java 源码解析--ArrayList
ArrayList
1.看源码前应该知道的知识点
/** * */package com.study.collection;import java.util.ArrayList;import java.util.Arrays;/** * * @className ：ArrayListTestLearn * @package : com.study.collection * @Description ：解读ArrayList前必须知道的几点知识点 * @author：lgf * @date ：2012 三月 12 15:31:48 * @version : 1.0 */public class ArrayListTestLearn {/** * 可以把Collection中的数据以Object[]的形式返回 * @author lgf * @Description: 接口Collection 的 toArray方法 * @CreateDate 2012 三月 12 16:18:12 * @lastModified 2012 三月 12 16:18:12 * @version 1.0 */public void collectionTOarray(){ArrayList<String> list = new ArrayList<String>();list.add("a");list.add("b");list.add("c");list.add("d");list.add("e");Object[] arr = list.toArray();print(arr);}/** * 对Object[] obj 进行复制功能的实现 * 进行截取或扩充 * @author lgf * @Description: Arrays.copyOf() 方法的使用 * @CreateDate 2012 三月 12 16:23:53 * @lastModified 2012 三月 12 16:23:53 * @version 1.0 */public void arraysCopyOf(){int length = 8;Object[] obj = new Object[5];obj[0] = "string";obj[1] = 1;//obj[2] = null;obj[3] = false;obj[4] = 1.2;print(obj);obj = Arrays.copyOf(obj, length);print(obj);}/** * public static void arraycopy( * Object src, 源数组 要复制的数组 * int srcPos, 源数组中的起始位置 * Object dest,目标数组 要复制到的数组 * int destPos,目标数据中的起始位置 * int length) 要复制的数组元素的数量 * @author lgf * @Description: System.arraycopy() 的使用方法 * @CreateDate 2012 三月 12 16:43:24 * @lastModified 2012 三月 12 16:43:24 * @version 1.0 */public void systemArraycopy(){Object[] objOne = new Object[10];Object[] obj = {0,1,2,3,4,5,6,7,8,9,10};System.arraycopy(obj, 3, objOne, 2, 4);print(objOne);}/** * @author lgf * @Description: 输出数组里面数据 * @CreateDate 2012 三月 12 16:37:21 * @lastModified 2012 三月 12 16:37:21 * @version 1.0 * @param obj */public void print(Object[] obj){for (Object o : obj) {System.out.println(o);}}public static void main(String[] args) {ArrayListTestLearn learn = new ArrayListTestLearn();learn.collectionTOarray();learn.arraysCopyOf();learn.systemArraycopy();}}2.源码
package java.util;public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable{ private static final long serialVersionUID = 8683452581122892189L; /** * 用来存放数据的Object[] 由此可知ArrayList是用过数组来存放数据的 */ private transient Object[] elementData; /** * 目前Object[] 存放的个数 ！= Object.length() */ private int size; /** * 构造函数,进行初始化 ，小于0则报错 */ public ArrayList(int initialCapacity) {super(); if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);this.elementData = new Object[initialCapacity]; } /** * 使用默认值进行构造,容器初始化大小为 10 . */ public ArrayList() {this(10); } /** * 通过Collection来初始化 */ 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); } /** * 将此 ArrayList 实例的容量调整为列表的当前大小 * modCount 每次进行修改时候都要调用 */ public void trimToSize() {modCount++;int oldCapacity = elementData.length;if (size < oldCapacity) {elementData = Arrays.copyOf(elementData, size);} } /** * 如有必要，增加此 ArrayList 实例的容量， * 以确保它至少能够容纳最小容量参数所指定的元素数 * 仅仅是必要，而且大小minCapacity 只是进行参考。 * @param minCapacity 要扩容的大小 */ public void ensureCapacity(int minCapacity) {modCount++;// 当前大小int oldCapacity = elementData.length;// 仅仅打算扩大的大小大于当前大小时候才需要扩容if (minCapacity > oldCapacity) {Object oldData[] = elementData;// 在原来基础上扩大1.5倍 + 1 作为新的扩容大小备选用int newCapacity = (oldCapacity * 3)/2 + 1;// 如果指定扩大的比基础上扩大1.5倍 + 1 还大则选用指定的if (newCapacity < minCapacity)newCapacity = minCapacity;// minCapacity is usually close to size, so this is a win:elementData = Arrays.copyOf(elementData, newCapacity);} } /** * 返回此列表中的元素数 * @return the number of elements in this list */ public int size() {return size; } /** * 如果此列表中没有元素，则返回 true * @return <tt>true</tt> if this list contains no elements */ public boolean isEmpty() {return size == 0; } /** * 如果此列表中包含指定的元素，则返回 true * 通过判断指定对象在Object[] 中的位置。