第二十讲：迭代模式

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我是靠谱客的博主震动跳跳糖，最近开发中收集的这篇文章主要介绍第二十讲：迭代模式，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

容器类的职责太多了.

Iterator迭代接口,JDK里面其实也提供了一个迭代接口.JDK里面已经很好地实现了这个迭代模式.

/*
 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.util;

/**
 * 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="{@docRoot}/../technotes/guides/collections/index.html">
 * 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.
     *
     * @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
     */
    void remove();
}

ConcreteIterator:迭代器的具体实现类,这个实现细节已经被JDK隐藏了(如果你没有很好地研究过JDK的话是不知道这个迭代器的实现类的.).

/*
 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.util;

/**
 * The root interface in the <i>collection hierarchy</i>.  A collection
 * represents a group of objects, known as its <i>elements</i>.  Some
 * collections allow duplicate elements and others do not.  Some are ordered
 * and others unordered.  The JDK does not provide any <i>direct</i>
 * implementations of this interface: it provides implementations of more
 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>.  This interface
 * is typically used to pass collections around and manipulate them where
 * maximum generality is desired.
 *
 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
 * duplicate elements) should implement this interface directly.
 *
 * <p>All general-purpose <tt>Collection</tt> implementation classes (which
 * typically implement <tt>Collection</tt> indirectly through one of its
 * subinterfaces) should provide two "standard" constructors: a void (no
 * arguments) constructor, which creates an empty collection, and a
 * constructor with a single argument of type <tt>Collection</tt>, which
 * creates a new collection with the same elements as its argument.  In
 * effect, the latter constructor allows the user to copy any collection,
 * producing an equivalent collection of the desired implementation type.
 * There is no way to enforce this convention (as interfaces cannot contain
 * constructors) but all of the general-purpose <tt>Collection</tt>
 * implementations in the Java platform libraries comply.
 *
 * <p>The "destructive" methods contained in this interface, that is, the
 * methods that modify the collection on which they operate, are specified to
 * throw <tt>UnsupportedOperationException</tt> if this collection does not
 * support the operation.  If this is the case, these methods may, but are not
 * required to, throw an <tt>UnsupportedOperationException</tt> if the
 * invocation would have no effect on the collection.  For example, invoking
 * the {@link #addAll(Collection)} method on an unmodifiable collection may,
 * but is not required to, throw the exception if the collection to be added
 * is empty.
 *
 * <p><a name="optional-restrictions"/>
 * Some collection implementations have restrictions on the elements that
 * they may contain.  For example, some implementations prohibit null elements,
 * and some have restrictions on the types of their elements.  Attempting to
 * add an ineligible element throws an unchecked exception, typically
 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
 * to query the presence of an ineligible element may throw an exception,
 * or it may simply return false; some implementations will exhibit the former
 * behavior and some will exhibit the latter.  More generally, attempting an
 * operation on an ineligible element whose completion would not result in
 * the insertion of an ineligible element into the collection may throw an
 * exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <p>It is up to each collection to determine its own synchronization
 * policy.  In the absence of a stronger guarantee by the
 * implementation, undefined behavior may result from the invocation
 * of any method on a collection that is being mutated by another
 * thread; this includes direct invocations, passing the collection to
 * a method that might perform invocations, and using an existing
 * iterator to examine the collection.
 *
 * <p>Many methods in Collections Framework interfaces are defined in
 * terms of the {@link Object#equals(Object) equals} method.  For example,
 * the specification for the {@link #contains(Object) contains(Object o)}
 * method says: "returns <tt>true</tt> if and only if this collection
 * contains at least one element <tt>e</tt> such that
 * <tt>(o==null ? e==null : o.equals(e))</tt>."  This specification should
 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
 * invoked for any element <tt>e</tt>.  Implementations are free to implement
 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
 * example, by first comparing the hash codes of the two elements.  (The
 * {@link Object#hashCode()} specification guarantees that two objects with
 * unequal hash codes cannot be equal.)  More generally, implementations of
 * the various Collections Framework interfaces are free to take advantage of
 * the specified behavior of underlying {@link Object} methods wherever the
 * implementor deems it appropriate.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements in this collection
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Set
 * @see     List
 * @see     Map
 * @see     SortedSet
 * @see     SortedMap
 * @see     HashSet
 * @see     TreeSet
 * @see     ArrayList
 * @see     LinkedList
 * @see     Vector
 * @see     Collections
 * @see     Arrays
 * @see     AbstractCollection
 * @since 1.2
 */

public interface Collection<E> extends Iterable<E> {
    // Query Operations

    /**
     * Returns the number of elements in this collection.  If this collection
     * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of elements in this collection
     */
    int size();

