迭代器为我们提供了统一的遍历容器的方式。
1、Iterator源码分析
public Iterator<E> iterator() {return new Itr();}private class Itr implements Iterator<E> {int cursor; // index of next element to returnint lastRet = -1; // index of last element returned; -1 if no suchpublic boolean hasNext() {return cursor != size;}@SuppressWarnings("unchecked")public E next() {int i = cursor;Object[] elementData = ArrayList.this.elementData;cursor = i + 1;return (E) elementData[lastRet = i];}public void remove() {ArrayList.this.remove(lastRet);cursor = lastRet;lastRet = -1;expectedModCount = modCount;}}private class ListItr extends Itr implements ListIterator<E> {ListItr(int index) {super();cursor = index;}public boolean hasPrevious() {return cursor != 0;}public int nextIndex() {return cursor;}public int previousIndex() {return cursor - 1;}@SuppressWarnings("unchecked")public E previous() {int i = cursor - 1;Object[] elementData = ArrayList.this.elementData;cursor = i;return (E) elementData[lastRet = i];}public void set(E e) {}public void add(E e) {}}
2、迭代器遍历List/Set集合
import java.util.ArrayList;import java.util.Iterator;import java.util.ListIterator;public class TestListItr {public static void main(String[] args) {ArrayList<String> list = new ArrayList<String>();for (int i = 0; i < 5; i++) {list.add("a" + i);}System.out.println(list);for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {String temp = iterator.next();System.out.print(temp + "t");if(temp.endsWith("3")) {iterator.remove();}}System.out.println();System.out.println(list);//for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {//String temp = iterator.next();//System.out.print(temp + "t");//if(temp.endsWith("4")) {//list.add("a5");//运行时异常ConcurrentModificationException并发修改异常//}//}//ListIterator是Iterator接口的扩展，用于解决并发修改异常 for (ListIterator<String> iterator = list.listIterator(); iterator.hasNext();) {String temp = iterator.next();System.out.print(temp + "t");if(temp.endsWith("4")) {iterator.add("a5");}}System.out.println();System.out.println(list);}}
运行结果：
注意：
如果遇到遍历容器时，判断删除元素的情况，使用Iterator遍历；如果是判断添加修改元素的情况，使用ListIterator遍历。
3、迭代器遍历Map集合
import java.util.HashMap;import java.util.Iterator;import java.util.Map.Entry;import java.util.Set;public class TestMapItr {public static void main(String[] args) {HashMap<String,Integer> map = new HashMap<String, Integer>();map.put("hello", 123);map.put("world", 456);map.put("java", 789);//方式一Set<String> keySet = map.keySet();for (Iterator<String> iterator = keySet.iterator(); iterator.hasNext();) {String key = iterator.next();System.out.print(key + "=" + map.get(key) + "t");}System.out.println();//方式二Entry<String, Integer>键值关系的类型Set<Entry<String, Integer>> entrySet = map.entrySet();for (Iterator<Entry<String, Integer>> iterator = entrySet.iterator(); iterator.hasNext();) {Entry<String, Integer> entry = iterator.next();System.out.print(entry.getKey() + "--" + entry.getValue() + "t");}System.out.println();}}
运行结果：
我们也可以通过map的keySet()、valueSet()获得key和value的集合，从而遍历它们。
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