源碼的解讀邏輯按照程序運行的軌跡展開
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Arraylist的繼承&實現關系
打開ArrayList源碼,會看到有如下的屬性定義,
ArrayList中定義的屬性
/**
* Default initial capacity.
* 初始容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
* 空數組
*/
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.
* 默認容量的空數組
*/
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.
* 真正存放對象的數組
*/
transient Object[] elementData;
/**
* The size of the ArrayList (the number of elements it contains).
* 實際數據的數量
* @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;
當運行 ArrayList<Integer> list = new ArrayList<>() ; ,因為它沒有指定初始容量,所以它調用的是它的無參構造
//無參構造,
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];//創建數組
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
}
}
當我們僅僅new出一個ArrayList時,它僅僅只會創建一個空數組,由此我們可以得知它的初始化操作被延遲到了第一次add()
//添加一個元素
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;
}
//判斷是否要擴容,minCapacity的值大于add數據之前的大小,就調用grow方法,進行擴容,否則什么也不做
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);//擴充capacity,將其向右一位再加上原來的數,實際上是擴充了1.5倍
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)//確保數組的容量不大于Integer的最大值
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);//復制
}
對源碼閱讀有問題的可以把以下代碼復制自行運行,這是一個簡版的ArrayList,是我從JDK源碼中抽取出來的,理解下面的代碼再去看JDK的源碼相信就很簡單了
package com.tanoak.list.arraylist;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
/**
* @Desc 自定義ArrayList集合類, 基于數組實現
*/
public class TkArrayList<E> implements Serializable {
/**
*
* 初始容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 空數組
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 默認容量的空數組
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 真正存放數據的數組
*/
transient Object[] elementData;
/**
* 實際數據的數量
*/
private int size;
/**
* 記錄了ArrayList結構性變化的次數
*/
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;
}
/**
* 指定數組大小
* @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);
}
}
/**
* 構造一個包含指定元素的list,這些元素的是按照Collection的迭代器返回的順序排列的
* @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);
}
/**
* 判斷是否擴容
* @param minCapacity
*/
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0){
grow(minCapacity);
}
}
//進行擴容
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//擴充capacity,將其向右一位再加上原來的數,實際上是擴充了1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0){
newCapacity = minCapacity;
}
//確保數組的容量不大于Integer類型最大值
if (newCapacity - MAX_ARRAY_SIZE > 0){
newCapacity = hugeCapacity(minCapacity);
}
// //復制數據
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
/**
* 根據索引 調用 elementData 返回值
* @param index
* @return
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
/**
* 根據索引取出值
* @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;
}
//刪
/**
*
* @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比較難理解的就是擴容,思路首先理清楚,但是只要理清楚幾個屬性在方法中所做的判斷,然后運行上面簡版的源碼,多熟悉幾次就不成問題了
- 如理解有誤,請指正