PriorityQueue Problem

[mohitkumar gupta]

http://nikojava.wordpress.com

PROBLEM-1
code1

public static void main(String[] args) {
    Queue<String> queue = new PriorityQueue<String>();
    queue.add("J");
    queue.add("A");
    queue.add("V");
    queue.add("A");
    Arrays.sort(queue.toArray());
    for (String element : queue) {
        System.out.format("%s ", element);
    }
}    

OPTIONS:

1.It prints J A V A.
2.It prints A A J V.
3.The output order is not guaranteed.

correct answer:3.

code2

  public static void main(String[] args) {
    Queue<String> queue = new PriorityQueue<String>();
    queue.add("J");
    queue.add("A");
    queue.add("V");
    queue.add("A");
    String[] array = queue.toArray(new String[0]);
    Arrays.sort(array);
    for (String element : array) {
        System.out.format("%s ", element);
    }
}  

OUTPUT:


It prints A A J V


MY Question is

why the code1 doesnot give same output as code2 because JLS says:
This class(PRIORITY QUEUE) and its iterator implement all of the optional methods of the Collection and Iterator interfaces. The Iterator provided in method iterator() is not guaranteed to traverse the elements of the priority queue in any particular order. If you need ordered traversal, consider using Arrays.sort(pq.toArray())


------------------------------------------------------------------------------------


PROBLEM-2
codeA

public static void main(String[] args) {
    Queue<String> queue = new PriorityQueue<String>();
    System.out.println(queue.remove());
}

the above throws Exception at run time.

now see this code
codeB

   Queue<String> queue = new PriorityQueue<String>();
    queue.add("J");
    queue.add("A");
    queue.add("V");
    queue.add("A");
   System.out.println(queue.remove());
   //queue.remove();
    Arrays.sort(queue.toArray());
    for (String element : queue) {
        System.out.format("%s ", element);} 

the output of this program is
A
A J V

--------

when i uncomment line 7,then output is
A
J V

QUERIES:

a.why the line 6 doesnot display true or false ?
b.why is not a exception thrown as in the program codeA ?
c.why is the line 7 removes element from the queue as JLS SAYS:

remove
public boolean remove(Object o)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).

Specified by:
remove in interface Collection<E>
Overrides:
remove in class AbstractCollection<E>
Parameters:
o - element to be removed from this queue, if present
Returns:
true if this queue changed as a result of the call

[Mehdi Ben Larbi]

I can answer your first question : the toArray method without any argument returns an Array of objects so there no natural order for sorting objects and we didn't specify any comparator,therefore the output is not guaranteed.
In the other case toArray(String[]) returns an array of strings and the sorting is done according to natural order.
Priority Queue allows duplicates so we get AAJV.

Hope it helps.

[Ankit Garg]

For problem 1, the toArray method of PriorityQueue class returns an Array which is not connected to the queue. So modifications to the array don't effect the PriorityQueue. When you iterate a PriorityQueue, the order of elements is not guaranteed. PriorityQueue maintains an order of elements when you call the peek or poll methods. While iterating a PriorityQueue, the elements are not iterated in sorted order.
In the second case you are iterating over an array. Since the array is sorted, the output is sorted.

For your second problem, you are looking at the description of boolean remove(Object o) method while you are calling the E remove() method of AbstractQueue class which says

Retrieves and removes the head of this queue. This method differs from poll only in that it throws an exception if this queue is empty.
This implementation returns the result of poll unless the queue is empty.

So the remove() method returns the removed element not true or false...