Iterator接口

源代碼

package java.util;import java.util.function.Consumer;/** * An iterator over a collection. {@code Iterator} takes the place of * {@link Enumeration} in the Java Collections Framework. Iterators * differ from enumerations in two ways: * * <ul> * <li> Iterators allow the caller to remove elements from the * underlying collection during the iteration with well-defined * semantics. * <li> Method names have been improved. * </ul> * * <p>This interface is a member of the * <a href='http://www.intensediesel.com/bcjs/technotes/guides/collections/index.html' rel='external nofollow' > * Java Collections Framework</a>. * * @param <E> the type of elements returned by this iterator * * @author Josh Bloch * @see Collection * @see ListIterator * @see Iterable * @since 1.2 */public interface Iterator<E> { /** * Returns {@code true} if the iteration has more elements. * (In other words, returns {@code true} if {@link #next} would * return an element rather than throwing an exception.) * * @return {@code true} if the iteration has more elements */ boolean hasNext(); /** * Returns the next element in the iteration. * * @return the next element in the iteration * @throws NoSuchElementException if the iteration has no more elements */ E next(); /** * Removes from the underlying collection the last element returned * by this iterator (optional operation). This method can be called * only once per call to {@link #next}. The behavior of an iterator * is unspecified if the underlying collection is modified while the * iteration is in progress in any way other than by calling this * method. * * @implSpec * The default implementation throws an instance of * {@link UnsupportedOperationException} and performs no other action. * * @throws UnsupportedOperationException if the {@code remove} * operation is not supported by this iterator * * @throws IllegalStateException if the {@code next} method has not * yet been called, or the {@code remove} method has already * been called after the last call to the {@code next} * method */ default void remove() { throw new UnsupportedOperationException('remove'); } /** * Performs the given action for each remaining element until all elements * have been processed or the action throws an exception. Actions are * performed in the order of iteration, if that order is specified. * Exceptions thrown by the action are relayed to the caller. * * @implSpec * <p>The default implementation behaves as if: * <pre>{@code * while (hasNext()) * action.accept(next()); * }</pre> * * @param action The action to be performed for each element * @throws NullPointerException if the specified action is null * @since 1.8 */ default void forEachRemaining(Consumer<? super E> action) { Objects.requireNonNull(action); while (hasNext()) action.accept(next()); }}

閱讀筆記

1. Iterator接口與Enumeration接口的關系/Iterator接口在Java集合庫中的作用

Iterator接口是Java集合框架的一部分，被用于替代原有的Enumeration接口。（“Iterator”比“Enumeration”更簡短、表意更清晰、功能更多，具體的信息下面Enumeration接口的注解中說的挺清楚，且Enumeration注解中也建議編程人員改用Iterator接口）

Java類庫中，集合類的基本接口是Collection接口，而Collection接口實現了Iterable接口，Iterable接口中有一個iterator()方法用于獲取Iterator對象。

package java.util;/** * An object that implements the Enumeration interface generates a * series of elements, one at a time. Successive calls to the * <code>nextElement</code> method return successive elements of the * series. * <p> * For example, to print all elements of a <tt>Vector&lt;E&gt;</tt> <i>v</i>: * <pre> * for (Enumeration&lt;E&gt; e = v.elements(); e.hasMoreElements();) * System.out.println(e.nextElement());</pre> * <p> * Methods are provided to enumerate through the elements of a * vector, the keys of a hashtable, and the values in a hashtable. * Enumerations are also used to specify the input streams to a * <code>SequenceInputStream</code>. * <p> * NOTE: The functionality of this interface is duplicated by the Iterator * interface. In addition, Iterator adds an optional remove operation, and * has shorter method names. New implementations should consider using * Iterator in preference to Enumeration. * * @see java.util.Iterator * @see java.io.SequenceInputStream * @see java.util.Enumeration#nextElement() * @see java.util.Hashtable * @see java.util.Hashtable#elements() * @see java.util.Hashtable#keys() * @see java.util.Vector * @see java.util.Vector#elements() * * @author Lee Boynton * @since JDK1.0 */public interface Enumeration<E> { /** * Tests if this enumeration contains more elements. * * @return <code>true</code> if and only if this enumeration object * contains at least one more element to provide; * <code>false</code> otherwise. */ boolean hasMoreElements(); /** * Returns the next element of this enumeration if this enumeration * object has at least one more element to provide. * * @return the next element of this enumeration. * @exception NoSuchElementException if no more elements exist. */ E nextElement();}

