代码如何优雅的去应用，如何去最佳实战，往往从一些概念，语法上的理解是做不到的，其实我们去看一些好代码，从中学习好的风格，那么理解源代码就是最好的路径了，ArrayList是最常用的集合类，我选择从ArrayList开头来看jdk源代码，一个不是原因的原因是应为ArrayList的位置不上不下，感觉刚刚好，在这里作为一个节点以其本质作为起点向不同的方向延伸。浏览并理解源代码不是一件简单的事情，但是是一件很有激情的事情，就像探险一样，惊艳视觉，扩展思维。只有不为艰难险阻，一步一个脚印，才能看见最终壮观独特的风景。愿自己不畏艰难，勇往直前。
1.ArrayList简介
可变的数组实现了List接口，并且实现了list所有选项的操作，包含null。除了实现List接口，ArrayList提供了方法操纵array的大小,使用内存存储list。ArrayList粗略的等于Vector，除了ArrayList是异步的，也就是ArrayList不是线程安全的，Vector是同步的，线程安全的。
size,isEmpty,get,set,iterator,listIterator操作在一个常量时间里运行，add操作平摊一个常量时间。其他的所有操作运行在线性时间。常量因子相比较于LinkedList的实现要低。
每一个ArrayList实例拥有一个容器，容器的大小是数组的长度，用来存储list的元素。至少和list的size一样大。当元素被添加到一个ArrayList时，它的容量自动增长，详细的增长策略没有被指定，事实上添加一个元素平摊常量时间。
使用ensureCapacity添加大量的元素操作之前应用程序可以增加容器容量，这样可以减少容器的增加在分配数量。
要注意的是ArrayList不是线程安全的，如果多线程同时访问ArrayList实例，至少一个线程修改了list结构，必须在外部使用synchronized关键字，结构上的改变指的是adds，delees一个或多个elements，或者resize，仅仅设置一个元素的值不是结构上的修改，应为设置是及时的，而修改结构是代码包围的一个过程。如果没有这样的对象存在，list在创建的时候也应该被Collections.synchronizedList包裹，来预防减少不安全的访问list。
在iterator创建之后，除了使用iterator自己的方法，如remove，add等外，list任何的结构改变，都会快速抛出ConcurrentModificationException异常，因此，面对并发式的修改，iterator将会快速失败，并且清空。导致iterator快速失败的行为最后的结果是不定的
相关联类：
* @see Collection * @see List * @see LinkedList
继承了一个类，实现了三个接口
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
/* * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.util; import java.util.function.Consumer; import java.util.function.Predicate; import java.util.function.UnaryOperator; /** * Resizable-array implementation of the <tt>List</tt> interface. Implements * all optional list operations, and permits all elements, including * <tt>null</tt>. In addition to implementing the <tt>List</tt> interface, * this class provides methods to manipulate the size of the array that is * used internally to store the list. (This class is roughly equivalent to * <tt>Vector</tt>, except that it is unsynchronized.) * * <p>The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>, * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant * time. The <tt>add</tt> operation runs in <i>amortized constant time</i>, * that is, adding n elements requires O(n) time. All of the other operations * run in linear time (roughly speaking). The constant factor is low compared * to that for the <tt>LinkedList</tt> implementation. * * <p>Each <tt>ArrayList</tt> instance has a <i>capacity</i>. The capacity is * the size of the array used to store the elements in the list. It is always * at least as large as the list size. As elements are added to an ArrayList, * its capacity grows automatically. The details of the growth policy are not * specified beyond the fact that adding an element has constant amortized * time cost. * * <p>An application can increase the capacity of an <tt>ArrayList</tt> instance * before adding a large number of elements using the <tt>ensureCapacity</tt> * operation. This may reduce the amount of incremental reallocation. * * <p><strong>Note that this implementation is not synchronized.</strong> * If multiple threads access an <tt>ArrayList</tt> instance concurrently, * and at least one of the threads modifies the list structurally, it * <i>must</i> be synchronized externally. (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.) This is typically accomplished by * synchronizing on some object that naturally encapsulates the list. * * If no such object exists, the list should be "wrapped" using the * {@link Collections#synchronizedList Collections.synchronizedList} * method. This is best done at creation time, to prevent accidental * unsynchronized access to the list:<pre> * List list = Collections.synchronizedList(new ArrayList(...));</pre> * * <p><a name="fail-fast"> * The iterators returned by this class's {@link #iterator() iterator} and * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:</a> * if the list is structurally modified at any time after the iterator is * created, in any way except through the iterator's own * {@link ListIterator#remove() remove} or * {@link ListIterator#add(Object) add} methods, the iterator will throw a * {@link ConcurrentModificationException}. Thus, in the face of * concurrent modification, the iterator fails quickly and cleanly, rather * than risking arbitrary, non-deterministic behavior at an undetermined * time in the future. * * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification. Fail-fast iterators * throw {@code ConcurrentModificationException} on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @author Josh Bloch * @author Neal Gafter * @see Collection * @see List * @see LinkedList * @see Vector * @since 1.2 */ public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { private static final long serialVersionUID = 8683452581122892189L; /** * Default initial capacity. */ 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. */ transient Object[] elementData; // non-private to simplify nested class access /** * The size of the ArrayList (the number of elements it contains). * * @serial */ private int size; /** * Constructs an empty list with the specified initial capacity. * * @param initialCapacity the initial capacity of the list * @throws IllegalArgumentException if the specified initial capacity * is negative */ 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; } /** * Constructs a list containing the elements of the specified * collection, in the order they are returned by the collection's * iterator. * * @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; } } /** * 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); } } /** * Increases the capacity of this <tt>ArrayList</tt> instance, if * necessary, to ensure that it can hold at least the number of elements * specified by the minimum capacity argument. * * @param minCapacity the desired minimum capacity */ public void ensureCapacity(int minCapacity) { int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) // any size if not default element table ? 