1.集合的由来：

我们学习的是面向对象的语言。而面向对象语言对事物的描述是通过对象体现的。为了方便对多个对象进行操作，我们就必须把多个对象进行存储。而要想存储多个对象，就不能是一个基本的变量，而应该是一个容器类型的变量。在我们目前所学过的知识里面，有哪些是容器类型的呢？数组和StringBuffer。但是StringBuffer的结果是一个字符串， 不一定符合我们的要求，所以我们只能选择数组，这就是对象数组。而对象数组又不能适应变化的需求。因为数组的长度是固定的。这个时候，为了适应变化的需求，java就提供了集合框架。

2.集合和数组的区别？

1.长度的区别

数组的长度固定，而集合的长度可变。

2.内容不同

数组存储的是同一种类型的元素，而集合可以存储不同类型的元素。

3.元素的数据类型

数组可以存储基本数据类型，也可以存储引用数据类型。

集合只能存储引用数据类型。

3.集合继承体系的由来

集合是存储多个元素的，但是，存储多个元素我们也是有不同需求的；比如说，我要这多个元素中不能有相同的元素。再比如说，我要这多个元素按照某种规则排序。针对不同的需求，java就提供了不同的集合类。这样呢，java就提供了很多个集合类，这多个集合类的数据结构不同。结构不同不重要，重要的是能存储数据，并且保证还能够使用这些东西，比如判断，获取等等。既然这样，这多个集合类是有共性的内容，我们把这些集合类的共性内容不断进行抽象，最终就能形成集合的继承体系结构。

4.数据结构解释

数据的物理结构不同，即数据的存储结构不同。

5.集合继承体系的图解

6.Collection接口概述

Collection层次结构中的根接口。Collection中存储的对象，称为Collection中的元素。一些Collection允许有重复的元素，而另一些则不允许有重复的元素。一些Collection是有序的，而另外一些则是无序的。

7.Collection的功能概述

添加功能：

public boolean add(E e):添加一个元素

public boolean addAll(Collection c):存储一个集合的元素

删除功能：

public void clear():移除所有元素

public boolean remove(E e):移除一个元素

public boolean removeAll(Collection c):移除一个集合的元素

判断功能：

public boolean contains(E e):判断集合中是否存在指定的元素

public boolean containsAll(Collection c):判断集合是否包含指定的集合元素

public boolean isEmpty():判断集合是否为空

获取功能：

public Iterator<E> iterator():返回在此集合的元素上的迭代的迭代器

交集功能:

public boolean retainAll(Collection c):两个集合都有的元素

长度功能

public int size():返回集合中元素的个数

把集合转换为数组

public Object[] toArray()

