Java編程中ArrayList源碼分析

之前看過一句話，說的特別好。有人問閱讀源碼有什麼用？學習別人實現某個功能的設計思路，提高自己的編程水平。

是的，大家都實現一個功能，不同的人有不同的設計思路，有的人用一萬行代碼，有的人用五千行。有的人代碼運行需要的幾十秒，有的人只需要的幾秒。 。下面進入正題了。

本文的主要內容：

・詳細註釋了ArrayList的實現，基於JDK 1.8 。
・迭代器SubList部分未詳細解釋，會放到其他源碼解讀裡面。此處重點關注ArrayList本身實現。
・沒有採用標準的註釋，並適當調整了代碼的縮進以方便介紹

import java.util.AbstractList;import java.util.Arrays;import java.util.BitSet;import java.util.Collection;import java.util.Comparator;import java.util.ConcurrentModificationException;import java.util.Iterator;import java.util.List;import java.util.ListIterator;import java.util.NoSuchElementException;import java.util.Objects;import java.util.RandomAccess;import java.util.Spliterator;import java.util.function.Consumer;import java.util.function.Predicate;import java.util.function.UnaryOperator;/** * 概述： * List接口可調整大小的數組實現。實現所有可選的List操作，並允許所有元素，包括null，元素可重複。 * 除了列表接口外，該類提供了一種方法來操作該數組的大小來存儲該列表中的數組的大小。 * * 時間複雜度： * 方法size、isEmpty、get、set、iterator和listIterator的調用是常數時間的。 * 添加刪除的時間複雜度為O(N)。其他所有操作也都是線性時間複雜度。 * * 容量： * 每個ArrayList都有容量，容量大小至少為List元素的長度，默認初始化為10。 * 容量可以自動增長。 * 如果提前知道數組元素較多，可以在添加元素前通過調用ensureCapacity()方法提前增加容量以減小後期容量自動增長的開銷。 * 也可以通過帶初始容量的構造器初始化這個容量。 * * 線程不安全： * ArrayList不是線程安全的。 * 如果需要應用到多線程中，需要在外部做同步* * modCount： * 定義在AbstractList中：protected transient int modCount = 0; * 已從結構上修改此列表的次數。從結構上修改是指更改列表的大小，或者打亂列表，從而使正在進行的迭代產生錯誤的結果。 * 此字段由iterator和listiterator方法返回的迭代器和列表迭代器實現使用。 * 如果意外更改了此字段中的值，則迭代器（或列表迭代器）將拋出concurrentmodificationexception來響應next、remove、previous、set或add操作。 * 在迭代期間面臨並發修改時，它提供了快速失敗行為，而不是非確定性行為。 * 子類是否使用此字段是可選的。 * 如果子類希望提供快速失敗迭代器（和列表迭代器），則它只需在其add(int,e)和remove(int)方法（以及它所重寫的、導致列表結構上修改的任何其他方法）中增加此字段。 * 對add(int, e)或remove(int)的單個調用向此字段添加的數量不得超過1，否則迭代器（和列表迭代器）將拋出虛假的concurrentmodificationexceptions。 * 如果某個實現不希望提供快速失敗迭代器，則可以忽略此字段。 * * transient： * 默認情況下,對象的所有成員變量都將被持久化.在某些情況下,如果你想避免持久化對象的一些成員變量,你可以使用transient關鍵字來標記他們,transient也是java中的保留字(JDK 1.8) */public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable{ private static final long serialVersionUID = 8683452581122892189L; //默認初始容量private static final int DEFAULT_CAPACITY = 10; //用於空實例共享空數組實例。 private static final Object[] EMPTY_ELEMENTDATA = {}; //默認的空數組private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; //對的，存放元素的數組，包訪問權限transient Object[] elementData; //大小，創建對象時Java會將int初始化為0 private int size; //用指定的數設置初始化容量的構造函數，負數會拋出異常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); } } //默認構造函數，使用控數組初始化public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } //以集合的迭代器返回順序，構造一個含有集合中元素的列表public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); if ((size = elementData.length) != 0) { // c.toarray可能（錯誤地）不返回對象[]（見JAVA BUG編號6260652） if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { // 使用空數組this.elementData = EMPTY_ELEMENTDATA; } } //因為容量常常會大於實際元素的數量。內存緊張時，可以調用該方法刪除預留的位置，調整容量為元素實際數量。 //如果確定不會再有元素添加進來時也可以調用該方法來節約空間public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } } //使用指定參數設置數組容量public void ensureCapacity(int minCapacity) { //如果數組為空，容量預取0，否則去默認值(10) int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)? 0: DEFAULT_CAPACITY; //若參數大於預設的容量，在使用該參數進一步設置數組容量if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } } //用於添加元素時，確保數組容量private void ensureCapacityInternal(int minCapacity) { //使用默認值和參數中較大者作為容量預設值if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); } //如果參數大於數組容量，就增加數組容量private void ensureExplicitCapacity(int minCapacity) { modCount++; if (minCapacity - elementData.length > 0) grow(minCapacity); } //數組的最大容量，可能會導致內存溢出(VM內存限制) private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; //增加容量，以確保它可以至少持有由參數指定的元素的數目private void grow(int minCapacity) { int oldCapacity = elementData.length; //預設容量增加一半int newCapacity = oldCapacity + (oldCapacity >> 1); //取與參數中的較大值if (newCapacity - minCapacity < 0)//即newCapacity<minCapacity newCapacity = minCapacity; //若預設值大於默認的最大值檢查是否溢出if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); elementData = Arrays.copyOf(elementData, newCapacity); } //檢查是否溢出，若沒有溢出，返回最大整數值(java中的int為4字節，所以最大為0x7fffffff)或默認最大值private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) //溢出throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } //返回數組大小public int size() { return size; } //是否為空public boolean isEmpty() { return size == 0; } //是否包含一個數返回bool public boolean contains(Object o) { return indexOf(o) >= 0; } //返回一個值在數組首次出現的位置，會根據是否為null使用不同方式判斷。