问题
Consider the following code:
import java.util.PriorityQueue;
public class Test {
public static void main(String argv[]) {
PriorityQueue<A> queue = new PriorityQueue<>();
System.out.println("Size of queue is " + queue.size()); // prints 0
try {
queue.add(new A());
} catch (ClassCastException ignored) { }
System.out.println("Size of queue is " + queue.size()); // prints 1
}
}
class A { } // non-comparable object
In this code, an object, which is explicitly non-comparable, is added to a PriorityQueue. As expected per PriorityQueue.add Javadoc, this code throws a ClassCastException because the object is not comparable.
However, it seems that the size of the queue is still increased, although an exception was thrown.
I would have expected both print statements to output 0 but the second one actually outputs 1, as if an object had been added to the queue.
What is happening here?
回答1:
I would not expect an exception to be thrown at all on that line. Read the docs more closely: an exception may be thrown
if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering
Since there are no other elements currently in the priority queue, I would not necessarily expect an exception to be thrown.
回答2:
This is the actual statement in the documentation:
A priority queue relying on natural ordering also does not permit insertion of non-comparable objects (doing so may result in
ClassCastException).
Doing something to the queue that isn't permitted implies no guarantees on how the queue is going to behave when it is done nevertheless. As an additional confirmation, look carefully at this part: "doing so may result in a ClassCastException." In other words, the implementation isn't even required to throw the exception.
回答3:
You claim that an exception is thrown by add(), but your test does not prove that claim. If in fact an exception were thrown, then you should expect the queue's size to remain 0. HOWEVER, you should not expect an exception to be thrown.
The docs for PriorityQueue.add() say:
ClassCastException- if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering
If there are no other elements, then that case does not arise. If you attempted to add another element, however, then an exception should be thrown, and the queue's size should remain 1.
来源：https://stackoverflow.com/questions/32318500/adding-a-non-comparable-object-to-a-priorityqueue