package com;import java.util.ArrayList;
import java.util.Collection;/*** 集合框架 *     *   集合的由来：*      我们学习的是面向对象的语言。而面向对象语言对事物的描述是通过对象体现的。为了方便对多个对象进行操作，*      我们就必须把多个对象进行存储。而要想存储多个对象，就不能是一个基本的变量，而应该是一个容器类型的变量。*      在我们目前所学过的知识里面，有哪些是容器类型的呢？数组和StringBuffer。*        但是StringBuffer的结果是一个字符串， 不一定符合我们的要求，所以我们只能选择数组，这就是对象数组。*       而对象数组又不能适应变化的需求。因为数组的长度是固定的。这个时候，为了适应变化的需求，java就提供了集合框架。*  *  集合和数组的区别？*   1.长度的区别*        数组的长度固定，而集合的长度可变。*      2.内容不同*          数组存储的是同一种类型的元素，而集合可以存储不同类型的元素。*      3.元素的数据类型*          数组可以存储基本数据类型，也可以存储引用数据类型*          集合只能存储引用数据类型*          *  集合是存储多个元素的，但是，存储多个元素我们也是有不同需求的；比如说，我要这多个元素中不能有相同的元素。 *  再比如说，我要这多个元素按照某种规则排序。针对不同的需求，java就提供了不同的集合类。*  这样呢，java就提供了很多个集合类，这多个集合类的数据结构不同。结构不同不重要，重要的是能存储数据，*  并且保证还能够使用这些东西，比如判断，获取等等。既然这样，这多个集合类是有共性的内容，我们把这些集合类的*  共性内容不断进行抽象，最终就能形成集合的继承体系结构。*  *  数据结构：数据的物理结构不同，即数据的存储结构不同。*  *  Collection的功能概述*  添加功能：*     public boolean add(E e):添加一个元素*     public boolean addAll(Collection c):存储一个集合的元素*  删除功能：*   public void clear():移除所有元素*     public boolean remove(E e):移除一个元素*      public boolean removeAll(Collection c):移除一个集合的元素*  判断功能：*    public boolean contains(E e):判断集合中是否存在指定的元素*      public boolean containsAll(Collection c):判断集合是否包含指定的集合元素*     public boolean isEmpty():判断集合是否为空*  获取功能：*      public Iterator<E> iterator():返回在此集合的元素上的迭代的迭代器*  交集功能*    public boolean retainAll(Collection c):两个集合都有的元素*  长度功能*    public int size():返回集合中元素的个数*  把集合转换为数组*    public Object[] toArray()*  *  *  * * */
public class CollectionDemo {public static void main(String[] args) {//创建集合对象Collection c = new ArrayList();System.out.println(c);//[]//public boolean add(E e):添加一个元素System.out.println(c.add("hello"));//trueSystem.out.println(c);//[hello]c.add("world");c.add("java");System.out.println(c);//[hello, world, java]//public boolean contains(E e):判断集合中是否存在指定的元素System.out.println(c.contains("hello"));//true//public boolean isEmpty():判断集合是否为空System.out.println(c.isEmpty());//false //public int size():返回集合中元素的个数System.out.println(c.size());//3        //public boolean remove(E e):移除一个元素c.remove("hello");System.out.println(c);//[world, java]//public void clear():移除所有元素c.clear();System.out.println(c);//[]}}

package com;import java.util.ArrayList;
import java.util.Collection;public class CollectionDemo2 {public static void main(String[] args) {//创建集合1Collection c1 = new ArrayList();c1.add("abc1");c1.add("abc2");c1.add("abc3");c1.add("abc4");//创建集合2Collection c2 = new ArrayList();c2.add("abc4");c2.add("abc5");c2.add("abc6");c2.add("abc7");System.out.println("c1:"+c1+",c2:"+c2);//c1:[abc1, abc2, abc3, abc4],c2:[abc4, abc5, abc6, abc7]//public boolean addAll(Collection c)c1.addAll(c2);System.out.println("c1:"+c1+",c2:"+c2);//c1:[abc1, abc2, abc3, abc4, abc4, abc5, abc6, abc7],c2:[abc4, abc5, abc6, abc7]//public boolean removeAll(Collection c) 只要有一个被移除了，就算移除c1.removeAll(c2);System.out.println("c1:"+c1+",c2:"+c2);//c1:[abc1, abc2, abc3],c2:[abc4, abc5, abc6, abc7]//public boolean containsAll(Collection c)//只有包含所有的元素，才叫包含System.out.println("containsAll:"+c1.containsAll(c2));//containsAll:false//public boolean retainAll(Collection c)求交集//假设有两个集合A,B//A对B的交集，最终的结果保存在A中，B不变//返回值表示的是A是否发生变化System.out.println("retainAll:"+c1.retainAll(c2));//retainAll:trueSystem.out.println("c1:"+c1);//c1:[]System.out.println("c2:"+c2);//c2:[abc4, abc5, abc6, abc7]}}

8.集合的遍历

package com;import java.util.ArrayList;
import java.util.Collection;/*** 集合的遍历，其实就是一次获取集合章的每一个元素*        public Object[] toArray() 把集合转成数组，可以实现集合的遍历**/
public class CollectionDemo3 {public static void main(String[] args) {//创建集合对象Collection<String> c = new ArrayList<String>();//添加元素c.add("hello");c.add("world");c.add("java");//遍历//public Object[] toArray()Object[] array = c.toArray();for (int i = 0; i < array.length; i++) {String str = (String)array[i];System.out.print(str+" ");//hello world java }}}