    /**
     * Returns <tt>true</tt> if this collection contains no elements.
     *
     * @return <tt>true</tt> if this collection contains no elements
     */
    boolean isEmpty();

    /**
     * Returns <tt>true</tt> if this collection contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this collection
     * contains at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this collection is to be tested
     * @return <tt>true</tt> if this collection contains the specified
     *         element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this collection
     *         (<a href="#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements
     *         (<a href="#optional-restrictions">optional</a>)
     */
    boolean contains(Object o);

    /**
     * Returns an iterator over the elements in this collection.  There are no
     * guarantees concerning the order in which the elements are returned
     * (unless this collection is an instance of some class that provides a
     * guarantee).
     *
     * @return an <tt>Iterator</tt> over the elements in this collection
     */
    Iterator<E> iterator();

    /**
     * Returns an array containing all of the elements in this collection.
     * If this collection makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the elements in
     * the same order.
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this collection.  (In other words, this method must
     * allocate a new array even if this collection is backed by an array).
     * The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this collection
     */
    Object[] toArray();

    /**
     * Returns an array containing all of the elements in this collection;
     * the runtime type of the returned array is that of the specified array.
     * If the collection fits in the specified array, it is returned therein.
     * Otherwise, a new array is allocated with the runtime type of the
     * specified array and the size of this collection.
     *
     * <p>If this collection fits in the specified array with room to spare
     * (i.e., the array has more elements than this collection), the element
     * in the array immediately following the end of the collection is set to
     * <tt>null</tt>.  (This is useful in determining the length of this
     * collection <i>only</i> if the caller knows that this collection does
     * not contain any <tt>null</tt> elements.)
     *
     * <p>If this collection makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the elements in
     * the same order.
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
     * The following code can be used to dump the collection into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of this collection are to be
     *        stored, if it is big enough; otherwise, a new array of the same
     *        runtime type is allocated for this purpose.
     * @return an array containing all of the elements in this collection
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this collection
     * @throws NullPointerException if the specified array is null
     */
    <T> T[] toArray(T[] a);

    // Modification Operations

    /**
     * Ensures that this collection contains the specified element (optional
     * operation).  Returns <tt>true</tt> if this collection changed as a
     * result of the call.  (Returns <tt>false</tt> if this collection does
     * not permit duplicates and already contains the specified element.)<p>
     *
     * Collections that support this operation may place limitations on what
     * elements may be added to this collection.  In particular, some
     * collections will refuse to add <tt>null</tt> elements, and others will
     * impose restrictions on the type of elements that may be added.
     * Collection classes should clearly specify in their documentation any
     * restrictions on what elements may be added.<p>
     *
     * If a collection refuses to add a particular element for any reason
     * other than that it already contains the element, it <i>must</i> throw
     * an exception (rather than returning <tt>false</tt>).  This preserves
     * the invariant that a collection always contains the specified element
     * after this call returns.
     *
     * @param e element whose presence in this collection is to be ensured
     * @return <tt>true</tt> if this collection changed as a result of the
     *         call
     * @throws UnsupportedOperationException if the <tt>add</tt> operation
     *         is not supported by this collection
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this collection
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements
     * @throws IllegalArgumentException if some property of the element
     *         prevents it from being added to this collection
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to insertion restrictions
     */
    boolean add(E e);

    /**
     * Removes a single instance of the specified element from this
     * collection, if it is present (optional operation).  More formally,
     * removes an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
     * this collection contains one or more such elements.  Returns
     * <tt>true</tt> if this collection contained the specified element (or
     * equivalently, if this collection changed as a result of the call).
     *
     * @param o element to be removed from this collection, if present
     * @return <tt>true</tt> if an element was removed as a result of this call
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this collection
     *         (<a href="#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements
     *         (<a href="#optional-restrictions">optional</a>)
     * @throws UnsupportedOperationException if the <tt>remove</tt> operation
     *         is not supported by this collection
     */
    boolean remove(Object o);


