Priority Queue to List. not adding properly

Question

I am trying to copy items out of a priority queue and into an ArrayList. for some reason, when there are three or four items, it stops after adding two items to the list. If there are 5 items, it stops after copying 3 items to the list. What am I doing wrong?

PriorityQueue<T> queue= new PriorityQueue<T> () ; 
List<T> list = new ArrayList<T>();

 for (int i = 0 ; i< queue.size(); i++)
        {
           list.add(0, queue.poll());
        }

Answer 1

Try the following code:

PriorityQueue<T> queue= new PriorityQueue<T> () ; 
List<T> list = new ArrayList<T>();

while(!queue.isEmpty()){
       list.add(0, queue.poll());
    }

An example:

import java.util.*;
import java.lang.*;
import java.io.*;

class Main{
    public static void main (String[] args) {
        PriorityQueue<Integer> queue = new PriorityQueue<>();
        queue.add(5);
        queue.add(4);
        queue.add(3);
        queue.add(2);
        queue.add(1);
        List<Integer> list = new ArrayList<>();
        while(!queue.isEmpty()) {
            list.add(0,queue.poll());
        }
        System.out.println(list);  // Prints [5, 4, 3, 2, 1]

    }
}

Why your for loop is not working:

Consider iteration 1: i = 0 and i <

queue.size() = 5 queue = {5,4,3,2,1} list = {}

After iteration 1:

queue = {5,4,3,2} list = {1} i = 1

At iteration 2: i = 1 i < queue.size() = 4 queue = {5,4,3,2} list = {1}

After iteration 2: queue = {5,4,3} list = {1,2} i = 2

At iteration 3: i = 2 i < queue.size() = 3 queue = {5,4,3,2} list = {1}

After iteration 3: queue = {5,4} list = {1,2,3} i = 3

At iteration 4: i = 3 i < queue.size() = 3? = > False

Quit the loop!

So you are quitting the for loop when still queue = {5,4} and all the elements are not added to list.