int DEFAULT_CAPACITY = 10：默认空间大小10
Object[] EMPTY_ELEMENTDATA = {}：初始化容量为0时使用
Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}：初始化容量默认时使用
transient Object[] elementData：元素数组，初始化不为0时new Object[n]
传入Collection的构造函数
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;
}
}
trimToSize()：修剪多余的空间，调用Arrays.copyOf()方法
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
add(E e)：尾插，返回true
public boolean add(E e) {
//当前数组长度+1进行判断，如果-数组长度小于0.调用grow扩容
ensureCapacityInternal(size + 1);
elementData[size++] = e;
return true;
}
grow(int minCapacity)：扩容方法，调用Arrays.copyOf()
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);//扩容为1.5
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);
}
Arrays.copyOf()：Arrays工具包的复制数据方法，调用native方法System.arraycopy()
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
@SuppressWarnings("unchecked")
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
add(int index, E element)：元素插入到索引位置，调用System.arraycopy()复制自己，把插入后置及以后的元素后移一位(索引加一)
public void add(int index, E element) {
rangeCheckForAdd(index);//检查index是否越界
ensureCapacityInternal(size + 1); //判断是否需要grow扩容
System.arraycopy(elementData, index, elementData, index + 1,size - index);
elementData[index] = element;
size++;
}
remove(int index)：删除指定索引处元素，并返回该元素。调用System.arraycopy()使后续元素索引-1
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);//获取该index的元素
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;
}
remove(Object o)：调用fastRemove()删除指定的元素，返回true。没有该元素返回false
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;
}
fastRemove(int index)：删除指定元素时掉用的方法，仍是调用System.arraycopy()方法
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
}
clear()：清空list，把size置为0
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
addAll(Collection<? extends E> c)：把集合中的元素添加到list尾部，返回true。目标集合长度为0时返回false。仍是调用System.arraycopy()
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;
}
addAll(int index, Collection<? extends E> c)：在指定index索引位置添加目标集合中的元素，同上
removeRange(int fromIndex, int toIndex)：移除[m,n)区间的元素，左闭右开
removeAll(Collection<?> c)：从list移除目标集合中的元素。调用batchRemove批量删除方法
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
batchRemove(Collection<?> c, boolean complement)：
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++)
//complement，false代表不在目标集合中时，备份到数组前面
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;
}
listIterator(int index)：返回索引位置开始的正确顺序迭代器，无参重载listIterator()默认为0
private class ListItr extends Itr implements ListIterator<E>
iterator()：返回一个Itr迭代器
private class Itr implements Iterator<E>
subList(int fromIndex, int toIndex)：返回索引区间的一个“视图”，SubList是ArrayList的内部类
private class SubList extends AbstractList<E> implements RandomAccess
sort(Comparator<? super E> c)：排序，调用Arrays.sort()
@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++;
}