    // Bulk Operations

    /**
     * Returns <tt>true</tt> if this collection contains all of the elements
     * in the specified collection.
     *
     * @param  c collection to be checked for containment in this collection
     * @return <tt>true</tt> if this collection contains all of the elements
     *         in the specified collection
     * @throws ClassCastException if the types of one or more elements
     *         in the specified collection are incompatible with this
     *         collection
     *         (<a href="#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this collection does not permit null
     *         elements
     *         (<a href="#optional-restrictions">optional</a>),
     *         or if the specified collection is null.
     * @see    #contains(Object)
     */
    boolean containsAll(Collection<?> c);

    /**
     * Adds all of the elements in the specified collection to this collection
     * (optional operation).  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in progress.
     * (This implies that the behavior of this call is undefined if the
     * specified collection is this collection, and this collection is
     * nonempty.)
     *
     * @param c collection containing elements to be added to this collection
     * @return <tt>true</tt> if this collection changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
     *         is not supported by this collection
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this collection
     * @throws NullPointerException if the specified collection contains a
     *         null element and this collection does not permit null elements,
     *         or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this
     *         collection
     * @throws IllegalStateException if not all the elements can be added at
     *         this time due to insertion restrictions
     * @see #add(Object)
     */
    boolean addAll(Collection<? extends E> c);

    /**
     * Removes all of this collection's elements that are also contained in the
     * specified collection (optional operation).  After this call returns,
     * this collection will contain no elements in common with the specified
     * collection.
     *
     * @param c collection containing elements to be removed from this collection
     * @return <tt>true</tt> if this collection changed as a result of the
     *         call
     * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
     *         is not supported by this collection
     * @throws ClassCastException if the types of one or more elements
     *         in this collection are incompatible with the specified
     *         collection
     *         (<a href="#optional-restrictions">optional</a>)
     * @throws NullPointerException if this collection contains one or more
     *         null elements and the specified collection does not support
     *         null elements
     *         (<a href="#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean removeAll(Collection<?> c);

    /**
     * Retains only the elements in this collection that are contained in the
     * specified collection (optional operation).  In other words, removes from
     * this collection all of its elements that are not contained in the
     * specified collection.
     *
     * @param c collection containing elements to be retained in this collection
     * @return <tt>true</tt> if this collection changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
     *         is not supported by this collection
     * @throws ClassCastException if the types of one or more elements
     *         in this collection are incompatible with the specified
     *         collection
     *         (<a href="#optional-restrictions">optional</a>)
     * @throws NullPointerException if this collection contains one or more
     *         null elements and the specified collection does not permit null
     *         elements
     *         (<a href="#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean retainAll(Collection<?> c);

    /**
     * Removes all of the elements from this collection (optional operation).
     * The collection will be empty after this method returns.
     *
     * @throws UnsupportedOperationException if the <tt>clear</tt> operation
     *         is not supported by this collection
     */
    void clear();


    // Comparison and hashing

    /**
     * Compares the specified object with this collection for equality. <p>
     *
     * While the <tt>Collection</tt> interface adds no stipulations to the
     * general contract for the <tt>Object.equals</tt>, programmers who
     * implement the <tt>Collection</tt> interface "directly" (in other words,
     * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
     * or a <tt>List</tt>) must exercise care if they choose to override the
     * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
     * course of action is to rely on <tt>Object</tt>'s implementation, but
     * the implementor may wish to implement a "value comparison" in place of
     * the default "reference comparison."  (The <tt>List</tt> and
     * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
     *
     * The general contract for the <tt>Object.equals</tt> method states that
     * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
     * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
     * and <tt>Set.equals</tt> state that lists are only equal to other lists,
     * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
     * collection class that implements neither the <tt>List</tt> nor
     * <tt>Set</tt> interface must return <tt>false</tt> when this collection
     * is compared to any list or set.  (By the same logic, it is not possible
     * to write a class that correctly implements both the <tt>Set</tt> and
     * <tt>List</tt> interfaces.)
     *
     * @param o object to be compared for equality with this collection
     * @return <tt>true</tt> if the specified object is equal to this
     * collection
     *
     * @see Object#equals(Object)
     * @see Set#equals(Object)
     * @see List#equals(Object)
     */
    boolean equals(Object o);

