java 堆栈，foreach，错误的顺序？
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Stack, foreach, wrong order?
提问by durron597
When using Java's foreach syntax, Stackdoesn't use LIFO ordering on the outputted elements. Consider the following code:
使用 Java 的foreach 语法时，Stack不对输出的元素使用 LIFO 排序。考虑以下代码：
import java.util.Queue;
import java.util.Stack;
import java.util.LinkedList;
public class QueueStackTest {
private static int[] numbers = {1, 2, 3, 4, 5};
public static void main(String[] args) {
Stack<Integer> s = new Stack<Integer>();
Queue<Integer> l = new LinkedList<Integer>();
for (int i : numbers) {
s.push(i);
l.offer(i);
}
System.out.println("Stack: ");
for(Integer i : s) {
System.out.println(i);
}
System.out.println();
System.out.println("Queue:");
for(Integer i : l) {
System.out.println(i);
}
}
}
Output:
输出：
Stack:
1
2
3
4
5
Queue:
1
2
3
4
5
Questions:
问题：
- Does this make sense? Is it a bug?
- Can I guarantee that this will, at least, return Queue elements in the correct order?
- When consuming (processing) a
Stackor aQueue, is this the best way to do it? Or should I make a more manual loop with something like:while(!s.isEmpty()) { handle(s.pop()); }orwhile(!l.isEmpty()) { handle(l.poll()); }
- 这有意义吗？这是一个错误吗？
- 我可以保证这至少会以正确的顺序返回 Queue 元素吗？
- 使用（处理） a
Stack或 a 时Queue，这是最好的方法吗？或者我应该使用以下内容进行更手动的循环：while(!s.isEmpty()) { handle(s.pop()); }或while(!l.isEmpty()) { handle(l.poll()); }
采纳答案by fvu
There is an interesting footnote in Stack's Javadoc:
Stack 的 Javadoc 中有一个有趣的脚注：
A more complete and consistent set of LIFO stack operations is provided by the Deque interface and its implementations, which should be used in preference to this class. For example:
Deque stack = new ArrayDeque();
Deque 接口及其实现提供了一组更完整和一致的 LIFO 堆栈操作，应优先使用此类。例如：
Deque stack = new ArrayDeque();
An extended version of your program:
程序的扩展版本：
public static void main(String[] args) {
Stack<Integer> s = new Stack<Integer>();
Deque<Integer> d = new ArrayDeque<Integer>();
Queue<Integer> l = new LinkedList<Integer>();
for (int i : numbers) {
s.push(i);
l.offer(i);
d.push(i);
}
System.out.println("Stack: ");
for(Integer i : s) {
System.out.println(i);
}
System.out.println();
System.out.println("Queue:");
for(Integer i : l) {
System.out.println(i);
}
System.out.println();
System.out.println("Deque:");
for(Integer i : d) {
System.out.println(i);
}
}
gives
给
....
Deque:
5
4
3
2
1
So maybe switch to Deque for a more consistent behavior.
所以也许切换到 Deque 以获得更一致的行为。
回答by Amit
You need to use pop() & poll() instead of for loop. Thats the API offered by Stack/Queue.
您需要使用 pop() 和 poll() 而不是 for 循环。这就是 Stack/Queue 提供的 API。
When you iterate, you are directly iterating the internal representation of Stack/Queue.
当你迭代的时候，你是在直接迭代 Stack/Queue 的内部表示。