package com;
/*** 学生类 **/
public class Student {private String name;private int age;public Student(){}public Student(String name,int age){this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "Student [name=" + name + ", age=" + age + "]";}}
package com;import java.util.ArrayList;
import java.util.Collection;/*** 练习：用集合存储5个学生对象，并把学生对象进行遍历* * 分析：*         1.创建学生类*        2.创建集合对象*       3.创建学生对象*       4.把学生添加到集合*         5.把集合转化成数组*         6.遍历数组* */
public class StudentDemo {public static void main(String[] args) {//创建集合对象Collection<Student> c = new ArrayList<Student>();//创建学生对象Student s1 = new Student("周星驰",35);Student s2 = new Student("风清扬",22);Student s3 = new Student("宋江",58);Student s4 = new Student("武松",63);Student s5 = new Student("马蓉",130);//把学生对象添加到集合中c.add(s1);c.add(s2);c.add(s3);c.add(s4);c.add(s5);//把集合转换成数组Object[] obj = c.toArray();//遍历数组，输出学生信息for (int i = 0; i < obj.length; i++) {System.out.println(obj[i]);}}}
Student [name=周星驰, age=35]
Student [name=风清扬, age=22]
Student [name=宋江, age=58]
Student [name=武松, age=63]
Student [name=马蓉, age=130]

9.集合之迭代器

package com;import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;/*** public Iterator<E> iterator() 迭代器，专门用于遍历集合的方式**/
public class CollectionDemo1 {public static void main(String[] args) {//创建集合对象Collection<String> c = new ArrayList<String>();//添加元素c.add("hello");c.add("world");c.add("java");//使用迭代器进行迭代for (Iterator<String>  iteraotr = c.iterator(); iteraotr.hasNext(); ) {String value = iteraotr.next();System.out.println(value);}}}
hello
world
java

package com;
/*** 学生类 **/
public class Student {private String name;private int age;public Student(){}public Student(String name,int age){this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "Student [name=" + name + ", age=" + age + "]";}}
package com;import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;/***  练习：用集合存储5个学生对象，并把学生对象进行遍历，用迭代进行比那里 **/
public class CollectionDemo4 {public static void main(String[] args) {//创建集合对象Collection<Student> c = new ArrayList<Student>();//创建学生对象Student s1 = new Student("周星驰",35);Student s2 = new Student("风清扬",22);Student s3 = new Student("宋江",58);Student s4 = new Student("武松",63);Student s5 = new Student("马蓉",130);//把学生对象存储到集合之中c.add(s1);c.add(s2);c.add(s3);c.add(s4);c.add(s5);//使用迭代器进行迭代for (Iterator<Student> iterator = c.iterator();iterator.hasNext(); ) {Student stu = iterator.next();System.out.println(stu);}}}
Student [name=周星驰, age=35]
Student [name=风清扬, age=22]
Student [name=宋江, age=58]
Student [name=武松, age=63]
Student [name=马蓉, age=130]

10.迭代器原理

迭代器，是遍历集合的一种方式。

迭代器是依赖于集合而存在的。

集合的使用步骤

1.创建集合对象

2.创建元素对象

3.把元素添加到集合

4.遍历集合

4.1通过集合对象获取迭代器对象

4.2通过迭代器对象的hasNext()方法判断是否有元素

4.3通过迭代器对象的next()方法获取元素并移动到下一个位置

为什么迭代器不定义成一个类，而定义成一个接口？

假设迭代器定义的是一个类，这样我们就可以创建该类的对象，调用该类的方法来实现集合的遍历。但是，我们知道，java提供了很多的集合类，而这些集合类的数据结构是不同的，所以，存储的方式和遍历的方式应该是不同的。进而它们的遍历方式也不一样的，最终，就没有迭代器类。

而无论你是哪种集合，你都应该具备获取元素的操作，并且，最好有辅助于判断功能。这样，在获取前，先判断。这样就更不容易出错。也就是说，判断和获取功能应该是一个集合遍历所具备，而每个集合的方式又不太一样，所以我们把判断和获取的功能提取出来，兵不提取具体实现，这种方式就是接口。

