JAVA中ArrayList的扩增源码解读

Posted 2020-09-30

今天某位大佬问了一下我关于ArrayList扩增的大小，本人甚是愚昧，用记忆之中的答案回复了一下，大佬大手一挥，去看源码再来回答我，所以就有了这篇观后感，个人愚见，共同进步吧。

然后先二话不说，上关于ArrayList的源码：

/*
* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;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&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;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&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;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++; 
} 
} 
 
 

 

我们直接看看什么时候会调用到扩增这个函数(grow())吧，先一步一步看，什么时候会出现扩增呢，一般就是在添加的时候，然后发现当前的数组下标和数组的大小发生了冲突，也就是当前下标大于数组的大小了，这时候为了避免溢出和不必要的异常，要进行扩增，扩增的大小怎么看呢，我们跟着源代码一步一步来看。

首先就是先看添加函数：

在添加元素之前，会先调用一个函数：

这个函数是用来确定当前这个数组的下标，将数组的默认大小10和当前下标进行比对，然后将比较大的值传入函数之中，判断是否要调用到扩增函数：

如果当前下标已经超过了数组的长度，那么就调用这个扩增函数。

接下来就是我们今天要说到的重点了，就是关于ArrayList的扩增问题，我们先看看这个函数：

将当前的下标传递进来，然后新的数组大小就变为原先大小的1.5倍，若是当前的数组大小还是小于当前的下标，那么直接将当前坐标设为数组的大小。若是数组大小太大了，已经超过Integer.MAX_VALUE – 8的话，那么再调用另外一个函数：

这时候进行以下判断，若是当前下标小于0，则不合法，提示超出内存大小的错误，因为太大的数组再加上另外一个整数，在二进制里面转为10进制之后，就会变为一个负数了，即超出了int的范围了。并且返回当前下标和Integer.MAX_VALUE – 8之间的。若是当前下标更大，那么返回int的最大值，若是比较大，则返回MAX_ARRAY_SIZE，两者都是避免了溢出。