源码的解读逻辑按照程序运行的轨迹展开
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Arraylist的继承&实现关系
打开ArrayList源码，会看到有如下的属性定义，
ArrayList中定义的属性
/**
* Default initial capacity.
* 初始容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
* 空数组
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
* 默认容量的空数组
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
* 真正存放对象的数组
*/
transient Object[] elementData;
/**
* The size of the ArrayList (the number of elements it contains).
* 实际数据的数量
* @serial
*/
private int size;
/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
* Integer 最大值
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
当运行 ArrayList<Integer> list = new ArrayList<>() ; ，因为它没有指定初始容量，所以它调用的是它的无参构造
//无参构造，
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
// 指定初始容量
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];//创建数组
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
}
}
当我们仅仅new出一个ArrayList时，它仅仅只会创建一个空数组，由此我们可以得知它的初始化操作被延迟到了第一次add()
//添加一个元素
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
public static int max(int a, int b) {
return (a >= b) ? a : b;
}
//判断是否要扩容,minCapacity的值大于add数据之前的大小，就调用grow方法，进行扩容，否则什么也不做
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);//扩充capacity,将其向右一位再加上原来的数，实际上是扩充了1.5倍
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)//确保数组的容量不大于Integer的最大值
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);//复制
}
对源码阅读有问题的可以把以下代码复制自行运行，这是一个简版的ArrayList,是我从JDK源码中抽取出来的，理解下面的代码再去看JDK的源码相信就很简单了
package com.tanoak.list.arraylist;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
/**
* @Desc 自定义ArrayList集合类， 基于数组实现
*/
public class TkArrayList<E> implements Serializable {
/**
*
* 初始容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 空数组
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 默认容量的空数组
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 真正存放数据的数组
*/
transient Object[] elementData;
/**
* 实际数据的数量
*/
private int size;
/**
* 记录了ArrayList结构性变化的次数
*/
protected transient int modCount = 0;
/**
* Integer 最大值
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
public TkArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 指定数组大小
* @param initialCapacity
*/
public TkArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* 构造一个包含指定元素的list，这些元素的是按照Collection的迭代器返回的顺序排列的
* @param c
*/
public TkArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class){
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
//增
/**
* 新增元素
* @param e
* @return
*/
public boolean add(E e) {
// Increments modCount!!
ensureCapacityInternal(size + 1);
elementData[size++] = e;
return true;
}
/**
*
* @param minCapacity
*/
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
/**
* 判断是否扩容
* @param minCapacity
*/
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0){
grow(minCapacity);
}
}
//进行扩容
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//扩充capacity,将其向右一位再加上原来的数，实际上是扩充了1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0){
newCapacity = minCapacity;
}
//确保数组的容量不大于Integer类型最大值
if (newCapacity - MAX_ARRAY_SIZE > 0){
newCapacity = hugeCapacity(minCapacity);
}
// //复制数据
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) {
// overflow
throw new OutOfMemoryError();
}
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
//查
/**
* 根据索引 调用 elementData 返回值
* @param index
* @return
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
/**
* 根据索引取出值
* @param index
* @return
*/
E elementData(int index) {
return (E) elementData[index];
}
private void rangeCheck(int index) {
if (index >= size){
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
}
/**
* 越界信息
* @param index
* @return
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
//删
/**
*
* @param index
* @return
*/
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);
}
// clear to let GC do its work
elementData[--size] = null;
return oldValue;
}
//改
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
}
ArrayList比较难理解的就是扩容，思路首先理清楚，但是只要理清楚几个属性在方法中所做的判断，然后运行上面简版的源码，多熟悉几次就不成问题了
- 如理解有误，请指正
