difference of add method between Queue class and Stack class

[oszc]

import java.util.*;

public class PriorityQueueTest {

	Queue<Number> queue = new PriorityQueue<Number>();
	Number a;
	
	public PriorityQueueTest(){
		queue.offer(1000);
		queue.offer(33.23);
		queue.offer(100.2);
		queue.offer(92);
		queue.offer(-21);
		
		while ( (a = queue.poll()) !=null )
		{
			System.out.println(a);
		}
	}
	
	
	public static void main(String[] args) {
		new PriorityQueueTest();
	}

}

error information:

Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Double
at java.lang.Double.compareTo(Unknown Source)
at java.util.PriorityQueue.siftUpComparable(Unknown Source)
at java.util.PriorityQueue.siftUp(Unknown Source)
at java.util.PriorityQueue.offer(Unknown Source)
at PriorityQueueTest.<init>(PriorityQueueTest.java:10 )
at PriorityQueueTest.main(PriorityQueueTest.java:24)

however, when i use Stack<Number> a = new Stack<Number>(); i could add all the Double, Integer and Float. So what's the diference between them? how can i figure out the difference in api doc?

[JosAH]

however, when i use Stack<Number> a = new Stack<Number>(); i could add all the Double, Integer and Float. So what's the diference between them? how can i figure out the difference in api doc?

Stacks and Queues can store objects only so autoboxing is applied when you attempt to store primitives (ints, longs etc.) A Queue needs its members to implement the Comparable interface because it has to store the members in a certain order; a Stack doesn't do that. The Number, Integer, Double etc. hierarchy is a bit stupid: Integers can only be compared to Integers, Doubles to Doubles etc. You put Integers and Doubles in your Queue and tries to compare them. You saw the result in your stack trace. A cheap hack would be to store doubles in your Queue only.

kind regards,

Jos