不存在就返回-1。時間複雜度為O(N) public int indexOf(Object o) { if (o == null) { for (int i = 0; i < size; i++) if (elementData[i]==null) return i; } else { for (int i = 0; i < size; i++) if (o.equals(elementData[i])) return i; } return -1; } //返回一個值在數組最後一次出現的位置，不存在就返回-1。時間複雜度為O(N) public int lastIndexOf(Object o) { if (o == null) { for (int i = size-1; i >= 0; i--) if (elementData[i]==null) return i; } else { for (int i = size-1; i >= 0; i--) if (o.equals(elementData[i])) return i; } return -1; } //返回副本，元素本身沒有被複製，複製過程數組發生改變會拋出異常public Object clone() { try { ArrayList<?> v = (ArrayList<?>) super.clone(); v.elementData = Arrays.copyOf(elementData, size); v.modCount = 0; return v; } catch (CloneNotSupportedException e) { throw new InternalError(e); } } //轉換為Object數組，使用Arrays.copyOf()方法public Object[] toArray() { return Arrays.copyOf(elementData, size); } //返回一個數組，使用運行時確定類型，該數組包含在這個列表中的所有元素（從第一到最後一個元素） //返回的數組容量由參數和本數組中較大值確定@SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { if (a.length < size) return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } //返回指定位置的值，因為是數組，所以速度特別快@SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } //返回指定位置的值，但是會檢查這個位置數否超出數組長度public E get(int index) { rangeCheck(index); return elementData(index); } //設置指定位置為一個新值，並返回之前的值，會檢查這個位置是否超出數組長度public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; } //添加一個值，首先會確保容量public boolean add(E e) { ensureCapacityInternal(size + 1); elementData[size++] = e; return true; } //指定位置添加一個值，會檢查添加的位置和容量public void add(int index, E element) { rangeCheckForAdd(index); ensureCapacityInternal(size + 1); //public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length) //src:源數組； srcPos:源數組要復制的起始位置； dest:目的數組； destPos:目的數組放置的起始位置； length:複製的長度System.arraycopy(elementData, index, elementData, index + 1,size - index); elementData[index] = element; size++; } //刪除指定位置的值，會檢查添加的位置，返回之前的值public E remove(int index) { rangeCheck(index); modCount++; E oldValue = elementData(index); int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index,numMoved); elementData[--size] = null; //便於垃圾回收期回收return oldValue; } //刪除指定元素首次出現的位置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; } //快速刪除指定位置的值，之所以叫快速，應該是不需要檢查和返回值，因為只內部使用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 } //清空數組，把每一個值設為null,方便垃圾回收(不同於reset，數組默認大小有改變的話不會重置) public void clear() { modCount++; for (int i = 0; i < size; i++) elementData[i] = null; size = 0; } //添加一個集合的元素到末端，若要添加的集合為空返回false public boolean addAll(Collection<? extends E> c) { Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); System.arraycopy(a, 0, elementData, size, numNew); size += numNew; return numNew != 0; } //功能同上，從指定位置開始添加public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); Object[] a = c.toArray(); //要添加的數組int numNew = a.length; //要添加的數組長度ensureCapacityInternal(size + numNew); //確保容量int numMoved = size - index;//不會移動的長度(前段部分) if (numMoved > 0) //有不需要移動的，就通過自身複製，把數組後部分需要移動的移動到正確位置System.arraycopy(elementData, index, elementData, index + numNew,numMoved); System.arraycopy(a, 0, elementData, index, numNew); //新的數組添加到改變後的原數組中間size += numNew; return numNew != 0; } //刪除指定範圍元素。