PriorityQueue的意外行为删除：为什么不使用compareTo？

Question

I am trying to use the priority queue, but the remove() is not working: My code:

PriorityQueue<OwnClass> pq=new PriorityQueue<OwnClass>();
OwnClass a=new OwnClass(1);
OwnClass b=new OwnClass(2);
OwnClass c=new OwnClass(3);
pq.add(a);
pq.add(b);
pq.add(c);
System.out.println("head:"+pq.peek());
pq.remove(new OwnClass(1));
System.out.println(pq.peek());

And the class implementation:

class OwnClass implements Comparable{

    int x;

    public OwnClass(int x){
        this.x=x;
    }

    public int compareTo(Object arg0) {

        OwnClass a=(OwnClass) arg0;
        if(a.x>this.x)
            return -1;
        if(a.x<x)
            return 1;
        return 0;
    }

    public String toString(){
        return ""+x;        
    }
}

I think the output final output should be 2, since I am removing the added '1'. The compareTo() should be used by priority queue remove() but his does not seem to be the case. What I am doing wrong? I know pq.remove(a) will work, but then my code should also work

Answer 1

remove() will not use compareTo() , rather it will use equals() to find the object to remove. You need to override equals() on your class as well.

Edit: Javadoc for PriorityQueue (Thanks, @templatetypedef)

Answer 2

The PriorityQueue class's remove method has the following description:

Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element e such that o.equals(e) , if this queue contains one or more such elements. Returns true if and only if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).

Thus compareTo is not used when determining whether to remove something. I believe that this is because PriorityQueue implements Collection , and the behavior of remove must therefore be consistent with the behavior specified in Collection , which is

Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if this collection contains one or more such elements.

In other words, I believe this design decision is motivated by trying to fit PriorityQueue into the Collection framework, even though it is a bit odd.

Hope this helps!

Answer 3

Please note that PriorityQueue has an other constructor that takes a Comparator. So it could be to keep the remove behavior consistent even if a Comparator is used or the objects in the priorityQueue implement the Comparable Interface, equals does not depend on any comparison except the base equals property of the Object.