概述
ArrayList可以简单的看作是动态数组，相对于普通的数组它可以动态的增加容量或者减少容量。要注意的是ArrayList并不是线程安全的，因此一般建议在单线程中使用ArrayList。
实现原理
继承关系
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
ArrayList继承AbstractList实现List, RandomAccess, Cloneable, java.io.Serializable接口
关键属性
/**
* 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.
*/
// 数据的数组
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
// 实际数据的数量
private int size;
底层使用数组保存所有元素
transient 如果用transient声明一个实例变量，当对象存储时，它的值不需要维持。换句话来说就是，用transient关键字标记的成员变量不参与序列化过程
构造方法
/**
* 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 = {};
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
默认情况下初始化空数组（长度为0的数组）
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 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);
}
}
指定数组的初始容量
当指定的初始容量大于0，初始化指定大小的数组
当指定的初始容量等于0，初始化空数组
当指定的初始容量小于0，抛出IllegalArgumentException异常
/**
* 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;
}
}
初始化指定集合的数组
当指定集合不为空即长度不为0，则复制该集合，否则初始化一个空数组
E get(int index) 获取index位置的元素
// Positional Access Operations
// 返回index下标的元素且强制转化为E（List<E>中的E）类型
@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) {
// 检查index是否越界
rangeCheck(index);
// 返回index下标的元素
return elementData(index);
}
/**
* 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) {
// 检查index是否大于等于size（数组的元素数量），因为数组下标从0开始计算，所以也不能等于元素数量
// 这里没有检查index < 0的情况，因为index < 0时数组会自动抛出异常，所以并未检查index<0的情况
if (index >= size)
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;
}
首先判断index是否越界，这里并没有判断是否小于0，因为下标小于0时数组会抛出异常。越界则抛出IndexOutOfBoundsException异常，反之返回数组对应index位置的元素
E set(int index, E element) 设置（覆盖）index位置的元素
/**
* 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;
}
和get一样先判断index（下标）是否越界，不越界则先获取原来index位置上的元素，接着设置（覆盖）index位置上的元素，然后返回原来的元素，反之抛出IndexOutOfBoundsException异常
boolean add(E e) 添加一个元素到列表尾
/**
* 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) {
// 检查当前容量是否还可以容纳一个元素，不够则扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
// 添加到数组末尾
// 这个语句可以分解为
// elementData[size] = e;
// size += 1;
elementData[size++] = e;
return true;
}
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10; // 默认容量为10
// 如果数据等于默认数据，返回默认容量和minCapacity（所需容量最小值）的最大值，反之返回所需容量最小值
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++; // 操作数+1
// overflow-conscious code
// 如果所需容量最小值大于实际数组的长度就扩大实际数组容量
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; // 数组最大容量为Integer最大值-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;
// 新的容量为旧的容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
// 如果扩充容量后还是不够，则新的容量等于所需容量最小值（一般就是数组实际元素个数）
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
// 如果新的容量大于数组最大容量，再调用hugeCapacity计算新的容量
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
// 复制原来的数据到新的数组，数组容量为新的容量
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
// 大于数组最大容量返回Integer最大值，反之返回数组最大容量
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
添加一个元素到列表尾，当列表容量不足时自动扩容（通常是扩容至原来的1.5倍），添加成功返回true
void add(int index, E element) 在index处放置元素
/**
* 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) {
// 检查下标是否越界
rangeCheckForAdd(index);
// 检查当前容量是否还可以在容纳一个元素，不够则扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
// 将elementData从index开始后面的元素往后移一位
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
/**
* A version of rangeCheck used by add and addAll.