0 // larger than default for default empty table. It's already // supposed to be at default size. : DEFAULT_CAPACITY; if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); } } private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); } private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } /** * The maximum size of array to allocate. * Some VMs reserve some header words in an array. * Attempts to allocate larger arrays may result in * OutOfMemoryError: Requested array size exceeds VM limit */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; /** * 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); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } /** * Returns the number of elements in this list. * * @return the number of elements in this list */ public int size() { return size; } /** * Returns <tt>true</tt> if this list contains no elements. * * @return <tt>true</tt> if this list contains no elements */ public boolean isEmpty() { return size == 0; } /** * Returns <tt>true</tt> if this list contains the specified element. * More formally, returns <tt>true</tt> if and only if this list contains * at least one element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>. * * @param o element whose presence in this list is to be tested * @return <tt>true</tt> if this list contains the specified element */ public boolean contains(Object o) { return indexOf(o) >= 0; } /** * Returns the index of the first occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the lowest index <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, * or -1 if there is no such index. */ 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; } /** * Returns the index of the last occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the highest index <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, * or -1 if there is no such index. */ 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; } /** * Returns a shallow copy of this <tt>ArrayList</tt> instance. (The * elements themselves are not copied.) * * @return a clone of this <tt>ArrayList</tt> instance */ public Object clone() { try { ArrayList<?> v = (ArrayList<?>) super.clone(); v.elementData = Arrays.copyOf(elementData, size); v.modCount = 0; return v; } catch (CloneNotSupportedException e) { // this shouldn't happen, since we are Cloneable throw new InternalError(e); } } /** * Returns an array containing all of the elements in this list * in proper sequence (from first to last element). * * <p>The returned array will be "safe" in that no references to it are * maintained by this list. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * * <p>This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all of the elements in this list in * proper sequence */ public Object[] toArray() { return Arrays.copyOf(elementData, size); } /** * Returns an array containing all of the elements in this list in proper * sequence (from first to last element); the runtime type of the returned * array is that of the specified array. If the list fits in the * specified array, it is returned therein. Otherwise, a new array is * allocated with the runtime type of the specified array and the size of * this list. * * <p>If the list fits in the specified array with room to spare * (i.e., the array has more elements than the list), the element in * the array immediately following the end of the collection is set to * <tt>null</tt>. (This is useful in determining the length of the * list <i>only</i> if the caller knows that the list does not contain * any null elements.) * * @param a the array into which the elements of the list are to * be stored, if it is big enough; otherwise, a new array of the * same runtime type is allocated for this purpose. * @return an array containing the elements of the list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws NullPointerException if the specified array is null */ @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { if (a.length < size) // Make a new array of a's runtime type, but my contents: return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } // Positional Access Operations @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } /** * 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); } /** * Replaces the element at the specified position in this list with * the specified element. * * @param index index of the element to replace * @param element element to be stored at the specified position * @return the element previously at the specified position * @throws IndexOutOfBoundsException {@inheritDoc} */ public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; } /** * 