參數為開始刪的位置和結束位置protected void removeRange(int fromIndex, int toIndex) { modCount++; int numMoved = size - toIndex; //後段保留的長度System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved); int newSize = size - (toIndex-fromIndex); for (int i = newSize; i < size; i++) { elementData[i] = null; } size = newSize; } //檢查數否超出數組長度用於添加元素時private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } //檢查是否溢出private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } //拋出的異常的詳情private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size; } //刪除指定集合的元素public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c);//檢查參數是否為null return batchRemove(c, false); } //僅保留指定集合的元素public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, true); } /** * 源碼解讀BY http://anxpp.com/ * @param complement true時從數組保留指定集合中元素的值，為false時從數組刪除指定集合中元素的值。 * @return 數組中重複的元素都會被刪除(而不是僅刪除一次或幾次)，有任何刪除操作都會返回true */ private boolean batchRemove(Collection<?> c, boolean complement) { final Object[] elementData = this.elementData; int r = 0, w = 0; boolean modified = false; try { //遍歷數組，並檢查這個集合是否包含對應的值，移動要保留的值到數組前面，w最後值為要保留的元素的數量//簡單點：若保留，就將相同元素移動到前段；若刪除，就將不同元素移動到前段for (; r < size; r++) if (c.contains(elementData[r]) == complement) elementData[w++] = elementData[r]; }finally {//確保異常拋出前的部分可以完成期望的操作，而未被遍歷的部分會被接到後面//r!=size表示可能出錯了：c.contains(elementData[r])拋出異常if (r != size) { System.arraycopy(elementData, r,elementData, w,size - r); w += size - r; } //如果w==size：表示全部元素都保留了，所以也就沒有刪除操作發生，所以會返回false；反之，返回true，並更改數組//而w!=size的時候，即使try塊拋出異常，也能正確處理異常拋出前的操作，因為w始終為要保留的前段部分的長度，數組也不會因此亂序if (w != size) { for (int i = w; i < size; i++) elementData[i] = null; modCount += size - w;//改變的次數size = w; //新的大小為保留的元素的個數modified = true; } } return modified; } //保存數組實例的狀態到一個流（即它序列化）。寫入過程數組被更改會拋出異常private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{ int expectedModCount = modCount; s.defaultWriteObject(); //執行默認的反序列化/序列化過程。將當前類的非靜態和非瞬態字段寫入此流// 寫入大小s.writeInt(size); // 按順序寫入所有元素for (int i=0; i<size; i++) { s.writeObject(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } //上面是寫，這個就是讀了。 private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { elementData = EMPTY_ELEMENTDATA; // 執行默認的序列化/反序列化過程s.defaultReadObject(); // 讀入數組長度s.readInt(); if (size > 0) { ensureCapacityInternal(size); Object[] a = elementData; //讀入所有元素for (int i=0; i<size; i++) { a[i] = s.readObject(); } } } //返回ListIterator，開始位置為指定參數public ListIterator<E> listIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index); } //返回ListIterator，開始位置為0 public ListIterator<E> listIterator() { return new ListItr(0); } //返回普通迭代器public Iterator<E> iterator() { return new Itr(); } //通用的迭代器實現private class Itr implements Iterator<E> { int cursor; //游標，下一個元素的索引，默認初始化為0 int lastRet = -1; //上次訪問的元素的位置int expectedModCount = modCount;//迭代過程不運行修改數組，否則就拋出異常//是否還有下一個public boolean hasNext() { return cursor != size; } //下一個元素@SuppressWarnings("unchecked") public E next() { checkForComodification();//檢查數組是否被修改int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; //向後移動游標return (E) elementData[lastRet = i]; //設置訪問的位置並返回這個值} //刪除元素public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification();//檢查數組是否被修改try { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } @Override @SuppressWarnings("unchecked") public void forEachRemaining(Consumer<? super E> consumer) { Objects.requireNonNull(consumer); final int size = ArrayList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[i++]); } cursor = i; lastRet = i - 1; checkForComodification(); } //檢查數組是否被修改final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } //ListIterator迭代器實現private class ListItr extends Itr implements ListIterator<E> { ListItr(int index) { super(); cursor = index; } public boolean hasPrevious() { return cursor != 0; } public int nextIndex() { return cursor; } public int previousIndex() { return cursor - 1; } @SuppressWarnings("unchecked") public E previous() { checkForComodification(); int i = cursor - 1; if (i < 0) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i; return (E) elementData[lastRet = i]; } public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification(); try { int i = cursor; ArrayList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } } //返回指定範圍的子數組public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, 0, fromIndex, toIndex); } //安全檢查static void subListRangeCheck(int fromIndex, int toIndex, int size) { if (fromIndex < 0) throw new IndexOutOfBoundsException("fromIndex = " + fromIndex); if (toIndex > size) throw new IndexOutOfBoundsException("toIndex = " + toIndex); if (fromIndex > toIndex) throw new IllegalArgumentException("fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")"); } //子數組private class SubList extends AbstractList<E> implements RandomAccess { private final AbstractList<E> parent; private final int parentOffset; private final int offset; int size; SubList(AbstractList<E> parent,int offset, int fromIndex, int toIndex) { this.parent = parent; this.parentOffset = fromIndex; this.offset = offset + fromIndex; this.size = toIndex - fromIndex; this.modCount = ArrayList.this.modCount; } public E set(int index, E e) { rangeCheck(index); checkForComodification(); E oldValue = ArrayList.this.elementData(offset + index); ArrayList.this.elementData[offset + index] = e; return oldValue; } public E get(int index) { rangeCheck(index); checkForComodification(); return ArrayList.this.elementData(offset + index); } public int size() { checkForComodification(); return this.size; } public void add(int index, E e) { rangeCheckForAdd(index); checkForComodification(); parent.add(parentOffset + index, e); this.modCount = parent.modCount; this.size++; } public E remove(int index) { rangeCheck(index); checkForComodification(); E result = parent.remove(parentOffset + index); this.modCount = parent.modCount; this.size--; return result; } protected void removeRange(int fromIndex, int toIndex) { checkForComodification(); parent.removeRange(parentOffset + fromIndex,parentOffset + toIndex); this.modCount = parent.modCount; this.size -= toIndex - fromIndex; } public boolean addAll(Collection<? extends E> c) { return addAll(this.size, c); } public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); int cSize = c.size(); if (cSize==0) return false; checkForComodification(); parent.addAll(parentOffset + index, c); this.modCount = parent.modCount; this.size += cSize; return true; } public Iterator<E> iterator() { return listIterator(); } public ListIterator<E> listIterator(final int index) { checkForComodification(); rangeCheckForAdd(index); final int offset = this.offset; return new ListIterator<E>() { int cursor = index; int lastRet = -1; int expectedModCount = ArrayList.this.modCount; public boolean hasNext() { return cursor != SubList.this.size; } @SuppressWarnings("unchecked") public E next() { checkForComodification(); int i = cursor; if (i >= SubList.this.size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[offset + (lastRet = i)]; } public boolean hasPrevious() { return cursor != 0; } @SuppressWarnings("unchecked") public E previous() { checkForComodification(); int i = cursor - 1; if (i < 0) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) throw new ConcurrentModificationException(); cursor = i; return (E) elementData[offset + (lastRet = i)]; } @SuppressWarnings("unchecked") public void forEachRemaining(Consumer<? super E> consumer) { Objects.requireNonNull(consumer); final int size = SubList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[offset + (i++)]); } // update once at end of iteration to reduce heap write traffic lastRet = cursor = i; checkForComodification(); } public int nextIndex() { return cursor; } public int previousIndex() { return cursor - 1; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { SubList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = ArrayList.this.modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(offset + lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification(); try { int i = cursor; SubList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = ArrayList.this.modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (expectedModCount != ArrayList.this.modCount) throw new ConcurrentModificationException(); } }; } public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, offset, fromIndex, toIndex); } private void rangeCheck(int index) { if (index < 0 || index >= this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private void rangeCheckForAdd(int index) { if (index < 0 || index > this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+this.size; } private void checkForComodification() { if (ArrayList.this.modCount != this.modCount) throw new ConcurrentModificationException(); } public Spliterator<E> spliterator() { checkForComodification(); return new ArrayListSpliterator<E>(ArrayList.this, offset,offset + this.size, this.modCount); } } @Override public void forEach(Consumer<? super E> action) { Objects.requireNonNull(action); final int expectedModCount = modCount; @SuppressWarnings("unchecked") final E[] elementData = (E[]) this.elementData; final int size = this.size; for (int i=0; modCount == expectedModCount && i < size; i++) { action.accept(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * Creates a <em><a href="Spliterator.html#binding" rel="external nofollow" >late-binding</a></em> * and <em>fail-fast</em> {@link Spliterator} over the elements in this * list. * * <p>The {@code Spliterator} reports {@link Spliterator#SIZED}, * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}. * Overriding implementations should document the reporting of additional * characteristic values. * * @return a {@code Spliterator} over the elements in this list * @since 1.8 */ @Override public Spliterator<E> spliterator() { return new ArrayListSpliterator<>(this, 0, -1, 0); } /** Index-based split-by-two, lazily initialized Spliterator */ static final class ArrayListSpliterator<E> implements Spliterator<E> { /* * If ArrayLists were immutable, or structurally immutable (no * adds, removes, etc), we could implement their spliterators * with Arrays.spliterator. Instead we detect as much * interference during traversal as practical without * sacrificing much performance. We rely primarily on * modCounts. These are not guaranteed to detect concurrency * violations, and are sometimes overly conservative about * within-thread interference, but detect enough problems to * be worthwhile in practice. To carry this out, we (1) lazily * initialize fence and expectedModCount until the latest * point that we need to commit to the state we are checking * against; thus improving precision. (This doesn't apply to * SubLists, that create spliterators with current non-lazy * values). (2) We perform only a single * ConcurrentModificationException check at the end of forEach * (the most performance-sensitive method). When using forEach * (as opposed to iterators), we can normally only detect * interference after actions, not before. Further * CME-triggering checks apply to all other possible * violations of assumptions for example null or too-small * elementData array given its size(), that could only have * occurred due to interference. This allows the inner loop * of forEach to run without any further checks, and * simplifies lambda-resolution. While this does entail a * number of checks, note that in the common case of * list.stream().forEach(a), no checks or other computation * occur anywhere other than inside forEach itself. The other * less-often-used methods cannot take advantage of most of * these streamlinings. */ private final ArrayList<E> list; private int index; // current