    /**
     * Returns the hash code value for this collection.  While the
     * <tt>Collection</tt> interface adds no stipulations to the general
     * contract for the <tt>Object.hashCode</tt> method, programmers should
     * take note that any class that overrides the <tt>Object.equals</tt>
     * method must also override the <tt>Object.hashCode</tt> method in order
     * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
     * In particular, <tt>c1.equals(c2)</tt> implies that
     * <tt>c1.hashCode()==c2.hashCode()</tt>.
     *
     * @return the hash code value for this collection
     *
     * @see Object#hashCode()
     * @see Object#equals(Object)
     */
    int hashCode();
}

Collection:容器接口实现了Iterator接口,表示它可以实现迭代.Iterator的实现类我们是不知道的.Iterator的实现细节是隐藏的很深的.

/*
 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.util;

/**
 * This class provides a skeletal implementation of the {@link List}
 * interface to minimize the effort required to implement this interface
 * backed by a "random access" data store (such as an array).  For sequential
 * access data (such as a linked list), {@link AbstractSequentialList} should
 * be used in preference to this class.
 *
 * <p>To implement an unmodifiable list, the programmer needs only to extend
 * this class and provide implementations for the {@link #get(int)} and
 * {@link List#size() size()} methods.
 *
 * <p>To implement a modifiable list, the programmer must additionally
 * override the {@link #set(int, Object) set(int, E)} method (which otherwise
 * throws an {@code UnsupportedOperationException}).  If the list is
 * variable-size the programmer must additionally override the
 * {@link #add(int, Object) add(int, E)} and {@link #remove(int)} methods.
 *
 * <p>The programmer should generally provide a void (no argument) and collection
 * constructor, as per the recommendation in the {@link Collection} interface
 * specification.
 *
 * <p>Unlike the other abstract collection implementations, the programmer does
 * <i>not</i> have to provide an iterator implementation; the iterator and
 * list iterator are implemented by this class, on top of the "random access"
 * methods:
 * {@link #get(int)},
 * {@link #set(int, Object) set(int, E)},
 * {@link #add(int, Object) add(int, E)} and
 * {@link #remove(int)}.
 *
 * <p>The documentation for each non-abstract method in this class describes its
 * implementation in detail.  Each of these methods may be overridden if the
 * collection being implemented admits a more efficient implementation.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @since 1.2
 */

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractList() {
    }

    /**
     * Appends the specified element to the end of this list (optional
     * operation).
     *
     * <p>Lists that support this operation may place limitations on what
     * elements may be added to this list.  In particular, some
     * lists will refuse to add null elements, and others will impose
     * restrictions on the type of elements that may be added.  List
     * classes should clearly specify in their documentation any restrictions
     * on what elements may be added.
     *
     * <p>This implementation calls {@code add(size(), e)}.
     *
     * <p>Note that this implementation throws an
     * {@code UnsupportedOperationException} unless
     * {@link #add(int, Object) add(int, E)} is overridden.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws UnsupportedOperationException if the {@code add} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this list
     */
    public boolean add(E e) {
        add(size(), e);
        return true;
    }

    /**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    abstract public E get(int index);

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public E set(int index, E element) {
        throw new UnsupportedOperationException();
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public E remove(int index) {
        throw new UnsupportedOperationException();
    }


    // Search Operations

    /**
     * {@inheritDoc}
     *
     * <p>This implementation first gets a list iterator (with
     * {@code listIterator()}).  Then, it iterates over the list until the
     * specified element is found or the end of the list is reached.
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public int indexOf(Object o) {
        ListIterator<E> it = listIterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return it.previousIndex();
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return it.previousIndex();
        }
        return -1;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation first gets a list iterator that points to the end
     * of the list (with {@code listIterator(size())}).  Then, it iterates
     * backwards over the list until the specified element is found, or the
     * beginning of the list is reached.
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public int lastIndexOf(Object o) {
        ListIterator<E> it = listIterator(size());
        if (o==null) {
            while (it.hasPrevious())
                if (it.previous()==null)
                    return it.nextIndex();
        } else {
            while (it.hasPrevious())
                if (o.equals(it.previous()))
                    return it.nextIndex();
        }
        return -1;
    }


