Java鏈接列表優先級隊列

Question

我想在java中使用鏈表創建一個優先級隊列，但是某些功能無法正常工作。我理解優先級隊列的一般功能，但在Java方面我是一個完整的初學者。我看了其他例子，似乎無法找到我的錯誤。有什麼建議？我注意到的一件事是使用類型或什麼，但我不完全確定它是什麼。

第一類：

public class Node {  //Node class structure 

    int data; //data contained in Node  
    Node next; //pointer to Next Node 

    public int getData() { 
     return data; 
    } 
    public void setData(int data) { 
     this.data = data; 
    } 
    public Node getNext() { 
     return next; 
    } 
    public void setNext(Node next) { 
     this.next = next; 
    } 

} 

二等：

import work.Node; 

//basic set-up of a singly linked list 
public class SLList{ 

    Node head; //head of SLList 
    Node tail; //tail of SLList 
    int n;  //number of elements in SLList 

} 

第三類：

import work.Node; 

public class PriorityQueue extends SLList{ 

//add a new node 
public void add(int x){ 
    Node y = new Node(); 
    y.data = x; 

    if (tail == null){ //if there is no existing tail, thus an empty list 
     tail = y;  //assign tail and head as new node y 
     head = y; 
    } 
    else if (y.data < head.data){ //if new node y is the smallest element, thus highest priority 
     y.next = head;    //assign y's next to be current head of queue 
     head = y;     //reassign head to be actual new head of queue (y) 
    } 
    else{    //if there is already a tail node 
     tail.next = y; //assign the tail's pointer to the new node 
     tail = y;  //reassign tail to actual new tail of queue (y) 
    } 
    n++;    //increment the queue's size 
} 

//delete the minimim (highest priority value) from the queue 
public Node deleteMin(){ 
    if (n == 0){   //if the list is of size 0, and thus empty 
     return null;  //do nothing 
    } 
    else{       //if there are node(s) in the list 
     Node min = head;   //assign min to the head 
     head = head.next;   //reassign head as next node,    
     n--;      //decrement list size 
     return min;    //return the minimum/highest priority value 
    }  
} 

//return the size of the queue 
public int size() { 
    return n;   
} 
} 

測試代碼：

import work.Node; 
import work.SLList; 
import work.PriorityQueue; 

public class Test { 

public static void main(String[] args){ 
    PriorityQueue PQueue1 = new PriorityQueue(); 

    PQueue1.add(3); 
    PQueue1.add(2); 
    PQueue1.add(8); 
    PQueue1.add(4); 

    System.out.println("Test add(x): " + PQueue1); 
    System.out.println("Test size() " + PQueue1.size()); 

    PriorityQueue PQueue2 = new PriorityQueue(); 
    PQueue2 = PQueue1.deleteMin();    //the data types don't line up but I don't know what should be changed 

    System.out.println("Test deleteMin() " + PQueue2); 
    System.out.println("Test size() " + PQueue2.size()); 
} 
} 

Answer 1

從您的測試類

PQueue2 = PQueue1.deleteMin();    //this line isn't working* 

此行

代碼不工作，因爲類型不匹配的：你不能轉換從節點到的PriorityQueue。

爲ClassCastException API規格：

拋出，表明代碼已經試圖對象轉換的子類，其中它不是一個實例。例如，下面的代碼生成一個ClassCastException：

在你的例子中，這將工作;

Node node = PQueue1.deleteMin();    //this will work