ArrayList源码分析

旧城等待， 2022-05-12 08:22 418阅读 0赞

*
 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 static int calculateCapacity(Object[] elementData, int minCapacity) {
 if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
 return Math.max(DEFAULT_CAPACITY, minCapacity);
 }
 return minCapacity;
 }
 
 private void ensureCapacityInternal(int minCapacity) {
 ensureExplicitCapacity(calculateCapacity(elementData, 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).
 *
 * 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.
 *
 * 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.
 *
 * 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 only 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
 int capacity = calculateCapacity(elementData, size);
 SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, 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.
 *
 * The returned list iterator is <a href="#fail-fast">fail-fast</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).
 *
 * The returned list iterator is <a href="#fail-fast">fail-fast</a>.
 *
 * @see #listIterator(int)
 */
 public ListIterator<E> listIterator() {
 return new ListItr(0);
 }
 
 /**
 * Returns an iterator over the elements in this list in proper sequence.
 *
 * The returned iterator is <a href="#fail-fast">fail-fast</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;
 
 Itr() {}
 
 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.
 *
 * 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.
 *
 * The semantics of the list returned by this method become undefined if
 * the backing list (i.e., this list) is structurally modified 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 <a href="Spliterator.html#binding">late-binding</a>
 * and fail-fast {@link Spliterator} over the elements in this
 * list.
 *
 * 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++;
 }

public class ArrayList<E> extends AbstractList<E>
 implements List<E>, RandomAccess, Cloneable, java.io.Serializable
 {

从以上源码

1.我们发现在进行扩容的时候，每次扩容都是在原来的基础上扩大原来的1.5倍；  
2.在进行扩容的时候都要进行复制数组，在进行复制数组的时候是用的是System.arraycopy(elementData, 0, a, 0, size);我们知道每次的搬运数组都会浪费大量的时间，所以要减少复制数组的行为；  
3.我们看到ArrayList默认的数组元素是10个，因此我们在创建数组的时候要注意了；  
4.ArrayList底层是用动态数组实现的；  
5.它继承了一些类，实现了一些接口，实现的接口中有一个RandomAccess  
这个是随机访问的接口，所以在对Arraylist用下标的方式快很多，  
6.我们在对其进行修改的时候用ArrayList也是比较快的。  
7.这个ArrayList只适合单线程，多线程用Vector。