    // Bulk Operations

    /**
     * Removes all of the elements from this list (optional operation).
     * The list will be empty after this call returns.
     *
     * <p>This implementation calls {@code removeRange(0, size())}.
     *
     * <p>Note that this implementation throws an
     * {@code UnsupportedOperationException} unless {@code remove(int
     * index)} or {@code removeRange(int fromIndex, int toIndex)} is
     * overridden.
     *
     * @throws UnsupportedOperationException if the {@code clear} operation
     *         is not supported by this list
     */
    public void clear() {
        removeRange(0, size());
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation gets an iterator over the specified collection
     * and iterates over it, inserting the elements obtained from the
     * iterator into this list at the appropriate position, one at a time,
     * using {@code add(int, E)}.
     * Many implementations will override this method for efficiency.
     *
     * <p>Note that this implementation throws an
     * {@code UnsupportedOperationException} unless
     * {@link #add(int, Object) add(int, E)} is overridden.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        boolean modified = false;
        for (E e : c) {
            add(index++, e);
            modified = true;
        }
        return modified;
    }


    // Iterators

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * <p>This implementation returns a straightforward implementation of the
     * iterator interface, relying on the backing list's {@code size()},
     * {@code get(int)}, and {@code remove(int)} methods.
     *
     * <p>Note that the iterator returned by this method will throw an
     * {@link UnsupportedOperationException} in response to its
     * {@code remove} method unless the list's {@code remove(int)} method is
     * overridden.
     *
     * <p>This implementation can be made to throw runtime exceptions in the
     * face of concurrent modification, as described in the specification
     * for the (protected) {@link #modCount} field.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation returns {@code listIterator(0)}.
     *
     * @see #listIterator(int)
     */
    public ListIterator<E> listIterator() {
        return listIterator(0);
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation returns a straightforward implementation of the
     * {@code ListIterator} interface that extends the implementation of the
     * {@code Iterator} interface returned by the {@code iterator()} method.
     * The {@code ListIterator} implementation relies on the backing list's
     * {@code get(int)}, {@code set(int, E)}, {@code add(int, E)}
     * and {@code remove(int)} methods.
     *
     * <p>Note that the list iterator returned by this implementation will
     * throw an {@link UnsupportedOperationException} in response to its
     * {@code remove}, {@code set} and {@code add} methods unless the
     * list's {@code remove(int)}, {@code set(int, E)}, and
     * {@code add(int, E)} methods are overridden.
     *
     * <p>This implementation can be made to throw runtime exceptions in the
     * face of concurrent modification, as described in the specification for
     * the (protected) {@link #modCount} field.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public ListIterator<E> listIterator(final int index) {
        rangeCheckForAdd(index);

        return new ListItr(index);
    }

    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;
                cursor = i + 1;
                return next;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.remove(lastRet);
                if (lastRet < cursor)
                    cursor--;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }
        }

