java 当类型未知时，如何迭代 Iterable 对象？
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How do I iterate over an Iterable object when the type is not known?
提问by Mirrana
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..
对于家庭作业，我需要实现自己的 PriorityQueue 和 PriorityQueueSort。我使用泛型让它在没有排序功能的情况下工作，但现在我被困在这里..
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?
我需要传入一个列表和一个空的 PriorityQueue，所以我对如何做到这一点的最佳猜测就在上面。我应该如何解决这个问题，以便我可以遍历未知类型的列表，并将该列表中具有正确类型的每个元素添加到优先级队列中？
Edit:
编辑：
Here are a few more details since it was determined that I didn't include enough information.
由于确定我没有包含足够的信息，因此这里有更多详细信息。
I have a custom PriorityQueue class, and a custom Entry class that holds a key of type K, and a value of type V.
我有一个自定义的 PriorityQueue 类和一个自定义的 Entry 类，其中包含一个 K 类型的键和一个 V 类型的值。
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.
我需要能够获取任何类型 T 的任何可迭代列表并遍历它，获取每个项目并将其添加到最初为空的 PriorityQueue 作为具有空值的键。然后我需要在我的 PriorityQueue 上连续调用 removeMin() 并将它按顺序添加回同一个列表对象。
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());
}
}
回答by Jon Skeet
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.
就方法签名而言，您目前拥有的内容没有意义 - 例如，它会让您传入 aList<Button>和 a PriorityQueue<String>。
I suspect you actually want something like:
我怀疑你真的想要这样的东西：
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.
请注意，这里的差异只是提供了更多的灵活性 -例如，您可以使用 a List<Circle>but a PriorityQueue<Shape>，它仍然是类型安全的。
EDIT: Now that we have more details, I think you want something like this:
编辑：现在我们有更多的细节，我想你想要这样的东西：
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 putmethod. We still don't know what your PriorityQueueclass looks like.)
（假设你有一个put方法。我们仍然不知道你的PriorityQueue类是什么样的。）
回答by SLaks
You need to make the method generic so that you can refer to the type:
您需要使方法通用，以便您可以引用类型：
public static <T> void PriorityQueueSort(Iterable<T> list,
PriorityQueue<?,T> pq) {
