Question

我正在尝试使用PriorityQueue实现堆，如下所示：

PriorityQueue<Node> heap = new PriorityQueue<Node>();
Set<String> allWords = codebook.getAllWords();
for(String word : allWords)
{
    heap.add(new Node(word, codebook.getProbability(word)));
    System.out.println(heap.toString());
}

我将Node定义为包含上述方法的同一类中的私有类。节点定义为：

private static class Node implements Comparable
{
    protected Node left;
    protected Node right;
    protected String name;
    protected double frequency;
    public Node(String n, double f)
    {
        name = n;
        frequency = f;
    }
    public Node(double f, Node l, Node r)
    {
        frequency = f;
        left = l;
        right = r;
    }

    @Override
    public int compareTo(Object arg0) 
    {
        Node other = (Node)(arg0);
        if(this.frequency < other.frequency)
        {
            System.out.println(name + " < " + other.name);
            return -1;
        }
        else if(this.frequency > other.frequency)
        {
            System.out.println(name + " > " + other.name);
            return 1;
        }
        System.out.println(name + " is equal to " + other.name);
        return 0;
    }

    public String toString()
    {return name;}
}

但是，当我向PriorityQueue添加节点时，它们不按频率排序。根据我的println语句的输出，Node.compareTo（）返回正确的值。例如，给定数据集：

名称，频率
需要，3
cat，1
neat，2

我的代码产生：
//添加需要
[需要]
//添加猫
猫＆lt;需要
[猫，需要]
//添加整齐整洁＆gt;猫
[猫，需要，整洁]
当PriorityQueue应该是[cat，neat，need]
有关为什么会发生这种情况的任何提示？

Answer 1

来自iterator for PriorityQueue的订单未定义;调用poll()时的顺序应该是比较器排序。来自API规范，

iterator()返回元素上的迭代器在这个队列中。迭代器没有返回任何特定的元素顺序。

如果你真正需要的是一个有序集，请使用SortedSet或将东西放入集合并使用Collections.sort()。但是，如果你真的需要一个pqueue，这是我的例子修复：

import java.util.HashMap;
import java.util.Map;
import java.util.PriorityQueue;
import java.util.Set;


public class TestPriorityQueue 
{
  static Map<String,Double> codebook = new HashMap<String, Double>();
  static {
    codebook.put("need", 3.0);
    codebook.put("cat", 1.0);
    codebook.put("neat", 2.0);
  }

  public static void main(String[] args)
  {
    test();
  }

  public static void test() {
    PriorityQueue<Node> heap = new PriorityQueue<Node>();
    Set<String> allWords = codebook.keySet();
    for (String word : allWords) {
      heap.add(new Node(word, codebook.get(word)));
      System.out.println(heap.toString());
    }

    System.out.println("In order now:");
    while(!heap.isEmpty()) {
      System.out.println(heap.poll());
    }
  }

  private static class Node implements Comparable<Node>
  {
      protected Node left;
      protected Node right;
      protected String name;
      protected double frequency;
      public Node(String n, double f)
      {
          name = n;
          frequency = f;
      }
      public Node(double f, Node l, Node r)
      {
          frequency = f;
          left = l;
          right = r;
      }

      @Override
      public int compareTo(Node arg0) 
      {
          if(this.frequency < arg0.frequency)
          {
              System.out.println(name + " < " + arg0.name);
              return -1;
          }
          else if(this.frequency > arg0.frequency)
          {
              System.out.println(name + " > " + arg0.name);
              return 1;
          }
          System.out.println(name + " is equal to " + arg0.name);
          return 0;
      }

      public String toString()
      {return name;}
  }

}

给出：

[need]
cat < need
[cat, need]
neat > cat
[cat, need, neat]
In order now:
neat < need
cat
neat
need

Answer 2

PriorityQueues以“及时”为基础开展工作。如果只显示其内容，则内容不会按排序顺序排列;他们在一堆（自从你提到之后，我认为你已经知道了。）无论如何，要按顺序获取内容，你必须使用i poll（）逐个删除一个项目。

添加到PriorityQueue的对象不按其优先级排序

2 个答案: