/**
* 无参构造函数，注意此处使用的数据缓存是DEFAULTCAPACITY_EMPTY_ELEMENTDATA
*/
public ArrayList() {
// 使用无参构造函数是，使用 DEFAULTCAPACITY_EMPTY_ELEMENTDATA
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 构造一定容量的ArrayList
* @param initialCapacity 容量大小
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
// 当容量大小为0时， 使用EMPTY_ELEMENTDATA
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* 通过一个已经存在的集合来构造ArrayList
* @param c 一个一个，该集合里面元素的类型为ArrayList元素类型的子类型
*/
public ArrayList(Collection<? extends E> c) {
Object[] a = c.toArray();
if ((size = a.length) != 0) {
// 如果二者类型一致，那么数组直接赋值
if (c.getClass() == ArrayList.class) {
elementData = a;
} else {
// 数据类型不一致，那么重新以Object[]类型复制一份
elementData = Arrays.copyOf(a, size, Object[].class);
}
} else {
// replace with empty array.
elementData = EMPTY_ELEMENTDATA;
}
}
/**
* 缩小数组到size大小，来减小占用的空间
*/
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
/**
* 扩容
* @param minCapacity 最终的结果为max(minCapacity, elementData.lenth * 1.5,DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
*/
public void ensureCapacity(int minCapacity) {
if (minCapacity > elementData.length
&& !(elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
&& minCapacity <= DEFAULT_CAPACITY)) {
modCount++;
grow(minCapacity);
}
}
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 将原来的数组扩容
* @param minCapacity
* @return
*/
private Object[] grow(int minCapacity) {
return elementData = Arrays.copyOf(elementData,
newCapacity(minCapacity));
}
private Object[] grow() {
return grow(size + 1);
}
/**
* 计算新的容量大小 = max(minCapacity, elementData.length * 1.5 , DEFAULT_CAPACITY)
* @param minCapacity
* @return
*/
private int newCapacity(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
// 新的大小为原来的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity <= 0) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
return Math.max(DEFAULT_CAPACITY, minCapacity);
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return minCapacity;
}
return (newCapacity - MAX_ARRAY_SIZE <= 0)
? newCapacity
: hugeCapacity(minCapacity);
}
// 大容量数组的大小， 与MAX_ARRAY_SIZE和Integer.MAX_VALUE比较
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE)
? Integer.MAX_VALUE
: MAX_ARRAY_SIZE;
}
// 元素的多少
public int size() {
return size;
}
// 判断是否为空
public boolean isEmpty() {
return size == 0;
}
/
// 判断是否包含某个元素， 复杂度O(n)
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
// 判断[0,size)区间内是否存在某个元素
public int indexOf(Object o) {
return indexOfRange(o, 0, size);
}
// 判读[begin,end)区间内是否存在某个元素
int indexOfRange(Object o, int start, int end) {
Object[] es = elementData;
if (o == null) {
for (int i = start; i < end; i++) {
if (es[i] == null) {
return i;
}
}
} else {
for (int i = start; i < end; i++) {
if (o.equals(es[i])) {
return i;
}
}
}
return -1;
/**
* 另一种写法
* for (int i = start; i < end; i ++)
* {
* if (Objects.equals(o,es[i]))
* return i;
* }
*
* return -1;
*/
}
// 返回最后出现的位置
public int lastIndexOf(Object o) {
return lastIndexOfRange(o, 0, size);
}
int lastIndexOfRange(Object o, int start, int end) {
Object[] es = elementData;
if (o == null) {
for (int i = end - 1; i >= start; i--) {
if (es[i] == null) {
return i;
}
}
} else {
for (int i = end - 1; i >= start; i--) {
if (o.equals(es[i])) {
return i;
}
}
}
return -1;
/**
* 另一种写法
* for (int i = end - 1; i >= 0; i --)
* {
* if (Objects.equals(o,es[i]))
* return i;
* }
*
* return -1;
*/
}
/**
* 浅复制
* @return 本实例的一个拷贝
*/
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);
}
}
/**
* 这个方法返回一个本实例数组的备份，因此是安全的，可以对其进行修改而不会伤害到
* 本实例的数组
* @return
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
/**
* 该函数返回本地数组的内容
* 如果a的大小小于本地数组的大小，那么则将本地数组的类型转换为a的类型之后返回
* 否则，将本地数组复制到a数组中后，将后面紧跟着的那个元素设置为null
* @param <T>
* @param a
* @return
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
// 返回本地数组的内容，但是类型进行转换，变为a中元素的类型
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
//否则，a的大小大于本地数组的大小，将本地数组的内容进行复制
System.arraycopy(elementData, 0, a, 0, size);
//将后面紧跟着的那个元素置为null
if (a.length > size)
a[size] = null;
return a;
}
// Positional Access Operations
// 返回一个元素, 将其强制转换为E类型
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
// 获取元素，静态方法
@SuppressWarnings("unchecked")
static <E> E elementAt(Object[] es, int index) {
return (E) es[index];
}
/**
* 返回元素
* @param index
* @return
*/
public E get(int index) {
Objects.checkIndex(index, size);
return elementData(index);
}
/**
* 设置特定位置的元素值
* @param index
* @param element
* @return 返回原先该位置的元素值
*/
public E set(int index, E element) {
Objects.checkIndex(index, size);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
//添加元素
private void add(E e, Object[] elementData, int s) {
if (s == elementData.length)
