ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
一 自動擴容機制
Resizable-array implementation of the List interface.
capacity: 存儲list元素的底層數組的大小；
list size：存儲的list元素的個數；
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初始化
不指明容量時，底層數組默認是DEFAULTCAPACITY_EMPTY_ELEMENTDATA，即{}；
指明容量為0時，底層數組為EMPTY_ELEMENTDATA，即{}；
指明容量>0時，this.elementData = new Object[initialCapacity];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); } } // Constructs an empty list with an initial capacity of ten. public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } -
add
public boolean add(E e) { ensureCapacityInternal(size + 1); // 確保底層數組的容量 elementData[size++] = e; return true; }先計算需要的容量，然后進行分配；
private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); }計算所需的最小容量
如果開始沒有指定容量大小，最低容量設為Math.max(10, minCapacity)；
為什么沒有指定容量時，不直接設為10呢？反序列化時也會調用，minCapacity可能>10;
如果開始制定了容量為0，最低容量是0；private static int calculateCapacity(Object[] elementData, int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { return Math.max(DEFAULT_CAPACITY, minCapacity); } return minCapacity; }底層數組容量是否改變，均執行modCount++; 表示list結構性變化；
private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); }實際的數組擴增函數:1.5倍擴增
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; //實際上小于MAX_ARRAY_SIZE，就報錯OutOfMemoryError /** * 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); }ArrayList<Integer> list =new ArrayList<Integer>();
list.add(1);默認數組分配容量為10 -
remove
remove函數并不會調節底層數組的大小；public E remove(int index) { rangeCheck(index); //范圍檢查 modCount++; //結構性改變 E oldValue = elementData(index); 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 return oldValue; }clear函數也并不會調節底層數組的大小；
/** * Removes all of the elements from this list. The list will * be empty after this call returns. */ public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }真正減少底層數組容量的函數調用
trimToSize()/** * Trims the capacity of this <tt>ArrayList</tt> instance to be the * list's current size. An application can use this operation to minimize * the storage of an <tt>ArrayList</tt> instance. */ public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } }
二: 線程不安全
This class is roughly equivalent to Vector, except that it is unsynchronized.
推薦方式：List list = Collections.synchronizedList(new ArrayList(...));
三: Fail-Fast 機制，ConcurrentModificationException
The iterators returned by this class's iterator methods are fail-fast:
if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own ListIterator.remove() methods, the iterator will throw a ConcurrentModificationException
A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification. （內部int值modCount負責記錄更改次數）
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一旦創建了Iterator后，Iterator會記錄當前list的modCount，并保存為expectedModCount；
此后外部list發生結構性更改，外部modCount更改，而內部expectedModCount不變，再次調用Iterator函數時無法通過檢查，報錯；private class Itr implements Iterator<E> { int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount; public boolean hasNext() { return cursor != size; } @SuppressWarnings("unchecked") public E next() { 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]; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }Iterator.remove是刪除前一次返回的值；
Iterator不能連續remove的原因，remove后 lastRet = -1; 解決方式：
3.1）Iterator創建之后，不能用ArrayList增刪元素。 只能用Iterator刪除元素；
因為Iterator.remove會同步更新list的modCount以及expectedModCount；
subList 和Iterator 一樣，創建后list就不能結構性修改了；list的增刪不要Iterator的操作混在一起
3.2）java1.8后，貌似removeIf函數式編程也可以避免，待研究；
注意
ArrayList線程不安全，改為Vector或者Collections.synchronizedList(list);并不能避免ConcurrentModificationException。
因為線程安全是多線程排隊訪問list，不同時訪問。仍然允許一個創建了Iterator，另一個修改，只要不是同時就沒問題。
四: 總結
- ArrayList默認擴增方式為1.5倍；
- ArrayList隨機訪問get，set是0(1)時間開銷，add有可能擴容，移動數組元素0(n)時間開銷；刪除移動數組元素0(n)時間開銷;
Iterator.remove移動數組元素0(n)時間開銷;
數組優勢，數組劣勢 - list轉array
List<String> v = new ArrayList<>(); return v.toArray(new String[v.size()]);
@夢工廠 2018.3.6
