一、ArrayList相關屬性
重要的部分都用了中文註釋
/**
* 類註釋
* 1、允許 put null 值,會自動擴容
* 2、size、isEmpty、get、set、add 等方法時間複雜度都是 O (1)
* 3、是非線程安全的,多線程情況下,推薦使用線程安全類:Collections#synchronizedList;
* 4、增強 for 循環,或者使用迭代器迭代過程中,如果數組大小被改變,會快速失敗,拋出異常。
*/
private static final long serialVersionUID = 8683452581122892189L;
/**
* Default initial capacity.
* 數組的初始化大小,默認是10
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
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.
*/
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.
* elementData表示數組本身
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*當前數組的大小,沒有用volatile修飾,不是線程安全的
* @serial
*/
private int size;
下面是ArrayList的父類AbstractList中定義了一個int型的屬性,記錄的是ArrayList的結構性變化次數,有變動就會+1
protected transient int modCount = 0;
二、初始化
/**
* Constructs an empty list with an initial capacity of ten.
* 無參數初始化,數組大小爲空
* 這裏注意ArrayList 無參構造器初始化時,默認大小是空數組,並不是 10,10 是在第一次 add 的時候擴容的數組值。
*/
public ArrayList() {
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.
* 有參數初始化
* @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保存數組
elementData = c.toArray();
//1、如果集合c數據有值,且集合元素類型不是Object的話,會被轉成Object的
//2、如果c沒有值,數組默認爲空
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;
}
}
三、新增和擴容
新增源碼
/**
* Appends the specified element to the end of this list.
* 確保數組大小是否足夠,不夠執行擴容。然後直接賦值,線程不安全
* 新增時,並沒有對值進行嚴格的校驗,所以 ArrayList 是允許 null 值的
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
擴容源碼
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
//modCount需要+1
modCount++;
// overflow-conscious code
//如果期望的容量大於目前數組的長度就擴容
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
//擴容
private void grow(int minCapacity) {
// overflow-conscious code
//記錄老的容量大小
int oldCapacity = elementData.length;
//新的容量大小=老的容量大小*1.5
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果新的容量大小 < 我們的期望值,擴容後的值就等於我們的期望值
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//如果擴容後的值 > jvm 所能分配的數組的最大值,那麼就用 Integer 的最大值
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
//通過複製進行擴容,copyOf內部通過System.arraycopy方法實現拷貝。
elementData = Arrays.copyOf(elementData, newCapacity);
}
四、刪除
/**
* Removes the first occurrence of the specified element from this list,
* if it is present. If the list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists). Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
* 新增的時候是沒有對 null 進行校驗的,所以刪除的時候也是允許刪除 null 值的
* 找到值在數組中的索引位置,是通過 equals 來判斷的,如果數組元素不是基本類型,需要我們關注 equals 的具體實現。
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
/*
* Private remove method that skips bounds checking and does not
*
* return the value removed.
*/
private void fastRemove(int index) {
//modCount需要+1
modCount++;
//numMoved 表示刪除 index 位置的元素後,需要從 index 後移動多少個元素到前面去
int numMoved = size - index - 1;
if (numMoved > 0)
//從 index +1 位置開始被拷貝,拷貝的起始位置是 index,長度是 numMoved
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
五、迭代器
/**
* An optimized version of AbstractList.Itr
* ArrayList這裏實現了Iterator類
*/
private class Itr implements Iterator<E> {
//迭代過程中,下一個元素的位置
int cursor; // index of next element to return
//迭代過程中,上一個元素的位置
int lastRet = -1; // index of last element returned; -1 if no such
//迭代過程中,期望的版本號,初試爲數組實際版本號modCount
int expectedModCount = modCount;
Itr() {}
//判斷還有沒有值可以迭代,有爲1,沒有爲0
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
//如果有值可以迭代,迭代的值是多少
//next方法檢驗能不能繼續迭代,然後找到迭代的值,併爲下一次迭代做準備(cursor+1)
public E next() {
//迭代過程中,判斷版本號有沒有修改,有被修改,則拋出ConcurrentModificationException異常
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
// 版本號比較
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
public void remove() {
//如lastRet < 0 了,說明元素已經被刪除了
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
// -1 表示元素已經被刪除,這裏也防止重複刪除
lastRet = -1;
//刪除元素成功,數組當前 modCount 就會發生變化,這裏會把 expectedModCount 重新賦值,下次迭代時兩者的值就會一致了
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
六、線程安全
ArrayList 有線程安全問題的本質,是因爲 ArrayList 自身的 elementData、size、modConut 在進行各種操作時,都沒有加鎖,而且這些變量的類型並非是可見(volatile)的,所以如果多個線程對這些變量進行操作時,可能會有值被覆蓋的情況。類註釋中推薦使用Collections#synchronizedList 來保證線程安全,SynchronizedList 是通過在每個方法上面加上鎖來實現,雖然實現了線程安全,但是性能大大降低,具體實現源碼:
public boolean add(E e) {
synchronized (mutex) {// synchronized 是一種輕量鎖,mutex 表示一個當前 SynchronizedList
return c.add(e);
}
}