elementData = grow();
elementData[s] = e;
size = s + 1;
}
/**
* 在末尾添加一个元素
* @param e
* @return
*/
public boolean add(E e) {
modCount++;
add(e, elementData, size);
return true;
}
/**
* 在特定位置加入一个元素
* @param index
* @param element
*/
public void add(int index, E element) {
rangeCheckForAdd(index);
modCount++;
final int s;
Object[] elementData;
if ((s = size) == (elementData = this.elementData).length)
elementData = grow();
//将该位置和之后的元素向后挪一个单位
System.arraycopy(elementData, index,
elementData, index + 1,
s - index);
//将该位置的元素进行替换
elementData[index] = element;
size = s + 1;
}
/**
* 删除某个特定位置的元素
* @param index
* @return
*/
public E remove(int index) {
Objects.checkIndex(index, size);
final Object[] es = elementData;
@SuppressWarnings("unchecked") E oldValue = (E) es[index];
//将该位置以后的元素向前移动一个单位
fastRemove(es, index);
return oldValue;
}
// 删掉元素
public boolean remove(Object o) {
final Object[] es = elementData;
final int size = this.size;
int i = 0;
found: {
if (o == null) {
for (; i < size; i++)
//如果相等，跳出循环
if (es[i] == null)
break found;
} else {
for (; i < size; i++)
//相等则跳出循环
if (o.equals(es[i]))
break found;
}
return false;
}
fastRemove(es, i);
return true;
}
// 将i之后的元素向前移动一个单元
private void fastRemove(Object[] es, int i) {
modCount++;
final int newSize;
if ((newSize = size - 1) > i)
System.arraycopy(es, i + 1, es, i, newSize - i);
es[size = newSize] = null;
}
// 清空所有元素，size变为0
public void clear() {
modCount++;
final Object[] es = elementData;
for (int to = size, i = size = 0; i < to; i++)
es[i] = null;
}
/**
* 添加一个集合里面的元素
* @param c
* @return
*/
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
modCount++;
int numNew = a.length;
if (numNew == 0)
return false;
Object[] elementData;
final int s;
// 如果此时的容量不够，那么应该扩容
if (numNew > (elementData = this.elementData).length - (s = size))
elementData = grow(s + numNew);
System.arraycopy(a, 0, elementData, s, numNew);
size = s + numNew;
return true;
}
/**
* 集合中的所有元素添加到指定位置开始的地方
* @param index
* @param c
* @return
*/
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
modCount++;
int numNew = a.length;
if (numNew == 0)
return false;
Object[] elementData;
final int s;
if (numNew > (elementData = this.elementData).length - (s = size))
elementData = grow(s + numNew);
int numMoved = s - index;
if (numMoved > 0)
System.arraycopy(elementData, index,
elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size = s + numNew;
return true;
}
/**
* 删掉范围内的元素
* @param fromIndex
* @param toIndex
*/
protected void removeRange(int fromIndex, int toIndex) {
if (fromIndex > toIndex) {
throw new IndexOutOfBoundsException(
outOfBoundsMsg(fromIndex, toIndex));
}
modCount++;
shiftTailOverGap(elementData, fromIndex, toIndex);
}
/**
* 通过简单的复制，覆盖掉范围内的元素，之后的位置置为null
* @param es
* @param lo
* @param hi
*/
/** Erases the gap from lo to hi, by sliding down following elements. */
private void shiftTailOverGap(Object[] es, int lo, int hi) {
System.arraycopy(es, hi, es, lo, size - hi);
for (int to = size, i = (size -= hi - lo); i < to; i++)
es[i] = null;
}
/**
* 删掉集合那些在集合c中的元素
* @param c
* @return
*/
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false, 0, size);
}
/**
* 留下那些在结合c中的元素
* @param c
* @return
*/
public boolean retainAll(Collection<?> c) {
return batchRemove(c, true, 0, size);
}
/**
* 工具函数
* @param c 集合c
* @param complement 判断是否留下在c中的元素
* @param from 起点
* @param end 终点
* @return
*/
boolean batchRemove(Collection<?> c, boolean complement,
final int from, final int end) {
Objects.requireNonNull(c);
final Object[] es = elementData;
int r;
// Optimize for initial run of survivors
for (r = from;; r++) {
if (r == end)
return false;
// 第一个不符合要求的元素
if (c.contains(es[r]) != complement)
break;
}
int w = r++;
try {
for (Object e; r < end; r++)
//只有符合要求，才将w往后移动，类似的做法还有去掉数组中的重复元素，都是双指针
if (c.contains(e = es[r]) == complement)
es[w++] = e;
} catch (Throwable ex) {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
System.arraycopy(es, r, es, w, end - r);
w += end - r;
throw ex;
} finally {
modCount += end - w;
// 此时，(w,end)之间的元素是不合法的，需要删除掉
shiftTailOverGap(es, w, end);
}
return true;
}
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() {}
// 判断是否还有元素
public boolean hasNext() {
return cursor != size;
}
// 获取下一个元素，同时将当前指针加1
@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();
// 当前指针加1
cursor = i + 1;
// 返回刚刚跳过的元素
return (E) elementData[lastRet = i];
}
public void remove() {
// 首先判断是否存在上一元素
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet);
// 此时的cursor指向的是被移除元素的下一元素
cursor = lastRet;
// 上一元素为-1
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();
}
}
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