概念

PriorityQueue 优先队列是一种先进先出（FIFO）的数据结构。和普通队列不同的是它出队的顺序是有优先级的

使用

构造方法

• Integer 或者 String类型

  // 小堆
  PriorityQueue<Integer> priorityQueue = new PriorityQueue<>();
  
  // 大堆
  PriorityQueue<Student> priorityQueue = new PriorityQueue<>(new Comparator<Student>() {  
          @Override  
          public int compare(Student o1, Student o2) {  
              return o1.age - o2.age;  
          }  
      }); 

• 实体类
  如果需要存储对象 需要重写 Comparator 否则会报错
  比如下面这个代码

  public static void main(String[] args) {  
      PriorityQueue<Student> priorityQueue = new PriorityQueue<>();  
      priorityQueue.offer(new Student( 19,"小明"));  
      priorityQueue.offer(new Student( 24,"大明"));  
  }  
    
    
  static class Student{  
      private int age;  
      private String name;  
      public Student(int age, String name){  
          this.age = age;  
          this.name = name;  
      }  
    
      public int getAge() {  
          return age;  
      }  
    
      public void setAge(int age) {  
          this.age = age;  
      }  
    
      public String getName() {  
          return name;  
      }  
    
      public void setName(String name) {  
          this.name = name;  
      }  
  }

会报

Exception in thread “main” java.lang.ClassCastException: Test$Student cannot be cast to java.lang.Comparable
at java.util.PriorityQueue.siftUpComparable(PriorityQueue.java:653)
at java.util.PriorityQueue.siftUp(PriorityQueue.java:648)
at java.util.PriorityQueue.offer(PriorityQueue.java:345)
at Test.main(SqlTest.java:9)

需要改写成这样

public static void main(String[] args) {  
    PriorityQueue<Student> priorityQueue = new PriorityQueue<>(new Comparator<Student>() {  
        @Override  
        public int compare(Student o1, Student o2) {  
            return o1.age - o2.age;  
        }  
    });  
    priorityQueue.offer(new Student( 19,"小明"));  
    priorityQueue.offer(new Student( 24,"大明"));  
}  
  
  
static class Student{  
    private int age;  
    private String name;  
    public Student(int age, String name){  
        this.age = age;  
        this.name = name;  
    }  
  
    public int getAge() {  
        return age;  
    }  
  
    public void setAge(int age) {  
        this.age = age;  
    }  
  
    public String getName() {  
        return name;  
    }  
  
    public void setName(String name) {  
        this.name = name;  
    }  
}

常用方法

public boolean add(E e); //在队尾添加元素，并调整堆结构
public E remove(); //在队头删除元素，并返回，再调整堆结构
public E element(); //返回队头元素（不删除）
public boolean isEmpty(); //判断队列是否为空
​
public int size(); //获取队列中元素个数
public void clear(); //清空队列
public boolean contains(Object o); //判断队列中是否包含指定元素（从队头到队尾遍历）
public Iterator<E> iterator(); //迭代器

public E peek(); //获取优先级最高的元素
public E poll(); //移除优先级最高的元素 并返回
public boolean offer(E e); //插入元素, 成功返回 true, 会自动扩容

参考 ![[1464-数组中两元素的最大乘积]]