那么，真正的具体的实现类在哪里？在真正的具体的子类中。以内部类的方式体现的。

那么我们可以从源代码中来看是否和上面分析的一样呢？欲知后事如何，且看下面分解。

package java.util;
import java.util.function.Predicate;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;
public interface Collection<E> extends Iterable<E> {int size();boolean isEmpty();boolean contains(Object o);Iterator<E> iterator();//获取迭代器接口Object[] toArray();<T> T[] toArray(T[] a);boolean add(E e);boolean remove(Object o);boolean containsAll(Collection<?> c);boolean addAll(Collection<? extends E> c);boolean removeAll(Collection<?> c);default boolean removeIf(Predicate<? super E> filter) {Objects.requireNonNull(filter);boolean removed = false;final Iterator<E> each = iterator();while (each.hasNext()) {if (filter.test(each.next())) {each.remove();removed = true;}}return removed;}boolean retainAll(Collection<?> c);void clear();boolean equals(Object o);int hashCode(); @Overridedefault Spliterator<E> spliterator() {return Spliterators.spliterator(this, 0);}  default Stream<E> stream() {return StreamSupport.stream(spliterator(), false);}default Stream<E> parallelStream() {return StreamSupport.stream(spliterator(), true);}
}

从上面我们知道Collection接口继承了Iterable接口，并且在Collection接口中有获取迭代器接口的方法。

Iterator<E> iterator();//获取迭代器接口

因为Collection接口继承了Iterable接口，所以可以推测在Iterable接口中也有一个获取迭代器接口的方法。因为接口是不能有实现方法的。

那么下面我们去看Iterable接口的源码。

package java.lang;
import java.util.Iterator;
import java.util.Objects;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;
public interface Iterable<T> {Iterator<T> iterator();//获取迭代器的接口的方法default void forEach(Consumer<? super T> action) {Objects.requireNonNull(action);for (T t : this) {action.accept(t);}}default Spliterator<T> spliterator() {return Spliterators.spliteratorUnknownSize(iterator(), 0);}
}

我们可以看到，在Iterable接口中有获取迭代器接口的方法。

而Iterable接口是没有父接口的，那么现在我们去看一下Iterator接口吧，看看它有什么方法

package java.util;
import java.util.function.Consumer;
public interface Iterator<E> {boolean hasNext();//判断是否包含下一个元素E next();//获取下一个元素的值default void remove() {throw new UnsupportedOperationException("remove");}default void forEachRemaining(Consumer<? super E> action) {Objects.requireNonNull(action);while (hasNext())action.accept(next());}
}

我们可以看出在Iterator接口中有我们经常用的方法，

    boolean hasNext();//判断是否包含下一个元素E next();//获取下一个元素的值

那么现在我们去看一下Collection的子接口List接口

/** Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
package java.util;
public interface List<E> extends Collection<E> {int size();boolean isEmpty();boolean contains(Object o);Iterator<E> iterator();//获取迭代器接口Object[] toArray();<T> T[] toArray(T[] a);boolean add(E e);boolean remove(Object o);boolean containsAll(Collection<?> c);boolean addAll(Collection<? extends E> c);boolean addAll(int index, Collection<? extends E> c);boolean removeAll(Collection<?> c);boolean retainAll(Collection<?> c);void clear();boolean equals(Object o);int hashCode();E get(int index);      E set(int index, E element);void add(int index, E element);E remove(int index);int indexOf(Object o);int lastIndexOf(Object o);ListIterator<E> listIterator();ListIterator<E> listIterator(int index);List<E> subList(int fromIndex, int toIndex);
}

那么我们可以看出在List接口中也有一个获取迭代器接口的方法，why?当然因为List接口继承了Collection啦，而Collection接口中就有一个获取迭代器接口的方法。