2.hasNext()、next()、remove()方法的關系

hasNext()方法：判斷是否還有元素可以進行迭代；

next()方法：迭代元素；

remove()方法：

/*** Remove from the underlying collection the last element returned by this iterator*（optional operation）. * 移除當前迭代器上一次從基礎集合中迭代的元素（可選操作）** This method can be called only once per call to next().* 調用remove()方法前必須先調用next()方法，調用完一次remove()方法后想要再次調用remove()方法，* 必須先調用next()方法。** The behavior of an iterator is unspecified if the underlying collection is modifyed while* the iteration is in progress is any way other than by call this method.* 如果在迭代進行過程中修改了基礎集合，則迭代器的行為是不確定的。*/public static void main(String[] args) { Collection<String> stringCollection = new ArrayList<>(); stringCollection.add('Hello'); stringCollection.add('World'); stringCollection.add('!'); Iterator<String> stringIterator = stringCollection.iterator(); stringIterator.next(); stringIterator.remove();//OK }public static void main(String[] args) { ...... stringIterator.next(); stringCollection.add('abc');//基本集合被改變 stringIterator.remove();//ERROR - java.util.ConcurrentModificationException }public static void main(String[] args) { ...... stringIterator.next(); stringCollection.add('abc');//基本集合被改變 stringIterator.next();//ERROR - java.util.ConcurrentModificationException }public static void main(String[] args) { ...... stringIterator.next(); stringCollection.add('abc');//基本集合改變 stringIterator = stringCollection.iterator();//重新獲取迭代器 stringIterator.next();//OK stringIterator.remove();//OK }

三者關系：調用remove()方法前必須先調用next()方法，調用next()方法前最好先調用hasNext()方法。

3.具體實現類

AbstractList類中定義了一個實現了Iterator接口的內部類：

private class Itr implements Iterator<E> { /** * Index of element to be returned by subsequent call to next. */ int cursor = 0; /** * Index of element returned by most recent call to next or * previous. Reset to -1 if this element is deleted by a call * to remove. */ int lastRet = -1; /** * The modCount value that the iterator believes that the backing * List should have. If this expectation is violated, the iterator * has detected concurrent modification. */ int expectedModCount = modCount; public boolean hasNext() { return cursor != size(); } public E next() { checkForComodification(); try { int i = cursor; E next = get(i); lastRet = i;//最近一次調用next()方法返回的元素的下標。 cursor = i + 1;//下一次調用next()方法返回的元素的下標。 return next; } catch (IndexOutOfBoundsException e) { checkForComodification(); throw new NoSuchElementException(); } } public void remove() { if (lastRet < 0) throw new IllegalStateException();//所以，調用remove()前必須先調用next() checkForComodification(); try { AbstractList.this.remove(lastRet); if (lastRet < cursor)cursor--;//因為移除了一個元素 lastRet = -1;//所以，不能連續調用兩次remove()方法 expectedModCount = modCount; } catch (IndexOutOfBoundsException e) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); }}

看完上面的代碼，我對modCount、expectedModCount變量以及checkForComodification()方法的作用比較好奇，所以嘗試著去搞清楚。

先來看modeCount變量，這個變量被聲明在內部類的外部：

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> { /** * The number of times this list has been <i>structurally modified</i>. * Structural modifications are those that change the size of the * list, or otherwise perturb it in such a fashion that iterations in * progress may yield incorrect results. * 用于表示該列表發生結構性修改的次數。結構性修改是指*更改列表的大小*或*以其他 * 方式干擾列表*，即正在進行的迭代可能會產生錯誤的結果。 * * <p>This field is used by the iterator and list iterator implementation * returned by the {@code iterator} and {@code listIterator} methods. * If the value of this field changes unexpectedly, the iterator (or list * iterator) will throw a {@code ConcurrentModificationException} in * response to the {@code next}, {@code remove}, {@code previous}, * {@code set} or {@code add} operations. This provides * <i>fail-fast</i> behavior, rather than non-deterministic behavior in * the face of concurrent modification during iteration. * 設計者認為，與其因為基本集合被并發修改從而使迭代產生不確定行為，不如盡早給出錯誤。 * * <p><b>Use of this field by subclasses is optional.</b> If a subclass * wishes to provide fail-fast iterators (and list iterators), then it * merely has to increment this field in its {@code add(int, E)} and * {@code remove(int)} methods (and any other methods that it overrides * that result in structural modifications to the list). A single call to * {@code add(int, E)} or {@code remove(int)} must add no more than * one to this field, or the iterators (and list iterators) will throw * bogus {@code ConcurrentModificationExceptions}. If an implementation * does not wish to provide fail-fast iterators, this field may be * ignored. * 是否使用應需求決定。 */ protected transient int modCount = 0;}

看完上面的源碼注解，已經大概能夠知道modCount、expectedModCount以及checkForComodification()的作用了。

假如把基礎集合當作一個銀行賬號，基礎集合中的元素表示存款。那么modCount就相當于銀行為每個賬號做的消費記錄，expectedModCount就相當于是賬號持有人自己做的一份消費記錄，一般銀行和賬號持有人自己做的消費記錄都不會出錯。

final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException();}

一旦銀行那邊的消費記錄和自己手里的那份消費記錄對不上，肯定是賬號被盜用了。

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