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

    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public E previous() {
            checkForComodification();
            try {
                int i = cursor - 1;
                E previous = get(i);
                lastRet = cursor = i;
                return previous;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor-1;
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.set(lastRet, e);
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                AbstractList.this.add(i, e);
                lastRet = -1;
                cursor = i + 1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation returns a list that subclasses
     * {@code AbstractList}.  The subclass stores, in private fields, the
     * offset of the subList within the backing list, the size of the subList
     * (which can change over its lifetime), and the expected
     * {@code modCount} value of the backing list.  There are two variants
     * of the subclass, one of which implements {@code RandomAccess}.
     * If this list implements {@code RandomAccess} the returned list will
     * be an instance of the subclass that implements {@code RandomAccess}.
     *
     * <p>The subclass's {@code set(int, E)}, {@code get(int)},
     * {@code add(int, E)}, {@code remove(int)}, {@code addAll(int,
     * Collection)} and {@code removeRange(int, int)} methods all
     * delegate to the corresponding methods on the backing abstract list,
     * after bounds-checking the index and adjusting for the offset.  The
     * {@code addAll(Collection c)} method merely returns {@code addAll(size,
     * c)}.
     *
     * <p>The {@code listIterator(int)} method returns a "wrapper object"
     * over a list iterator on the backing list, which is created with the
     * corresponding method on the backing list.  The {@code iterator} method
     * merely returns {@code listIterator()}, and the {@code size} method
     * merely returns the subclass's {@code size} field.
     *
     * <p>All methods first check to see if the actual {@code modCount} of
     * the backing list is equal to its expected value, and throw a
     * {@code ConcurrentModificationException} if it is not.
     *
     * @throws IndexOutOfBoundsException if an endpoint index value is out of range
     *         {@code (fromIndex < 0 || toIndex > size)}
     * @throws IllegalArgumentException if the endpoint indices are out of order
     *         {@code (fromIndex > toIndex)}
     */
    public List<E> subList(int fromIndex, int toIndex) {
        return (this instanceof RandomAccess ?
                new RandomAccessSubList<>(this, fromIndex, toIndex) :
                new SubList<>(this, fromIndex, toIndex));
    }

    // Comparison and hashing

    /**
     * Compares the specified object with this list for equality.  Returns
     * {@code true} if and only if the specified object is also a list, both
     * lists have the same size, and all corresponding pairs of elements in
     * the two lists are <i>equal</i>.  (Two elements {@code e1} and
     * {@code e2} are <i>equal</i> if {@code (e1==null ? e2==null :
     * e1.equals(e2))}.)  In other words, two lists are defined to be
     * equal if they contain the same elements in the same order.<p>
     *
     * This implementation first checks if the specified object is this
     * list. If so, it returns {@code true}; if not, it checks if the
     * specified object is a list. If not, it returns {@code false}; if so,
     * it iterates over both lists, comparing corresponding pairs of elements.
     * If any comparison returns {@code false}, this method returns
     * {@code false}.  If either iterator runs out of elements before the
     * other it returns {@code false} (as the lists are of unequal length);
     * otherwise it returns {@code true} when the iterations complete.
     *
     * @param o the object to be compared for equality with this list
     * @return {@code true} if the specified object is equal to this list
     */
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof List))
            return false;

        ListIterator<E> e1 = listIterator();
        ListIterator e2 = ((List) o).listIterator();
        while (e1.hasNext() && e2.hasNext()) {
            E o1 = e1.next();
            Object o2 = e2.next();
            if (!(o1==null ? o2==null : o1.equals(o2)))
                return false;
        }
        return !(e1.hasNext() || e2.hasNext());
    }

    /**
     * Returns the hash code value for this list.
     *
     * <p>This implementation uses exactly the code that is used to define the
     * list hash function in the documentation for the {@link List#hashCode}
     * method.
     *
     * @return the hash code value for this list
     */
    public int hashCode() {
        int hashCode = 1;
        for (E e : this)
            hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
        return hashCode;
    }

    /**
     * Removes from this list all of the elements whose index is between
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
     * Shifts any succeeding elements to the left (reduces their index).
     * This call shortens the list by {@code (toIndex - fromIndex)} elements.
     * (If {@code toIndex==fromIndex}, this operation has no effect.)
     *
     * <p>This method is called by the {@code clear} operation on this list
     * and its subLists.  Overriding this method to take advantage of
     * the internals of the list implementation can <i>substantially</i>
     * improve the performance of the {@code clear} operation on this list
     * and its subLists.
     *
     * <p>This implementation gets a list iterator positioned before
     * {@code fromIndex}, and repeatedly calls {@code ListIterator.next}
     * followed by {@code ListIterator.remove} until the entire range has
     * been removed.  <b>Note: if {@code ListIterator.remove} requires linear
     * time, this implementation requires quadratic time.</b>
     *
     * @param fromIndex index of first element to be removed
     * @param toIndex index after last element to be removed
     */
    protected void removeRange(int fromIndex, int toIndex) {
        ListIterator<E> it = listIterator(fromIndex);
        for (int i=0, n=toIndex-fromIndex; i<n; i++) {
            it.next();
            it.remove();
        }
    }

    /**
     * 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.
     *
     * <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.
     *
     * <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.
     */
    protected transient int modCount = 0;