index, modified on advance/split private int fence; // -1 until used; then one past last index private int expectedModCount; // initialized when fence set /** Create new spliterator covering the given range */ ArrayListSpliterator(ArrayList<E> list, int origin, int fence, int expectedModCount) { this.list = list; // OK if null unless traversed this.index = origin; this.fence = fence; this.expectedModCount = expectedModCount; } private int getFence() { // initialize fence to size on first use int hi; // (a specialized variant appears in method forEach) ArrayList<E> lst; if ((hi = fence) < 0) { if ((lst = list) == null) hi = fence = 0; else { expectedModCount = lst.modCount; hi = fence = lst.size; } } return hi; } public ArrayListSpliterator<E> trySplit() { int hi = getFence(), lo = index, mid = (lo + hi) >>> 1; return (lo >= mid) ? null : // divide range in half unless too small new ArrayListSpliterator<E>(list, lo, index = mid, expectedModCount); } public boolean tryAdvance(Consumer<? super E> action) { if (action == null) throw new NullPointerException(); int hi = getFence(), i = index; if (i < hi) { index = i + 1; @SuppressWarnings("unchecked") E e = (E)list.elementData[i]; action.accept(e); if (list.modCount != expectedModCount) throw new ConcurrentModificationException(); return true; } return false; } public void forEachRemaining(Consumer<? super E> action) { int i, hi, mc; // hoist accesses and checks from loop ArrayList<E> lst; Object[] a; if (action == null) throw new NullPointerException(); if ((lst = list) != null && (a = lst.elementData) != null) { if ((hi = fence) < 0) { mc = lst.modCount; hi = lst.size; } else mc = expectedModCount; if ((i = index) >= 0 && (index = hi) <= a.length) { for (; i < hi; ++i) { @SuppressWarnings("unchecked") E e = (E) a[i]; action.accept(e); } if (lst.modCount == mc) return; } } throw new ConcurrentModificationException(); } public long estimateSize() { return (long) (getFence() - index); } public int characteristics() { return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED; } } @Override public boolean removeIf(Predicate<? super E> filter) { Objects.requireNonNull(filter); // figure out which elements are to be removed // any exception thrown from the filter predicate at this stage // will leave the collection unmodified int removeCount = 0; final BitSet removeSet = new BitSet(size); final int expectedModCount = modCount; final int size = this.size; for (int i=0; modCount == expectedModCount && i < size; i++) { @SuppressWarnings("unchecked") final E element = (E) elementData[i]; if (filter.test(element)) { removeSet.set(i); removeCount++; } } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } // shift surviving elements left over the spaces left by removed elements final boolean anyToRemove = removeCount > 0; if (anyToRemove) { final int newSize = size - removeCount; for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) { i = removeSet.nextClearBit(i); elementData[j] = elementData[i]; } for (int k=newSize; k < size; k++) { elementData[k] = null; // Let gc do its work } this.size = newSize; if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; } return anyToRemove; } @Override @SuppressWarnings("unchecked") public void replaceAll(UnaryOperator<E> operator) { Objects.requireNonNull(operator); final int expectedModCount = modCount; final int size = this.size; for (int i=0; modCount == expectedModCount && i < size; i++) { elementData[i] = operator.apply((E) elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; } @Override @SuppressWarnings("unchecked") public void sort(Comparator<? super E> c) { final int expectedModCount = modCount; Arrays.sort((E[]) elementData, 0, size, c); if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; }}

總結

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