java arraylist 源代码_Java中ArrayList源码浅析
ArrayList基本使用
public class ArrayListTest {
public static void main(String[] args) {
List list = new ArrayList<>();
list.add("5");
System.out.println(list.get(0));
}
}
ArrayList繼承層次
public class ArrayList extends AbstractList
implements List, RandomAccess, Cloneable, java.io.Serializable
基本字段
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
*/
//默認的初始化容量
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
//空數組,會在構造函數中給予0參數的情況下，賦值給elementData
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.
*/
//在一個個元素加入進來的時候，會自動擴展成為DEFAULTCAPACITY
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.
*/
//真正的數組的引用
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
//添加的元素數量
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
*/
//是因為有header words?才導致要Integer的最大值減8
//其實這個變量只在后面使用一次，比較了之后，然后還是按照MAX_VALUE來擴容了...為啥
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
構造函數
/**
* Constructs an empty list with the specified initial capacity.
*
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
//帶有初始化容量的構造函數
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
//初始化一個initialCapacity大小的Object數組
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
//若初始化為0,則使用默認的空數組賦值
this.elementData = EMPTY_ELEMENTDATA;
} else {
//若為負數，拋出非法參數異常
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* Constructs an empty list with an initial capacity of ten.
*/
//最基本的構造函數
public ArrayList() {
//注意這里被賦值為DEFAULTCAPACITY_EMPTY_ELEMENTDATA
//而不是EMPTY_ELEMENTDATA，表示的意思就是當add的時候
//會默認擴容為DEFAULT_CAPACITY
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @param c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
public ArrayList(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;
}
}
add操作
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return true (as specified by {@link Collection#add})
*/
public boolean add(E e) {
//添加元素的時候，調用內部確保容量的方法
//為什么是內部呢？因為還有一個共有的ensureCapacity方法
ensureCapacityInternal(size + 1); // Increments modCount!!
//因為本身是一個數組，所以在下一個數組索引位置添加上元素
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
//如果elementDate等于DEFAULTCAPACITY_EMPTY_ELEMENTDATA
//表示第一次擴容時，起碼要擴容至DEFAULT_CAPACITY
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
//取DEFAULT_CAPACITY=10和minCapacity(可以在初始化函數中初始化)的最大值
//也就是說若調用默認構造函數，第一次會起碼擴展為10的大小
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
//確保顯示的容量
ensureExplicitCapacity(minCapacity);
}
其實ensureCapacityInternal一個任務是算出不論是第一次添加導致的擴容還是后面添加導致的擴容的最小的容量值。然后將這個最小擴容值傳遞給ensureExplicitCapacity,由ensureExplicitCapacity實現擴容。
private void ensureExplicitCapacity(int minCapacity) {
//用于迭代器的fail fast機制
modCount++;
// overflow-conscious code
//如果最小容量大于數組的長度,那么擴容。
//否則，則不擴容。
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//新的容量為舊的容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果擴了1.5倍之后,還是小，那么以最小的容量進行擴容
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:
//將elementData擴展為newCapacity大小，使用復制數組的方式
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
//如果minCapacity小于0(而minCapacity是由某數+1得到)
//其實也剛開始想錯了,minCapacity也有可能是addAll()調用導致的
//反正minCapacity是一個需要保證的最小的容量值，不需要去理解
//其他函數是如何調用的
//所以minCapacity是由于Integer溢出的
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
//這里表示將最大容量擴展為Integer.MAX_VALUE,那么MAX_ARRAY_SIZE還有什么意義呢?
//這里minCapacity沒有溢出說明是小于MAX_ARRAY_SIZE，但是分為兩種情況，如果小于MAX_ARRAY_SIZE
//那么就直接擴容為MAX_ARRAY_SIZE
//否則比如說是MAX_VALUE-2,那么最多擴容為MAX_VALUE
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
get操作
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
//判斷是否越界
rangeCheck(index);
//返回數組的值
return elementData(index);
}
/**
* Checks if the given index is in range. If not, throws an appropriate
* runtime exception. This method does *not* check if the index is
* negative: It is always used immediately prior to an array access,
* which throws an ArrayIndexOutOfBoundsException if index is negative.
*/
private void rangeCheck(int index) {
if (index >= size)
//如果下標大于元素的數量，那么拋出異常
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
remove操作
/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
*
* @param index the index of the element to be removed
* @return the element that was removed from the list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
rangeCheck(index);
//add和remove都需要讓modCount加一
modCount++;
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//最大位的引用置為null,讓GC回收
elementData[--size] = null; // clear to let GC do its work
//返回刪除的值
return oldValue;
}
參考
總結
以上是生活随笔為你收集整理的java arraylist 源代码_Java中ArrayList源码浅析的全部內容，希望文章能夠幫你解決所遇到的問題。
- 上一篇： java中xpath_java-xpat
- 下一篇： java entries_Java le