java集合: ArrayList源碼淺析
阿新 • • 發佈:2019-02-18
const hat 序列化 nal ref tca 添加 進行 position
ArrayList 是一個動態數組,線程不安全 ,允許元素為null。
ArrayList的數據結構是數組,查詢比較方便。
ArrayList類的接口
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable{
RandomAccess:RandmoAccess是一個標記接口,用於被List相關類實現。他主要的作用表明這個相關類支持快速隨機訪問。在ArrayList中,我們即可以通過元素的序號快速獲取元素對象——這就是快速隨機訪問。除了List的“快速隨機訪問”,還可以“通過Iterator叠代器訪問”。
Cloneable:實現該接口的類可以對該類的實例進行克隆(按字段進行復制)。
Serializable:ArrayList支持序列化,能通過序列化去傳輸。
構造方法
ArrayList(),初始化的時候,先分配一個空數組。添加一個元素時,容量就會擴展到DEFAULT_CAPACITY,也就是10。
/** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this.elementData = DEFAULTCAPACITY_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.*/ 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. * * 初始化的時候,分配一個空數組。添加一個元素時,容量就會擴展到DEFAULT_CAPACITY,也就是10。 * 關鍵字transient表示屬性不會被序列化。*/ transient Object[] elementData; // non-private to simplify nested class access /** * Default initial capacity.
* 默認容量為10 */ private static final int DEFAULT_CAPACITY = 10;
往ArrayList中添加數據的方法add() 如下:
/** * Appends the specified element to the end of this list. * * @param e element to be appended to this list * @return <tt>true</tt> (as specified by {@link Collection#add}) */ public boolean add(E e) {
// 擴容 ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; }
擴容時,ensureCapacityInternal()方法內部調用的是grow()方法。
數組擴容。如果插入數據時容量不夠,就將容量擴大為1.5倍。
擴容的過程就是數組拷貝 Arrays.copyOf的過程,每一次擴容就會開辟一塊新的內存空間和數據的復制移動
grow()方法 如下所示:
/** * 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;
// 左移一位表示原來的0.5倍,以下是將容量擴大為1.5倍 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:
// 復制數組 elementData = Arrays.copyOf(elementData, newCapacity); }
get(int index)方法很簡單,就是檢查一下小心數組越界,然後根據下標返回數組元素
/** * 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) { rangeCheck(index); return elementData(index); } @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; }
參考博客 :
https://www.jianshu.com/p/02f8696bf4cf
java集合: ArrayList源碼淺析