Java實現PriorityQueue

發現本身的知識面水平實在是過低了，以前有這個概念，可是不知道Java已經實現了 PriorityQueue了，經別人的提醒，回來看了一下Thinking In Java，裏面的介紹確實很是詳細，並且代碼寫的很明朗。主要是一個實現了Comparable接口來對Priority進行控制。

In computer science, a priority queue is an abstract data type which is like a regular queue or stack data structure, but additionally, each element is associated with a 「priority」.
stack — elements are pulled in last-in first-out-order (e.g. a stack of papers)
queue — elements are pulled in first-in first-out-order (e.g. a line in a cafeteria)
priority queue — elements are pulled highest-priority-first (e.g. cutting in line, or VIP service).

A priority queue must at least support the following operations:
insert_with_priority: add an element to the queue with an associated priority
pull_highest_priority_element: remove the element from the queue that has the highest priority, and return it (also known as 「pop_element(Off)」, 「get_maximum_element」, or 「get_front(most)_element」; some conventions consider lower priorities to be higher, so this may also be known as 「get_minimum_element」, and is often referred to as 「get-min」 in the literature; the literature also sometimes implement separate 「peek_at_highest_priority_element」 and 「delete_element」 functions, which can be combined to produce 「pull_highest_priority_element」)

下面就是其中的代碼：

package com.bjwilly.test;
 
import java.util.PriorityQueue;
 
public class PriorityQueueTest extends PriorityQueue<PriorityQueueTest.TodoItem>{
    static class TodoItem implements Comparable<TodoItem>{
        private char primary;
        private int secondary;
        private String item;
         
        public TodoItem(String td,char pri,int sec){
            primary = pri;
            secondary = sec;
            item = td;
        }
        @Override
        public int compareTo(TodoItem arg) {
            if(primary > arg.primary)
                return +1;
            if(primary == arg.primary)
                if(secondary > arg.secondary)
                    return +1;
                else if(secondary == arg.secondary)
                    return 0;
            return -1;
        }
         
        public String toString(){
            return Character.toString(primary)+ secondary + ": " + item;
        }
         
    }
     
    public void add(String td,char pri,int sec){
        super.add(new TodoItem(td,pri,sec));
    }
    public static void main(String[] args) {
        PriorityQueueTest toDoList = new PriorityQueueTest();
        toDoList.add("Empty trash",'C',4);
        toDoList.add("Feed dog",'A',2);
        toDoList.add("Feed bird",'B',7);
        toDoList.add("Mow lawn",'C',3);
        toDoList.add("Water lawn",'A',1);
        toDoList.add("Feed cat",'B',1);
        while(!toDoList.isEmpty())
            System.out.println(toDoList.remove());
    }
}

輸出結果

A1: Water lawn
A2: Feed dog
B1: Feed cat
B7: Feed bird
C3: Mow lawn
C4: Empty trash