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简介
ArrayList是Java集合框架中比较常用的数据结构，继承自AbstractList，实现了List接口，底层基于数组实现容量大小动态变化，允许null成员，同时还实现了RandomAccess、Cloneable、Serializable接口，所有ArrayList是支持快速访问、复制、序列化的。
成员变量
ArrayList底层是基于数组来实现容量动态变化的。
/**
* The size of the ArrayList (the number of elements it contains).
*/
private int size; // 实际元素个数
transient Object[] elementData;
注：size表示数组中实际的元素个数，elementData.length表示可容纳多少个元素（容量），
默认初始容量为10，
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
modCount定义在AbstractList中，记录对List操作的次数
protected transient int modCount = 0;
下面两个变量是用在构造函数里面的
/**
* 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 = {};
两个空的数组有什么区别呢？ We distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when first element is added. 简单来讲就是第一次添加元素时知道该 elementData 从空的构造函数还是有参构造函数被初始化的。以便确认如何扩容。
构造函数
无参构造函数
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
注：构造一个初始容量为10的List集合，构造函数给elementData赋值了一个空的数组，在第一次添加元素时容量扩大至10
构造一个初始容量大小为initialCapacity的ArrayList
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);
}
}
使用collection来构造ArrayList函数
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;
}
}
将collection转化为数组，并赋值给elementData
主要操作方法解析
Add操作
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
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);
}
//扩容
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);//1.5倍
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);
}
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++;
}
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;
}
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;
}
Remove操作
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;
}
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 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
}
Get操作
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
迭代器Iterator
集合中，for循环遍历的时候不可对集合进行remove操作，因为remove会改变集合的大小，从而造成结果不准确或数组越界
public Iterator<E> iterator() {
return new 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;
// prevent creating a synthetic constructor
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
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
final int size = ArrayList.this.size;
int i = cursor;
if (i < size) {
final Object[] es = elementData;
if (i >= es.length)
throw new ConcurrentModificationException();
for (; i < size && modCount == expectedModCount; i++)
action.accept(elementAt(es, i));
// update once at end to reduce heap write traffic
cursor = i;
lastRet = i - 1;
checkForComodification();
}
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
成员变量：
- cursor：代表下一个要访问的元素下标
- lastRet：代表上一个要访问的元素下标
- expectedModCount：代表对ArrayList修改次数的期望值，初始值为modCount
函数：
hasNext：如果下一个元素的下标等于集合大小，说明到最后了
next：首先判断expectedModCount与modCount是否相等，然后判断cursor是否超过集合长度，然后将cursor赋值给lastRet并返回下标为lastRet的元素，最后cursor自增1.
remove：首先判断lastRet是否小于0（未开始获取值），然后判断expectedModCount与modCount是否相等，然后直接调用ArrayList的删除方法，指针指向上一个操作对象。
初始化状态
调用next方法
调用remove方法
总结
- ArrayList 底层基于数组实现容量大小动态可变。
- 扩容机制为首先扩容为原始容量的 1.5 倍。如果1.5倍太小的话，则将我们所需的容量大小赋值给 newCapacity，如果1.5倍太大或者我们需要的容量太大，那就直接拿 newCapacity = (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE 来扩容。
- 扩容之后是通过数组的拷贝来确保元素的准确性的，所以尽可能减少扩容操作。
- ArrayList 的最大存储能力：Integer.MAX_VALUE。
- size 为集合中存储的元素的个数。
- elementData.length 为数组长度，表示最多可以存储多少个元素。
- 如果需要边遍历边 remove ，必须使用 iterator。且 remove 之前必须先 next，next 之后只能用一次 remove。