Iterator<E> iterator();//获取迭代器接口

到现在距离我们推测的应该很接近了，下面我们看看List接口的实现类ArrayList吧。看一下是否和我们推测的差不多。

package java.util;
public class ArrayList<E> extends AbstractList<E>implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{private static final long serialVersionUID = 8683452581122892189L;private static final int DEFAULT_CAPACITY = 10;private static final Object[] EMPTY_ELEMENTDATA = {};private transient Object[] elementData;private int size;public ArrayList(int initialCapacity) {super();if (initialCapacity < 0)throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);this.elementData = new Object[initialCapacity];}public ArrayList() {super();this.elementData = EMPTY_ELEMENTDATA;}public ArrayList(Collection<? extends E> c) {elementData = c.toArray();size = elementData.length;// c.toArray might (incorrectly) not return Object[] (see 6260652)if (elementData.getClass() != Object[].class)elementData = Arrays.copyOf(elementData, size, Object[].class);}public void trimToSize() {modCount++;if (size < elementData.length) {elementData = Arrays.copyOf(elementData, size);}}public void ensureCapacity(int minCapacity) {int minExpand = (elementData != EMPTY_ELEMENTDATA)// any size if real element table? 0// larger than default for empty table. It's already supposed to be// at default size.: DEFAULT_CAPACITY;if (minCapacity > minExpand) {ensureExplicitCapacity(minCapacity);}}private void ensureCapacityInternal(int minCapacity) {if (elementData == EMPTY_ELEMENTDATA) {minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}ensureExplicitCapacity(minCapacity);}private void ensureExplicitCapacity(int minCapacity) {modCount++;// overflow-conscious codeif (minCapacity - elementData.length > 0)grow(minCapacity);}private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;private void grow(int minCapacity) {// overflow-conscious codeint oldCapacity = elementData.length;int newCapacity = oldCapacity + (oldCapacity >> 1);if (newCapacity - minCapacity < 0)newCapacity = minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);// minCapacity is usually close to size, so this is a win:elementData = Arrays.copyOf(elementData, newCapacity);}private static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}public int size() {return size;}public boolean isEmpty() {return size == 0;}public boolean contains(Object o) {return indexOf(o) >= 0;}public int indexOf(Object o) {if (o == null) {for (int i = 0; i < size; i++)if (elementData[i]==null)return i;} else {for (int i = 0; i < size; i++)if (o.equals(elementData[i]))return i;}return -1;}public int lastIndexOf(Object o) {if (o == null) {for (int i = size-1; i >= 0; i--)if (elementData[i]==null)return i;} else {for (int i = size-1; i >= 0; i--)if (o.equals(elementData[i]))return i;}return -1;}public Object clone() {try {@SuppressWarnings("unchecked")ArrayList<E> v = (ArrayList<E>) super.clone();v.elementData = Arrays.copyOf(elementData, size);v.modCount = 0;return v;} catch (CloneNotSupportedException e) {// this shouldn't happen, since we are Cloneablethrow new InternalError();}}public Object[] toArray() {return Arrays.copyOf(elementData, size);}@SuppressWarnings("unchecked")public <T> T[] toArray(T[] a) {if (a.length < size)// Make a new array of a's runtime type, but my contents:return (T[]) Arrays.copyOf(elementData, size, a.getClass());System.arraycopy(elementData, 0, a, 0, size);if (a.length > size)a[size] = null;return a;}@SuppressWarnings("unchecked")E elementData(int index) {return (E) elementData[index];}public E get(int index) {rangeCheck(index);return elementData(index);}public E set(int index, E element) {rangeCheck(index);E oldValue = elementData(index);elementData[index] = element;return oldValue;}public boolean add(E e) {ensureCapacityInternal(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}public void add(int index, E element) {rangeCheckForAdd(index);ensureCapacityInternal(size + 1);  // Increments modCount!!System.arraycopy(elementData, index, elementData, index + 1,size - index);elementData[index] = element;size++;}public E remove(int index) {rangeCheck(index);modCount++;E oldValue = elementData(index);int numMoved = size - index - 1;if (numMoved > 0)System.arraycopy(elementData, index+1, elementData, index,numMoved);elementData[--size] = null; return oldValue;}public boolean remove(Object o) {if (o == null) {for (int index = 0; index < size; index++)if (elementData[index] == null) {fastRemove(index);return true;}} else {for (int index = 0; index < size; index++)if (o.equals(elementData[index])) {fastRemove(index);return true;}}return false;}private void fastRemove(int index) {modCount++;int numMoved = size - index - 1;if (numMoved > 0)System.arraycopy(elementData, index+1, elementData, index,numMoved);elementData[--size] = null; // clear to let GC do its work}public void clear() {modCount++;for (int i = 0; i < size; i++)elementData[i] = null;size = 0;}public boolean addAll(Collection<? extends E> c) {Object[] a = c.toArray();int numNew = a.length;ensureCapacityInternal(size + numNew);  // Increments modCountSystem.arraycopy(a, 0, elementData, size, numNew);size += numNew;return numNew != 0;}public boolean addAll(int index, Collection<? extends E> c) {rangeCheckForAdd(index);Object[] a = c.toArray();int numNew = a.length;ensureCapacityInternal(size + numNew);  // Increments modCountint numMoved = size - index;if (numMoved > 0)System.arraycopy(elementData, index, elementData, index + numNew,numMoved);System.arraycopy(a, 0, elementData, index, numNew);size += numNew;return numNew != 0;}protected void removeRange(int fromIndex, int toIndex) {modCount++;int numMoved = size - toIndex;System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);int newSize = size - (toIndex-fromIndex);for (int i = newSize; i < size; i++) {elementData[i] = null;}size = newSize;}private void rangeCheck(int index) {if (index >= size)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}private void rangeCheckForAdd(int index) {if (index > size || index < 0)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}private String outOfBoundsMsg(int index) {return "Index: "+index+", Size: "+size;}public boolean removeAll(Collection<?> c) {return batchRemove(c, false);}public boolean retainAll(Collection<?> c) {return batchRemove(c, true);}private boolean batchRemove(Collection<?> c, boolean complement) {final Object[] elementData = this.elementData;int r = 0, w = 0;boolean modified = false;try {for (; r < size; r++)if (c.contains(elementData[r]) == complement)elementData[w++] = elementData[r];} finally {// Preserve behavioral compatibility with AbstractCollection,// even if c.contains() throws.if (r != size) {System.arraycopy(elementData, r,elementData, w,size - r);w += size - r;}if (w != size) {// clear to let GC do its workfor (int i = w; i < size; i++)elementData[i] = null;modCount += size - w;size = w;modified = true;}}return modified;}private void writeObject(java.io.ObjectOutputStream s)throws java.io.IOException{int expectedModCount = modCount;s.defaultWriteObject();s.writeInt(size);for (int i=0; i<size; i++) {s.writeObject(elementData[i]);}if (modCount != expectedModCount) {throw new ConcurrentModificationException();}}private void readObject(java.io.ObjectInputStream s)throws java.io.IOException, ClassNotFoundException {elementData = EMPTY_ELEMENTDATA;// Read in size, and any hidden stuffs.defaultReadObject();// Read in capacitys.readInt(); // ignoredif (size > 0) {// be like clone(), allocate array based upon size not capacityensureCapacityInternal(size);Object[] a = elementData;// Read in all elements in the proper order.for (int i=0; i<size; i++) {a[i] = s.readObject();}}}public ListIterator<E> listIterator(int index) {if (index < 0 || index > size)throw new IndexOutOfBoundsException("Index: "+index);return new ListItr(index);}public ListIterator<E> listIterator() {return new ListItr(0);}public Iterator<E> iterator() {//1.这边就是实现父接口的抽象方法return new Itr();//2.这边是new的Iterator的子类，那么这个Itr是什么鬼，内部类啊}//3.看这里，是不是，内部类实现了Iterator接口，那么就是意味着每个子类，//比如ArrayList，LinkedList应该都有类似的实现啦，太厉害了，jdk的作者大神，膜拜吧private class Itr implements Iterator<E> {int cursor;       // index of next element to returnint lastRet = -1; // index of last element returned; -1 if no suchint expectedModCount = modCount;public boolean hasNext() {return cursor != size;}@SuppressWarnings("unchecked")public E next() {checkForComodification();int i = cursor;if (i >= size)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length)throw new ConcurrentModificationException();cursor = i + 1;return (E) elementData[lastRet = i];}public void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.remove(lastRet);cursor = lastRet;lastRet = -1;expectedModCount = modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}}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() {checkForComodification();int i = cursor - 1;if (i < 0)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length)throw new ConcurrentModificationException();cursor = i;return (E) elementData[lastRet = i];}public void set(E e) {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.set(lastRet, e);} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}public void add(E e) {checkForComodification();try {int i = cursor;ArrayList.this.add(i, e);cursor = i + 1;lastRet = -1;expectedModCount = modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}}public List<E> subList(int fromIndex, int toIndex) {subListRangeCheck(fromIndex, toIndex, size);return new SubList(this, 0, fromIndex, toIndex);}static void subListRangeCheck(int fromIndex, int toIndex, int size) {if (fromIndex < 0)throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);if (toIndex > size)throw new IndexOutOfBoundsException("toIndex = " + toIndex);if (fromIndex > toIndex)throw new IllegalArgumentException("fromIndex(" + fromIndex +") > toIndex(" + toIndex + ")");}private class SubList extends AbstractList<E> implements RandomAccess {private final AbstractList<E> parent;private final int parentOffset;private final int offset;int size;SubList(AbstractList<E> parent,int offset, int fromIndex, int toIndex) {this.parent = parent;this.parentOffset = fromIndex;this.offset = offset + fromIndex;this.size = toIndex - fromIndex;this.modCount = ArrayList.this.modCount;}public E set(int index, E e) {rangeCheck(index);checkForComodification();E oldValue = ArrayList.this.elementData(offset + index);ArrayList.this.elementData[offset + index] = e;return oldValue;}public E get(int index) {rangeCheck(index);checkForComodification();return ArrayList.this.elementData(offset + index);}public int size() {checkForComodification();return this.size;}public void add(int index, E e) {rangeCheckForAdd(index);checkForComodification();parent.add(parentOffset + index, e);this.modCount = parent.modCount;this.size++;}public E remove(int index) {rangeCheck(index);checkForComodification();E result = parent.remove(parentOffset + index);this.modCount = parent.modCount;this.size--;return result;}protected void removeRange(int fromIndex, int toIndex) {checkForComodification();parent.removeRange(parentOffset + fromIndex,parentOffset + toIndex);this.modCount = parent.modCount;this.size -= toIndex - fromIndex;}public boolean addAll(Collection<? extends E> c) {return addAll(this.size, c);}public boolean addAll(int index, Collection<? extends E> c) {rangeCheckForAdd(index);int cSize = c.size();if (cSize==0)return false;checkForComodification();parent.addAll(parentOffset + index, c);this.modCount = parent.modCount;this.size += cSize;return true;}public Iterator<E> iterator() {return listIterator();}public ListIterator<E> listIterator(final int index) {checkForComodification();rangeCheckForAdd(index);final int offset = this.offset;return new ListIterator<E>() {int cursor = index;int lastRet = -1;int expectedModCount = ArrayList.this.modCount;public boolean hasNext() {return cursor != SubList.this.size;}@SuppressWarnings("unchecked")public E next() {checkForComodification();int i = cursor;if (i >= SubList.this.size)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (offset + i >= elementData.length)throw new ConcurrentModificationException();cursor = i + 1;return (E) elementData[offset + (lastRet = i)];}public boolean hasPrevious() {return cursor != 0;}@SuppressWarnings("unchecked")public E previous() {checkForComodification();int i = cursor - 1;if (i < 0)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (offset + i >= elementData.length)throw new ConcurrentModificationException();cursor = i;return (E) elementData[offset + (lastRet = i)];}public int nextIndex() {return cursor;}public int previousIndex() {return cursor - 1;}public void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {SubList.this.remove(lastRet);cursor = lastRet;lastRet = -1;expectedModCount = ArrayList.this.modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}public void set(E e) {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.set(offset + lastRet, e);} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}public void add(E e) {checkForComodification();try {int i = cursor;SubList.this.add(i, e);cursor = i + 1;lastRet = -1;expectedModCount = ArrayList.this.modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}final void checkForComodification() {if (expectedModCount != ArrayList.this.modCount)throw new ConcurrentModificationException();}};}public List<E> subList(int fromIndex, int toIndex) {subListRangeCheck(fromIndex, toIndex, size);return new SubList(this, offset, fromIndex, toIndex);}private void rangeCheck(int index) {if (index < 0 || index >= this.size)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}private void rangeCheckForAdd(int index) {if (index < 0 || index > this.size)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}private String outOfBoundsMsg(int index) {return "Index: "+index+", Size: "+this.size;}private void checkForComodification() {if (ArrayList.this.modCount != this.modCount)throw new ConcurrentModificationException();}}
}