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) { 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). * * @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++; } /** * Removes the element at the specified position in this list. * Shifts any subsequent elements to the left (subtracts one from their * indices). * * @param index the index of the element to be removed * @return the element that was removed from the list * @throws IndexOutOfBoundsException {@inheritDoc} */ 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; // clear to let GC do its work return oldValue; } /** * 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). * * @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++; 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 } /** * 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; } /** * Appends all of the elements in the specified collection to the end of * this list, in the order that they are returned by the * specified collection's Iterator. The behavior of this operation is * undefined if the specified collection is modified while the operation * is in progress. (This implies that the behavior of this call is * undefined if the specified collection is this list, and this * list is nonempty.) * * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws NullPointerException if the specified collection is null */ public boolean addAll(Collection<? extends E> c) { Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); // Increments modCount System.arraycopy(a, 0, elementData, size, numNew); size += numNew; return numNew != 0; } /** * Inserts all of the elements in the specified collection into this * list, starting at the specified position. Shifts the element * currently at that position (if any) and any subsequent elements to * the right (increases their indices). The new elements will appear * in the list in the order that they are returned by the * specified collection's iterator. * * @param index index at which to insert the first element from the * specified collection * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws IndexOutOfBoundsException {@inheritDoc} * @throws NullPointerException if the specified collection is null */ public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); // Increments modCount int numMoved = size - index; if (numMoved > 0) System.arraycopy(elementData, index, elementData, index + numNew, numMoved); System.arraycopy(a, 0, elementData, index, numNew); size += numNew; return numNew != 0; } /** * Removes from this list all of the elements whose index is between * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. * Shifts any succeeding elements to the left (reduces their index). * This call shortens the list by {@code (toIndex - fromIndex)} elements. * (If {@code toIndex==fromIndex}, this operation has no effect.) * * @throws IndexOutOfBoundsException if {@code fromIndex} or * {@code toIndex} is out of range * ({@code fromIndex < 0 || * fromIndex >= size() || * toIndex > size() || * toIndex < fromIndex}) */ protected void removeRange(int fromIndex, int toIndex) { modCount++; int numMoved = size - toIndex; System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved); // clear to let GC do its work int newSize = size - (toIndex-fromIndex); for (int i = newSize; i < size; i++) { elementData[i] = null; } size = newSize; } /** * Checks if the given index is in range. If not, throws an appropriate * runtime exception. This method does *not* check if the index is * negative: It is always used immediately prior to an array access, * which throws an ArrayIndexOutOfBoundsException if index is negative. */ private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } /** * A version of rangeCheck used by add and addAll. */ private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } /** * Constructs an IndexOutOfBoundsException detail message. * Of the many possible refactorings of the error handling code, * this "outlining" performs best with both server and client VMs. */ private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size; } /** * Removes from this list all of its elements that are contained in the * specified collection. * * @param c collection containing elements to be removed from this list * @return {@code true} if this list changed as a result of the call * @throws ClassCastException if the class of an element of this list * is incompatible with the specified collection * (<a href="Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if this list contains a null element and the * specified collection does not permit null elements * (<a href="Collection.html#optional-restrictions">optional</a>), * or if the specified collection is null * @see Collection#contains(Object) */ public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, false); } /** * Retains only the elements in this list that are contained in the * specified collection. In other words, removes from this list all * of its elements that are not contained in the specified collection. * * @param c collection containing elements to be retained in this list * @return {@code true} if this list changed as a result of the call * @throws ClassCastException if the class of an element of this list * is incompatible with the specified collection * (<a href="Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if this list contains a null element and the * specified collection does not permit null elements * (<a href="Collection.html#optional-restrictions">optional</a>), * or if the specified collection is null * @see Collection#contains(Object) */ public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, true); } private boolean batchRemove(Collection<?