AnyClass newObj)
    {

     
        if (currNodes < maxNodes)
        {
            Node newNode = new Node(newObj);
           /* if(full){
                return false;
            }else{
                rear=rear.next;
                rear.obj=newObj;
                                
                if(rear.next==front){
                    full=true;
                }
                currNodes++;
                return true;
            } */
        
            if (rear == null)
            {
               front = newNode;
                rear = newNode;
            }
            else
            {
                
                rear.next=newNode;
                rear = rear.next;
            }

            currNodes++;

            return true;
        }
        else
            return false;
           
   
}

    // Returns reference to the front object of AnyClass or null if queue is empty. First node is removed from the Queue.
    public AnyClass serve()
    {
        if(front==null){
            return null;
        }
        else{
            AnyClass obj=front.obj;
            Node firstNode=front;
            front=front.next;
            firstNode=null;
            return obj;
        }
    }

    // Part 3
    public static void listAll()
    {    
       if(rear!=null){
           temp=front;
  do{
        for (int i= 0; i<currNodes; i++)
        {
            System.out.println(temp.obj.getData()); 
            temp = temp.next;
        }
    
    }while(!(temp==rear.next));
   } else System.out.println("Empty queue!");
   }
    public AnyClass searchKey(String key)
    {
        temp=front; // first node
         //CQueue queue = new CQueue(); 
        do
        {
            if ((temp.obj.getKey()).equals(key))
                return temp.obj;
            temp = temp.next;
        } while (temp!=rear.next);
        return null;
    }

    public AnyClass editObject(String key){   
        int choice, newSeqNo;
        double newPay;
        boolean exit = false;
        Employee etemp = new Employee();
      
        AnyClass searchResult = searchKey(key);

        if(searchKey(key)!= null){
            temp.obj.getData();
            System.out.println();
            System.out.println("------------------------");
            System.out.print("Enter new Salary: ");
            newPay = input.nextDouble();
            System.out.println("------------------------");
     
           if(searchResult != null ){
        // Check if searchResult is an Employee object
        if( searchResult instanceof Employee )
            // Cast AnyClass to Employee
            etemp = (Employee) searchResult;

            // Your logic here...

            // Set properties of the Employee object
            etemp.setPay(newPay);
            System.out.print("edited:"+etemp.getData());

            // ... System.out.print("edited:"+etemp.getData);

           return etemp;
        }

        }else{
            System.out.println("NO OBJECT WAS FOUND!");
            return null;
        } return etemp;
    }

}

我创建了一个循环队列。但是，当我尝试对队列进行操作时，它返回 null，当我列出队列中的所有项目时，列表是正确的，但是当我尝试编辑功能时，它显示队列为空。我不知道这怎么可能。

Answer 1

您正在 switch 语句中声明队列和树。在外部声明它们:

CQueue queue = new CQueue(); 
BinarySearchTree tree = new BinarySearchTree();
switch(menu){
    // here you can re-create them if the user chooses to do so
    case 1:
        queue = new CQueue(); 
        tree = new BinarySearchTree();
        System.out.println("A new Queue and BST have been constructed!");
        break;
    ....