    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size())
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size();
    }
}

class SubList<E> extends AbstractList<E> {
    private final AbstractList<E> l;
    private final int offset;
    private int size;

    SubList(AbstractList<E> list, int fromIndex, int toIndex) {
        if (fromIndex < 0)
            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
        if (toIndex > list.size())
            throw new IndexOutOfBoundsException("toIndex = " + toIndex);
        if (fromIndex > toIndex)
            throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")");
        l = list;
        offset = fromIndex;
        size = toIndex - fromIndex;
        this.modCount = l.modCount;
    }

    public E set(int index, E element) {
        rangeCheck(index);
        checkForComodification();
        return l.set(index+offset, element);
    }

    public E get(int index) {
        rangeCheck(index);
        checkForComodification();
        return l.get(index+offset);
    }

    public int size() {
        checkForComodification();
        return size;
    }

    public void add(int index, E element) {
        rangeCheckForAdd(index);
        checkForComodification();
        l.add(index+offset, element);
        this.modCount = l.modCount;
        size++;
    }

    public E remove(int index) {
        rangeCheck(index);
        checkForComodification();
        E result = l.remove(index+offset);
        this.modCount = l.modCount;
        size--;
        return result;
    }

    protected void removeRange(int fromIndex, int toIndex) {
        checkForComodification();
        l.removeRange(fromIndex+offset, toIndex+offset);
        this.modCount = l.modCount;
        size -= (toIndex-fromIndex);
    }

    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        int cSize = c.size();
        if (cSize==0)
            return false;

        checkForComodification();
        l.addAll(offset+index, c);
        this.modCount = l.modCount;
        size += cSize;
        return true;
    }

    public Iterator<E> iterator() {
        return listIterator();
    }

    public ListIterator<E> listIterator(final int index) {
        checkForComodification();
        rangeCheckForAdd(index);

        return new ListIterator<E>() {
            private final ListIterator<E> i = l.listIterator(index+offset);

            public boolean hasNext() {
                return nextIndex() < size;
            }

            public E next() {
                if (hasNext())
                    return i.next();
                else
                    throw new NoSuchElementException();
            }

            public boolean hasPrevious() {
                return previousIndex() >= 0;
            }

            public E previous() {
                if (hasPrevious())
                    return i.previous();
                else
                    throw new NoSuchElementException();
            }

            public int nextIndex() {
                return i.nextIndex() - offset;
            }

            public int previousIndex() {
                return i.previousIndex() - offset;
            }

            public void remove() {
                i.remove();
                SubList.this.modCount = l.modCount;
                size--;
            }

            public void set(E e) {
                i.set(e);
            }

            public void add(E e) {
                i.add(e);
                SubList.this.modCount = l.modCount;
                size++;
            }
        };
    }

    public List<E> subList(int fromIndex, int toIndex) {
        return new SubList<>(this, fromIndex, toIndex);
    }

    private void rangeCheck(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    private void checkForComodification() {
        if (this.modCount != l.modCount)
            throw new ConcurrentModificationException();
    }
}

class RandomAccessSubList<E> extends SubList<E> implements RandomAccess {
    RandomAccessSubList(AbstractList<E> list, int fromIndex, int toIndex) {
        super(list, fromIndex, toIndex);
    }

    public List<E> subList(int fromIndex, int toIndex) {
        return new RandomAccessSubList<>(this, fromIndex, toIndex);
    }
}

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;
cursor = i + 1;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}

public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
AbstractList.this.remove(lastRet);
if (lastRet < cursor)
cursor--;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException e) {
throw new ConcurrentModificationException();
}
}

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

    public Iterator<E> iterator() {
        return new Itr();
    }

AbstractList的内部类Itr其实就是Iterator的实现类ConcreteIterator.

AbstractList相当于ConcreteAggregate(容器实现类),它持有了ConcreteIterator(迭代器实现类)Itr的引用.因为AbstractList持有了Itr的实例化对象,可以通过AbstractList的iterator()方法return Itr的实例.

这就是为什么我们平时获得一个内部迭代器的时候不是new()而是iterator()

Itr是AbstractList的内部迭代器.因为Itr是private私有的而且是一个内部类.

