How do I iterate over an Iterable object when the type is not known?

algorithm, iterable, java

Solution

What you've got at the moment doesn't make sense in terms of the method signature - it would let you pass in a `List<Button>` and a `PriorityQueue<String>` for example.

I suspect you actually want something like:

public static <T> void prioritySortQueue(Iterable<? extends T> iterable,
    PriorityQueue<? super T> queue) {

    for (T item : iterable) {
        queue.add(item);
    }
}

Note that the variance here just gives more flexibility - you could have a `List<Circle>` but a `PriorityQueue<Shape>` for example, and it's still type-safe.

EDIT: Now that we have more details, I think you want something like this:

public static <K> void prioritySortQueue(Iterable<? extends K> iterable,
    PriorityQueue<? super K, ?> queue) {

    for (T item : iterable) {
        queue.put(item, null);
    }
}

(Assuming you have a `put` method. We still don't know what your `PriorityQueue` class looks like.)

Problem

For a homework assignment, I need to implement my own PriorityQueue and PriorityQueueSort. I used generics to get it working without the sort function, but now I'm stuck here.. ``` public static void PriorityQueueSort(Iterable<?> list, PriorityQueue<?,?> pq) { if (!pq.isEmpty()) { throw new IllegalArgumentException("Non-Empty PriorityQueue"); } for (Object obj : list) { } } ``` I need to pass in a list and an empty PriorityQueue, so my best guess at how to do this is just above. How should I attack this so that I can iterate through the list with unknown type, and add each element in that list with the proper type into the priority queue? Edit: Here are a few more details since it was determined that I didn't include enough information. I have a custom PriorityQueue class, and a custom Entry class that holds a key of type K, and a value of type V. I need to be able to take any iterable list with any type T and iterate through it, taking each item and add it to an initially empty PriorityQueue as a key with null value. I then need to continuously call removeMin() on my PriorityQueue and add it in order back into the same list object. ``` public class PriorityQueue<K extends Comparable<? super K>,V> { private Entry<K,V> _head; private Entry<K,V> _tail; private int _size; public PriorityQueue() { this._head = null; this._tail = null; this._size = 0; } public int size() { return _size; } public boolean isEmpty() { return (size() == 0); } public Entry<K,V> min() { if (_head == null) { return null; } Entry<K,V> current = _head; Entry<K,V> min = _head;; while (current != null) { if (current.compareTo(min) < 0) { min = current; } current = current.getNext(); } return min; } public Entry<K,V> insert(K k, V x) { Entry<K,V> temp = new Entry<K,V>(k,x); if (_tail == null) { _tail = temp; _head = temp; } else { _tail.setNext(temp); temp.setPrev(_tail); _tail = temp; } return temp; } public Entry<K,V> removeMin() { Entry<K,V> smallest = min(); smallest.getPrev().setNext(smallest.getNext()); smallest.getNext().setPrev(smallest.getPrev()); return smallest; } public String toString() { return null; } public static <K> void PriorityQueueSort(Iterable<? extends K> list, PriorityQueue<? super K, ?> queue) { for (K item : list) { queue.insert(item, null); } list.clear(); } public static void main(String[] args) { PriorityQueue<Integer, Integer> pq = new PriorityQueue<Integer, Integer>(); pq.insert(4, 2); pq.insert(5, 1); System.out.println(pq.min().toString()); } } ```

Original source