Java集合中迭代实现（foreach语句）

 对于集合的一个基本的操作利用foreach语句遍历集合以处理集合中的每个元素。看下面的代码：

 

Java代码

1. // 下面是关于foreach语句的使用，代码非常简洁和紧凑
2. Stack<String> collection = new Stack<String>();
3. // ....
4. for (String s : collection) {
5. System.out.println(s);
6. }
7. // ...
8.  
9. // 下面使用while语句来代替上面的foreach语句来实现相同的功能
10. Stack<String> collection = new Stack<String>();
11. Iterator<String> iterator = collection.iterator();
12. while (iterator.hasNext()) {
13. System.out.println(iterator.next());
14. }

// 下面是关于foreach语句的使用，代码非常简洁和紧凑
Stack<String> collection = new Stack<String>();
// ....
for (String s : collection) {
    System.out.println(s);
}
// ...

// 下面使用while语句来代替上面的foreach语句来实现相同的功能
Stack<String> collection = new Stack<String>();
Iterator<String> iterator = collection.iterator();
while (iterator.hasNext()) {
    System.out.println(iterator.next());
}

上面的代码说明为了使用foreach语句我们必须实现可迭代的集合，在Java中就是实现Iterable<T>接口：

 

Java代码

1. // java.lang.Iterable
2. public interface Iterable<T> {
3. Iterator<T> iterator();
4. }

// java.lang.Iterable
public interface Iterable<T> {
    Iterator<T> iterator();
}

其实，我们要实现的有两个方面：（1）集合必须实现iterator()方法，并返回一个Iterator（这在Java中也是个接口）对象。（2）Iterator类必须包括两个方法：1.hasNext()，返回一个布尔值。2.next()，返回集合中的一个元素。下面是一个例子：

 

Java代码

1. import java.util.Iterator;
2. import java.util.NoSuchElementException;
3. public class Stack<E> implements Iterable<E>
4. {
5. private int size; // size of the stack
6. private Node first; // top of stack
7.  
8. // helper linked list class
9. private class Node
10. {
11. private E element;
12. private Node next;
13. }
14. // create an empty stack
15. public Stack() {
16. size = 0;
17. first = null;
18. assert check();
19. }
20. // Is the stack empty?
21. public boolean isEmpty() {
22. return (first == null);
23. }
24. // return the number of items in the stack
25. public int size() {
26. return size;
27. }
28.  
29. /*
30. Add the element to the stack.
31. */
32. public void push(E element) {
33. Node oldfirst = first;
34. first = new Node();
35. first.element = element;
36. first.next = oldfirst;
37. size++;
38. assert check();
39. }
40. /*
41. Delete and return the item most recently added to the stack.
42. @throws java.util.NoSuchElementException if the stack is Empty
43. */
44.  
45. public E pop() {
46. if (isEmpty())
47. {
48. throw new NoSuchElementException("Stack underflow");
49. }
50. E element = first.element; // save element to return
51. first = first.next; // delete first node
52. size--;
53. assert check();
54. return element; // return the saved element
55. }
56.  
57. /*
58. Return the element most recently added to the stack.
59. @throws java.util.NoSuchElementException if stack is Empty.
60. */
61. public E peek() {
62. if (isEmpty())
63. {
64. throw new NoSuchElementException("Stack underflow");
65. }
66. return first.element;
67. }
68.  
69. /*
70. Return string representation.
71. */
72.  
73. public String toString() {
74. StringBuilder sb = new StringBuilder();
75. for (E element: this)
76. {
77. sb.append(element + "-");
78. }
79. return sb.toString();
80. }
81.  
82. /* check internal invariants */
83. private boolean check() {
84. if (size == 0)
85. {
86. if (first != null)
87. {
88. return false;
89. }
90. }
91.  
92. return true;
93. }
94. /* return an iterator to the stack that iterates
95. through the elemets in LIFO order */
96. public Iterator<E> iterator() {
97. return new ListIterator();
98. }
99. // an iterator,doesn't implement remove() since it's optional
100. private class ListIterator implements Iterator<E> {
101. private Node current = first;
102. public boolean hasNext() {
103. return current != null;
104. }
105.  
106. public void remove() {
107. throw new UnsupportedOperationException();
108. }
109.  
110. public E next() {
111. if (!hasNext()) {
112. throw new NoSuchElementException();
113. }
114. E element = current.element;
115. current = current.next;
116.  
117. return element;
118. }
119. }
120.  
121. public static void main(String[] args)
122. {
123. Stack<String> s = new Stack<String>();
124. while (!StdIn.isEmpty())
125. {
126. String element = StdIn.readString();
127. if (!element.equals("-"))
128. {
129. s.push(element);
130. }else if(!s.isEmpty()) {
131. StdOut.print(s.pop() + " ");
132. }
133. }
134.  
135. StdOut.println("(" + s.size() + "left on stack)");
136. }
137. }