迭代器设计模式结构图上半部分不做.因为抽象容器接口是可有可无的.

了解了JDK的容器内部的Iterator是如何进行遍历的.

ConcreteIterator一般是私有的,而且是容器里面的内部类.因为内部类可以访问外部类的数据,例如BookList的bookList和index.不然迭代器怎么可能知道容器类里面有什么东西.就是因为迭代器是容器类的内部类所以可以访问容器类的内部数据.

容器接口:Collection

容器实现类:AbstractList和BookList.

迭代器模式隐藏了容器的实现细节.到底容器里面是ArrayList还是LinkedList还是Map外部调用者都不知道.

无论是ArrayList还是LinkedList都有一个iterator()方法来返回它的内部迭代器.一方面是方便调用,一方面是调用者不必关心迭代器的实现细节.有了iterator这个方法,外部调用者可以很方便地获得容器的内部迭代器.而这个内部迭代器都有hasNext()和next(),很容易判断是否有下一个和获得下一个.

    public ListIterator<E> listIterator() {
        return listIterator(0);
    }

private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public E previous() {
            checkForComodification();
            try {
                int i = cursor - 1;
                E previous = get(i);
                lastRet = cursor = i;
                return previous;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor-1;
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.set(lastRet, e);
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                AbstractList.this.add(i, e);
                lastRet = -1;
                cursor = i + 1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

这是另外一种迭代方法.JDK的容器类AbstractList的另外一种迭代方法.

public class Book {
     private String ISBN;
     private String name;
     private double price;
    public String getISBN() {
        return ISBN;
    }
    public void setISBN(String iSBN) {
        ISBN = iSBN;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    public Book(String iSBN, String name, double price) {
        super();
        this.ISBN = iSBN;
        this.name = name;
        this.price = price;
    }
    public void display(){
        System.out.println("ISBN=" +ISBN + ",name=" + name +",price=" + price);
    }
     
}

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;


public class BookList {
    private List<Book> bookList;
    private int index;
    private Iterator iterator;
    
    
    public BookList() {
        super();
        // TODO Auto-generated constructor stub
        bookList = new ArrayList<Book>();
    }




/*    public BookList(List<Book> bookList) {
        super();
        this.bookList = bookList;
    }
*/



    //添加书籍
    public void addBook(Book book){
        bookList.add(book);      
    }
    //删除书籍
    public void deleteBook(Book book){
        //int bookIndex = bookList.indexOf(book);
        //bookList.remove(bookIndex);
        bookList.remove(book);
    }
    
/*    //判断是否有下一本书
    public boolean hasNext(){
        if(index >= bookList.size()){
            return false;
        }
        return true;
    }
    
    //获得下一本书
    public  Book getNext(){
        return bookList.get(index++);
    }*/
/*    public List<Book> getBookList(){
        return bookList;
    }*/
    public Iterator  Iterator(){
        return new Itr();//ConcreteIterator的实现类
        
    }
    
    private class Itr implements Iterator{

        @Override
        public boolean hasNext() {
            // TODO Auto-generated method stub
            if(index >= bookList.size()){
                return false;
            }
            return true;
        }

        @Override
        public Object next() {//因为这里是object类型的.
            // TODO Auto-generated method stub
            return bookList.get(index++);
        }

        @Override
        public void remove() {
            // TODO Auto-generated method stub
            
        }
        
    }
}

import java.util.Iterator;
import java.util.List;


public class MainClass {
    public static void main(String[] args) {
        BookList bookList = new BookList();
        
        Book book1  =   new Book("010203","Java编程思想",90);
        Book book2  =   new Book("010204","Java从入门到精通",60);
        
        bookList.addBook(book1);
        bookList.addBook(book2);
        
    /*    while(bookList.hasNext()){
            Book book = bookList.getNext();
            book.display();
        }*/
        
/*        List<Book> bookDateList = bookList.getBookList();//获得bookList里面的一个数据
        for(int i=0;i<bookDateList.size();i++){
             Book book = bookDateList.get(i);
             book.display();
        }*/
        Iterator iter = bookList.Iterator();
        
        while(iter.hasNext()){
            Book book = (Book) iter.next();
            book.display();
        }
        
        
    }
}