版權聲明:本文為部落客原創文章,未經部落客允許不得轉載。 https://blog.csdn.net/weixin_40254498/article/details/81383693
ArrayList
ArrayList是基于
數組
實作的,是一個
動态數組
,其容量能自動增長,類似于C語言中的動态申請記憶體,動态增長記憶體。
-
的,隻能用在不是線程安全
下單線程環境
- 多線程環境下可以考慮用
傳回一個線程安全的ArrayList類Collections.synchronizedList(List l)函數
- 也可以使用concurrent并發包下的
CopyOnWriteArrayList類
-
,是以它支援序列化,能夠通過序列化傳輸實作了Serializable接口
- 實作了
,支援快速随機通路,實際上就是通過下标序号進行快速通路RandomAccess接口
-
,能被克隆Cloneable接口
根據JDK版本的不同 ,構造方法也不同
/**
* JDK 1.8
*/
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 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.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 上面這個對象數組就是其存儲元素的資料結構,前面有一個java關鍵字transient
* 這個關鍵字是去序列化的意思,即,在這個類序列化後儲存到磁盤或者輸出到輸出流的時候
* 這個對象數組是不被儲存或者輸出的。(這個不是下面的翻譯,對transient解釋)
*
* 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.
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* ArrayList帶容量大小的構造函數。
* 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
*/
public ArrayList(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);
}
}
/**
* 無參構造方法構造的ArrayList的預設傳回空數組
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 帶有Collection參數的構造方法
* 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;
}
}
/**
* JDK 1.7/1.6
*/
/**
* 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
*/
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
/**
* 無參構造直接傳回了this(10);預設10 這也是與1.8不同的地方
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this(10);
}
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
* 在這1.8也多了一個判斷
* @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();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
為什麼用到 transient?
這就跟這個ArrayList的特性有關,我們知道ArrayList的容量,也就是這個數組的容量,一般都是預留一些容量,等到容量不夠時再拓展,那麼就會出現容量還有備援的情況,如果這時候進行序列化,整個數組都會被序列化,連後面沒有意義空元素的也被序列化。這些是不應該被存儲的。是以java的設計者,就為這個類提供了一個writeObject方法,在實作了Serializable接口的類,如果這個類提供了writeObject方法,那麼在進行序列化的時候就會通過writeObject方法進行序列化,是以ArrayList的writeObject方法就會顯式的為每個實際的數組元素進行序列化,隻序列化有用的元素。
為什麼源碼中大量地調用了Arrays.copyof()和System.arraycopy()方法?
public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
最後都調用了System.arraycopy()方法。
/**
* 該方法被标記了native,調用了系統的C/C++代碼,在JDK中是看不到的,
* 但在openJDK中可以看到其源碼。該函數實際上最終調用了C語言的memmove()函數,
* 是以它可以保證同一個數組内元素的正确複制 和移動,比一般的複制方法的實作效率要高很多,
* 很适合用來批量處理數組。Java強烈推薦在複制大量數組元素時用該方法,以取得更高的效率。
* 這也說明了ArrayList 與數組
*/
public static native void arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);
JDK1.6 開始擴容辦法也不一樣
/**
* JDK1.6
* Increases the capacity of this <tt>ArrayList</tt> instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
public void ensureCapacity(int minCapacity) {
modCount++;
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) {
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3)/2 + 1;
if (newCapacity < minCapacity)
newCapacity = minCapacity;
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
}
/**
* JDK1.8
*/
/**
* 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
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 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;
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) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
- 1.7JDK開始使用的是位運算
- 在算出newCapacity時,其沒有和ArrayList所定義的MAX_ARRAY_SIZE作比較,為什麼沒有進行比較呢,原因是jdk1.6沒有定義這個MAX_ARRAY_SIZE最大容量,也就是說,其沒有最大容量限制的,但是jdk1.7以上做了一個改進,進行了容量限制。
Java 集合中常見 checkForComodification()方法的作用? modCount和expectedModCount作用?
ArrayList 快速通路
ArrayList基于數組實作,可以通過下标索引直接查找到指定位置的元素,是以查找效率高,但每次插入或删除元素,就要大量地移動元素,插入删除元素的效率低。
/**
* 先檢查索引 然後校驗操作正确
*/
public E get(int var1) {
this.rangeCheck(var1);
return this.elementData(var1);
}