我们可以看到这样的代码

public Iterator<E> iterator() {//1.这边就是实现父接口的抽象方法return new Itr();//2.这边是new的Iterator的子类，那么这个Itr是什么鬼，内部类啊}//3.看这里，是不是，内部类实现了Iterator接口，那么就是意味着每个子类，//比如ArrayList，LinkedList应该都有类似的实现啦，太厉害了，jdk的作者大神，膜拜吧private class Itr implements Iterator<E> {int cursor;       // index of next element to returnint lastRet = -1; // index of last element returned; -1 if no suchint expectedModCount = modCount;public boolean hasNext() {return cursor != size;}@SuppressWarnings("unchecked")public E next() {checkForComodification();int i = cursor;if (i >= size)throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length)throw new ConcurrentModificationException();cursor = i + 1;return (E) elementData[lastRet = i];}public void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.remove(lastRet);cursor = lastRet;lastRet = -1;expectedModCount = modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}}

那么到了这里，就和我们推测的一样了。

那么图解如下所示

11.练习

11.1存储字符串并遍历

package com;import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;/***  需求：存储字符串并遍历* 分析：*     1.创建集合对象*      2.创建字符串对象*      3.把字符串对象添加到集合中*      4.遍历集合 */
public class CollectionTest {public static void main(String[] args) {//创建集合对象Collection<String> c = new ArrayList<String>();//创建字符串对象String s1 = "hello";String s2 = "world";String s3 = "java";//把字符串对象添加到集合中c.add(s1);c.add(s2);c.add(s3);//遍历集合 方式一,将集合转换为数组，再遍历
//      String[] strs = (String[]) c.toArray();
//      for (int i = 0; i < strs.length; i++) {
//          System.out.println(strs[i]);
//      }
//      上面的这种方式是错误的，因为Object[]怎么可以转换成String[]数组呢
//      何况在Collection中的toArray()的定义是这样的
/***        Object[] toArray();*///所以，只能先将集合转换为Object数组，然后通过for循环将每个元素取出来，//再转换为StringObject[] obj = c.toArray();for (int i = 0; i < obj.length; i++) {String str = (String)obj[i];System.out.println(str);}//遍历集合方式二：直接通过迭代器的方式输出数组for (Iterator<String> iterator = c.iterator(); iterator.hasNext(); ) {String str = iterator.next();System.out.println(str);}}
}

11.2存储学生对象并遍历

package com;
/*** 学生类 **/
public class Student {private String name;private int age;public Student(){}public Student(String name,int age){this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "Student [name=" + name + ", age=" + age + "]";}}
package com;import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;/*** 练习：用集合存储5个学生对象，并把学生对象进行遍历* * 分析：*       1.创建学生类*        2.创建集合对象*       3.创建学生对象*       4.把学生添加到集合*         5.把集合转化成数组*         6.遍历数组* */
public class StudentDemo {public static void main(String[] args) {//创建集合对象Collection<Student> c = new ArrayList<Student>();//创建学生对象Student s1 = new Student("周星驰",35);Student s2 = new Student("风清扬",22);Student s3 = new Student("宋江",58);Student s4 = new Student("武松",63);Student s5 = new Student("马蓉",130);//把学生对象添加到集合中c.add(s1);c.add(s2);c.add(s3);c.add(s4);c.add(s5);//把集合转换成数组Object[] obj = c.toArray();//遍历集合方式一，输出学生信息for (int i = 0; i < obj.length; i++) {System.out.println(obj[i]);}//遍历集合方式二：通过迭代器for (Iterator<Student> iterator = c.iterator(); iterator.hasNext(); ) {Student student = iterator.next();System.out.println(student);}}}