如果>=0 说明存在 * @param o 要判断的对象 */ public boolean contains(Object o) {return indexOf(o) >= 0; } /** * 返回此列表中首次出现的指定元素的索引，或如果此列表不包含元素，则返回 -1 * 如果传递的是null 则判断数组里面是否存在null * 否则就是根据equals方法进行判断 */ 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; } /** * 返回此列表中最后一次出现的指定元素的索引， * 或如果此列表不包含索引，则返回 -1 * 如果传递的是null 则判断数组里面是否存在null * 否则就是根据equals方法进行判断 */ 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; } /** * 返回此 ArrayList 实例的浅表副本 * @return a clone of this <tt>ArrayList</tt> instance */ public Object clone() {try {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();} } /** * 按适当顺序（从第一个到最后一个元素）返回包含此列表中所有元素的数组 * 就是调用Arrays.copyOf(Object[] o,int size) 方法 * @return包含此列表中所有元素的数组（按适当顺序） */ public Object[] toArray() { return Arrays.copyOf(elementData, size); } /** * 按适当顺序（从第一个到最后一个元素）返回包含此列表中所有元素的数组 * * @param a the array into which the elements of the list 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 the elements of the list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws NullPointerException if the specified array is null */ 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; } // Positional Access Operations /** * 返回此列表中指定位置上的元素。 * @param index 要返回元素的索引 * @return 此列表中指定位置上的元素 * @throws 如果索引超出范围 (index < 0 || index >= size()) */ public E get(int index) {// 进行判断是否越界RangeCheck(index);return (E) elementData[index]; } /** * 用指定的元素替代此列表中指定位置上的元素 * @param index 要替代的元素的索引 * @param element 存储在指定位置上的元素 * @return 以前位于该指定位置上的元素，旧值 * @throws IndexOutOfBoundsException - 如果索引超出范围 (index < 0 || index >= size()) */ public E set(int index, E element) {//判断越界和保存旧值RangeCheck(index);E oldValue = (E) elementData[index];elementData[index] = element;return oldValue; } /** *将指定的元素添加到此列表的尾部 * @param e 要添加的元素 * @return 是否成功添加 */ public boolean add(E e) {ensureCapacity(size + 1); // Increments modCount!!elementData[size++] = e;return true; } /** * 将指定的元素插入此列表中的指定位置。 * 向右移动当前位于该位置的元素（如果有）以及所有后续元素（将其索引加 1）。 * @param index 指定元素所插入位置的索引 * @param element 要插入的元素 * @throws IndexOutOfBoundsException 如果索引超出范围 (index < 0 || index > size()) */ public void add(int index, E element) {if (index > size || index < 0)throw new IndexOutOfBoundsException("Index: "+index+", Size: "+size);ensureCapacity(size+1); // Increments modCount!!//对elementData中index以后的所有元素向后移动一位//通过对同一个elementData进行操作实现的System.arraycopy(elementData, index, elementData, index + 1,size - index);elementData[index] = element;size++; } /** * 移除此列表中指定位置上的元素。向左移动所有后续元素（将其索引减 1） * @param index 要移除的元素的索引 * @return 从列表中移除的元素 * @throws IndexOutOfBoundsException如果索引超出范围 (index < 0 || index >= size()) */ public E remove(int index) {//越界判断RangeCheck(index);modCount++;// 获得要移除的元素E oldValue = (E) elementData[index];//移除点开始到最后一个元素之间的元素个数int numMoved = size - index - 1;if (numMoved > 0)// 在index后所有元素往前移动一位System.arraycopy(elementData, index+1, elementData, index,numMoved);elementData[--size] = null; // Let gc do its work设置为null 好回收return oldValue; } /** * 移除此列表中首次出现的指定元素（如果存在）。如果列表不包含此元素，则列表不做改动 * @param o element to be removed from this list, if present * @return <tt>true</tt> if this list contained the specified element */ public boolean remove(Object o) {if (o == null) {for (int index = 0; index < size; index++)if (elementData[index] == null) {//进行快速移除，不进行判断越界fastRemove(index);return true;}} else {for (int index = 0; index < size; index++)if (o.equals(elementData[index])) {//进行快速移除，不进行判断越界fastRemove(index);return true;}}return false; } /** * 内置快速移除方法 * Private remove method that skips bounds checking and does not * return the value removed. */ 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; // Let gc do its work } /** * 清空数组对象 */ public void clear() {modCount++;// Let gc do its work全部设置为null 大小size=0for (int i = 0; i < size; i++)elementData[i] = null;size = 0; } /** * Appends all of the elements in the specified collection to the end of * this list, in the order that they are returned by the * specified collection's Iterator. 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 list, and this * list is nonempty.) * 按照指定 collection 的迭代器所返回的元素顺序， * 将该 collection 中的所有元素添加到此列表的尾部。 * 如果正在进行此操作时修改指定的 collection ，那么此操作的行为是不确定的 * @param c 包含要添加到此列表中的元素的 collection * @return 如果此列表由于调用而发生更改，则返回 true * @throws NullPointerException 如果指定的 collection 为 null */ public boolean addAll(Collection<? extends E> c) {//新增加对象Object[] a = c.toArray();//扩容 int numNew = a.length;ensureCapacity(size + numNew); // Increments modCount System.arraycopy(a, 0, elementData, size, numNew); size += numNew;return numNew != 0; } /** * Inserts all of the elements in the specified collection into this * list, starting at the specified position. Shifts the element * currently at that position (if any) and any subsequent elements to * the right (increases their indices). The new elements will appear * in the list in the order that they are returned by the * specified collection's iterator. * * @param index index at which to insert the first element from the * specified collection * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws IndexOutOfBoundsException {@inheritDoc} * @throws NullPointerException if the specified collection is null */ public boolean addAll(int index, Collection<? extends E> c) {if (index > size || index < 0)throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);Object[] a = c.toArray();// 要插入的个数和进行扩容int numNew = a.length;ensureCapacity(size + numNew); // Increments modCount// 要向后移动的个数int 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; } /** * Removes from this list all of the elements whose index is between * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive. * Shifts any succeeding elements to the left (reduces their index). * This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements. * (If <tt>toIndex==fromIndex</tt>, this operation has no effect.) * * @param fromIndex index of first element to be removed * @param toIndex index after last element to be removed * @throws IndexOutOfBoundsException if fromIndex or toIndex out of * range (fromIndex < 0 || fromIndex >= size() || toIndex * > size() || toIndex < fromIndex) */ protected void removeRange(int fromIndex, int toIndex) {modCount++;int numMoved = size - toIndex;System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);// Let gc do its workint newSize = size - (toIndex-fromIndex);while (size != newSize)elementData[--size] = null; } /** * 判断是否越界 */ private void RangeCheck(int index) {if (index >= size)throw new IndexOutOfBoundsException("Index: "+index+", Size: "+size); } /** * Save the state of the <tt>ArrayList</tt> instance to a stream (that * is, serialize it). * * @serialData The length of the array backing the <tt>ArrayList</tt> * instance is emitted (int), followed by all of its elements * (each an <tt>Object</tt>) in the proper order. */ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{// Write out element count, and any hidden stuffint expectedModCount = modCount;s.defaultWriteObject(); // Write out array length s.writeInt(elementData.length);// Write out all elements in the proper order.for (int i=0; i<size; i++) s.writeObject(elementData[i]);if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, * deserialize it). */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {// Read in size, and any hidden stuffs.defaultReadObject(); // Read in array length and allocate array int arrayLength = s.readInt(); Object[] a = elementData = new Object[arrayLength];// Read in all elements in the proper order.for (int i=0; i<size; i++) a[i] = s.readObject(); }}3.重要点
1.默认初始化大小
/** * 使用默认值进行构造,容器初始化大小为 10 . */ public ArrayList() {this(10); }2.进行扩容
/** * 如有必要，增加此 ArrayList 实例的容量， * 以确保它至少能够容纳最小容量参数所指定的元素数 * 仅仅是必要，而且大小minCapacity 只是进行参考。 * @param minCapacity 要扩容的大小 */ public void ensureCapacity(int minCapacity) {modCount++;// 当前大小int oldCapacity = elementData.length;// 仅仅打算扩大的大小大于当前大小时候才需要扩容if (minCapacity > oldCapacity) {Object oldData[] = elementData;// 在原来基础上扩大1.5倍 + 1 作为新的扩容大小备选用int newCapacity = (oldCapacity * 3)/2 + 1;// 如果指定扩大的比基础上扩大1.5倍 + 1 还大则选用指定的if (newCapacity < minCapacity)newCapacity = minCapacity;// minCapacity is usually close to size, so this is a win:elementData = Arrays.copyOf(elementData, newCapacity);} }3.每次进行判断都要分null和非null
/** * 返回此列表中首次出现的指定元素的索引，或如果此列表不包含元素，则返回 -1 * 如果传递的是null 则判断数组里面是否存在null * 否则就是根据equals方法进行判断 */ 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; }4.移动和插入数据方法
public boolean addAll(int index, Collection<? extends E> c) {if (index > size || index < 0)throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);Object[] a = c.toArray();// 要插入的个数和进行扩容int numNew = a.length;ensureCapacity(size + numNew); // Increments modCount// 要向后移动的个数int 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; }