ArrayList源碼解析
-
簡介
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。
ArrayList不是線程安全的,隻能用在單線程環境下,多線程環境下可以考慮用Collections.synchronizedList(List l)函數傳回一個線程安全的ArrayList類,也可以使用concurrent并發包下的CopyOnWriteArrayList類。
ArrayList實作了Serializable接口,是以它支援序列化,能夠通過序列化傳輸,實作了RandomAccess接口,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,實作了Cloneable接口,能被克隆。
*由add()方法中檢查擴容操作可以看出:ArrayList初始化時最好指定可預測的數組長度,避免擴容導緻系統消耗
- 構造函數
/**
Constructs an empty list with an initial capacity of ten.
構造初始容量為10的空清單。
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* Constructs an empty list with the specified initial capacity.
*用指定的初始容量構造空清單。
* @param initialCapacity the initial capacity of the list
初始容量清單的初始容量
* @throws IllegalArgumentException if the specified initial capacity is negative
指定的初始容量為負
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
//建立指定長度的數組
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
//空數組
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);
}
}
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*構造包含指定集合的元素的清單,按集合的疊代器傳回這些元素的順序
* @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;
}
}
- 插入資料
/**
* Appends the specified element to the end of this list.
*将指定的元素追加到此清單的末尾。
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
//檢測是否擴容
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
/**
* Inserts the specified element at the specified position in this
* list. Shifts the element currently at that position (if any) and
* any subsequent elements to the right (adds one to their indices).
*在此清單中的指定位置插入指定元素。将目前位于該位置的元素(如果有)和任何後續元素向右移動(将一 個元素添加到其索引中)。
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
- 擴容檢查
/**
檢查arrayList數組是否需要擴容
*/
private void ensureCapacityInternal(int minCapacity) {
//目前數組是否是空數組
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
//空數組,傳回預設長度
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
//檢查擴容
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
//記錄操作次數
modCount++;
// overflow-conscious code
//如果長度-數組長度>0,進行擴容處理
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;
//新的數組長度 = 舊長度+舊長度*0.5(即:arrayList預設擴容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:
//複制舊數組到新數組,并對新數組擴容
elementData = Arrays.copyOf(elementData, newCapacity);
}
- 删除方法
-
//根據下标删除 public E remove(int index) { //檢查下标是否越界 rangeCheck(index); //增加操作次數 modCount++; //擷取下标對應的元素 E oldValue = elementData(index); //資料移動量 //将index+1及之後的元素向前移動一位 //例: size =10 目前數組最大index =9 //對應index為0,1,2,3,4,5,6,7,8,9 //index =2 則需要将index為3,4,5,6,7,8,9的資料向前移動一位 //即:對應的移動量為 size-index-1 =7 int numMoved = size - index - 1; if (numMoved > 0)//删除的不是最後一個 //複制數組 System.arraycopy(elementData, index+1, elementData, index, numMoved); //删除的是最後一位 elementData[--size] = null; // clear to let GC do its work return oldValue; } //删除第一次出現的元素 public boolean remove(Object o) { //判斷元素是否為null 如果為null 則删除第一個為null的元素下标 if (o == null) { for (int index = 0; index < size; index++) //找出第一個為null的下标,根據下标進行删除 if (elementData[index] == null) { fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) //找出第一個元素為o的下标,根據下标進行删除 if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; } private void fastRemove(int index) { modCount++; int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work }