> c, boolean complement) { final Object[] elementData = this.elementData; int r = 0, w = 0; boolean modified = false; try { for (; r < size; r++) if (c.contains(elementData[r]) == complement) elementData[w++] = elementData[r]; } finally { // Preserve behavioral compatibility with AbstractCollection, // even if c.contains() throws. if (r != size) { System.arraycopy(elementData, r, elementData, w, size - r); w += size - r; } if (w != size) { // clear to let GC do its work for (int i = w; i < size; i++) elementData[i] = null; modCount += size - w; size = w; modified = true; } } return modified; } /** * Save the state of the <tt>ArrayList</tt> instance to a stream (that * is, serialize it). * * @serialData The length of the array backing the <tt>ArrayList</tt> * instance is emitted (int), followed by all of its elements * (each an <tt>Object</tt>) in the proper order. */ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{ // Write out element count, and any hidden stuff int expectedModCount = modCount; s.defaultWriteObject(); // Write out size as capacity for behavioural compatibility with clone() s.writeInt(size); // Write out all elements in the proper order. for (int i=0; i<size; i++) { s.writeObject(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, * deserialize it). */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { elementData = EMPTY_ELEMENTDATA; // Read in size, and any hidden stuff s.defaultReadObject(); // Read in capacity s.readInt(); // ignored if (size > 0) { // be like clone(), allocate array based upon size not capacity ensureCapacityInternal(size); Object[] a = elementData; // Read in all elements in the proper order. for (int i=0; i<size; i++) { a[i] = s.readObject(); } } } /** * Returns a list iterator over the elements in this list (in proper * sequence), starting at the specified position in the list. * The specified index indicates the first element that would be * returned by an initial call to {@link ListIterator#next next}. * An initial call to {@link ListIterator#previous previous} would * return the element with the specified index minus one. * * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @throws IndexOutOfBoundsException {@inheritDoc} */ public ListIterator<E> listIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index); } /** * Returns a list iterator over the elements in this list (in proper * sequence). * * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @see #listIterator(int) */ public ListIterator<E> listIterator() { return new ListItr(0); } /** * Returns an iterator over the elements in this list in proper sequence. * * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @return an iterator over the elements in this list in proper sequence */ public Iterator<E> iterator() { return new Itr(); } /** * An optimized version of AbstractList.Itr */ 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(); } } @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++]); } // update once at end of iteration to reduce heap write traffic cursor = i; lastRet = i - 1; checkForComodification(); } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } /** * An optimized version of AbstractList.ListItr */ 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(); } } } /** * Returns a view of the portion of this list between the specified * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. (If * {@code fromIndex} and {@code toIndex} are equal, the returned list is * empty.) The returned list is backed by this list, so non-structural * changes in the returned list are reflected in this list, and vice-versa. * The returned list supports all of the optional list operations. * * <p>This method eliminates the need for explicit range operations (of * the sort that commonly exist for arrays). Any operation that expects * a list can be used as a range operation by passing a subList view * instead of a whole list. For example, the following idiom * removes a range of elements from a list: * <pre> * list.subList(from, to).clear(); * </pre> * Similar idioms may be constructed for {@link #indexOf(Object)} and * {@link #lastIndexOf(Object)}, and all of the algorithms in the * {@link Collections} class can be applied to a subList. * * <p>The semantics of the list returned by this method become undefined if * the backing list (i.e., this list) is <i>structurally modified</i> in * any way other than via the returned list. (Structural modifications are * those that change the size of this list, or otherwise perturb it in such * a fashion that iterations in progress may yield incorrect results.) * * @throws IndexOutOfBoundsException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, 0, fromIndex, toIndex); } 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 + ")"); } 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">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++; } }