*/
private void rangeCheckForAdd(int index) {
// 当index等于size时相当于添加元素到列表尾
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
将elementData数组从index开始后面的元素往后移一位，接着在index处放置元素
模拟添加数据(lierabbit)到index=4过程如下
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boolean addAll(Collection<? extends E> c) 添加一个集合里的所有元素到列表尾
/**
* 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;
// 检查当前容量是否还可以在容纳a数组的元素，不够则扩容
ensureCapacityInternal(size + numNew); // Increments modCount
// 将a数组里的元素添加到elementData末尾
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
// a数组不为空（长度不为0）时返回true，反之false
return numNew != 0;
}
将要添加的集合变为数组，然后将其复制到elementData数组末尾
boolean addAll(int index, Collection<? extends E> c) 添加一个集合里的所有元素到index位置
/**
* 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) {
// 检查下标是否越界
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
// 检查当前容量是否还可以在容纳a数组的元素，不够则扩容
ensureCapacityInternal(size + numNew); // Increments modCount
// 需要往后移动几个位置
int numMoved = size - index;
if (numMoved > 0)
// 从index开始，往后的元素向后移动numMoved个位置
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
// 将a数组里的所有元素复制到elementData从index到index + numNew -1的位置上
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
// a数组不为空（长度不为0）时返回true，反之false
return numNew != 0;
}
将要添加的集合变为数组，然后把elementData数组从index开始，往后的元素向后移动numMoved个位置，接着将要添加的数组里的所有元素复制到elementData从index到index + numNew -1的位置上
void trimToSize() 改变列表内部数组容量至列表实际元素数量
/**
* 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++; // 操作数+1
// 如果数组实际元素数量小于数组长度
if (size < elementData.length) {
// 如果数组实际元素数量等于0则数组被赋值为空数组，反之创建一个新的元素数量等于数组长度的数组
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
当数据稳定了之后可以使用这个方法来减少内存的使用
int indexOf(Object o) 查找o元素在列表第一次出现的位置
/**
* 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) {
//元素可以为null，如果为null返回null的下标
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;
}
// 没有找到对应的元素返回-1
return -1;
}
ArrayList中可以存放null元素，indexof是返回elementData数组中值相同的首个元素的下标，indexof中比较方法是equals而equals是比较元素的值，因此必须对null单独查找。如果未找到该元素则返回-1
E remove(int index) 删除index位置上的元素
/**
* 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) {
// 检查下标是否越界
rangeCheck(index);
modCount++; // 操作数+1
E oldValue = elementData(index); // 获取index位置上的元素
int numMoved = size - index - 1; // 需要往前移动几个位置
if (numMoved > 0)
// 从index + 1开始，往后的元素向前移动1个位置
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
// 将数组末尾元素置空
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
模拟删除index=4（值为lierabbit）过程如下
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boolean remove(Object o) 删除o元素
/**
* 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) {
// 元素可以为null，分开搜索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;
}
}
// 没有找到返回false
return false;
}
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
// 由于已经找到元素，则元素必定存在，则index必定合理，所以不需要在检查index是否越界
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
}
通过寻找o元素，可以获得其下标，再根据下标删除o元素
forEach(Consumer<? super E> action)遍历列表
/**
* The number of times this list has been <i>structurally modified</i>.
* Structural modifications are those that change the size of the
* list, or otherwise perturb it in such a fashion that iterations in
* progress may yield incorrect results.
*
* <p>This field is used by the iterator and list iterator implementation
* returned by the {@code iterator} and {@code listIterator} methods.
* If the value of this field changes unexpectedly, the iterator (or list
* iterator) will throw a {@code ConcurrentModificationException} in
* response to the {@code next}, {@code remove}, {@code previous},
* {@code set} or {@code add} operations. This provides
* <i>fail-fast</i> behavior, rather than non-deterministic behavior in
* the face of concurrent modification during iteration.
*
* <p><b>Use of this field by subclasses is optional.</b> If a subclass
* wishes to provide fail-fast iterators (and list iterators), then it
* merely has to increment this field in its {@code add(int, E)} and
* {@code remove(int)} methods (and any other methods that it overrides
* that result in structural modifications to the list). A single call to
* {@code add(int, E)} or {@code remove(int)} must add no more than
* one to this field, or the iterators (and list iterators) will throw
* bogus {@code ConcurrentModificationExceptions}. If an implementation
* does not wish to provide fail-fast iterators, this field may be
* ignored.
*/
protected transient int modCount = 0;//操作数
@Override
public void forEach(Consumer<? super E> action) {
// 确保不为空
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();
}
}
/**
* Checks that the specified object reference is not {@code null}. This
* method is designed primarily for doing parameter validation in methods
* and constructors, as demonstrated below:
* <blockquote><pre>
* public Foo(Bar bar) {
* this.bar = Objects.requireNonNull(bar);
* }
* </pre></blockquote>
*
* @param obj the object reference to check for nullity
* @param <T> the type of the reference
* @return {@code obj} if not {@code null}
* @throws NullPointerException if {@code obj} is {@code null}
*/
public static <T> T requireNonNull(T obj) {
if (obj == null)
throw new NullPointerException();
return obj;
}
这里可以看到modCount的用处，当modCount发生改变后，立刻抛出ConcurrentModificationException异常。通过之前的分析可以知道当列表内容被修改时modCount会增加。也就是说如果在遍历ArrayList的过程中有其他线程修改了ArrayList，那么将抛出ConcurrentModificationException异常
ArrayList小结
- ArrayList是List接口的一个可变大小的数组的实现
- ArrayList的内部是使用一个Object对象数组来存储元素的
- 初始化ArrayList的时候，可以指定初始化容量的大小，如果不指定，就会使用默认大小，为10
- 当添加一个新元素的时候，首先会检查容量是否足够添加这个元素，如果够就直接添加，如果不够就进行扩容，扩容为原数组容量的1.5倍
- 当在index处放置一个元素的时候，会将数组index处右边的元素全部右移
- 当在index处删除一个元素的时候，会将数组index处右边的元素全部左移
本文首发：https://lierabbit.cn/2018/01/23/ArrayList%E5%AE%9E%E7%8E%B0%E5%8E%9F%E7%90%86/
