public class ArrayList
implements List
0.ArrayList元素定义:
//存储数组元素的缓冲区;也就是真实的ArrayList的盒子大小
transient Object[] elementData;
//默认空数组元素
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//默认初始化容量
private static final int DEFAULT_CAPACITY = 10;
//数组的大小,也就是真实ArrayList里面存的东西多少
private int size;
//记录被修改的次数
protected transient int modCount = 0;
//数组的最大值
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8
1.ArrayList的构造函数:
无参构造函数:
public ArrayList() {
//初始化一个空的数组,在JDK1.8以后默认数组长度10,不在构造函数时创建,实在add操作时初始化
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
定义长度的构造函数:
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);
}
}
输入集合的构造函数:
public ArrayList(Collection c) {
//将集合定义为数组复制给缓冲区(arrayList底层)
elementData = c.toArray();
//集合不为空
if ((size = elementData.length) != 0) {
//不为空,如果类型不一样,调用复制方法;将集合复制给新的arrayList
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
//集合为空;定义一个空数组
this.elementData = EMPTY_ELEMENTDATA;
}
}
2.添加元素add:
添加元素:
public boolean add(E e) {
//扩充长度,在原来的大小上面加1
ensureCapacityInternal(size + 1); // Increments modCount!!
//添加元素
elementData[size++] = e;
return true;
}
或者
public void add(int index, E element) {
//判断index是否越界
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1, size - index);
elementData[index] = element;
size++;
}
判断是否越界:
private void rangeCheckForAdd(int index) {
//小于0或者大于size都数组越界
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
计算容量:
//minCapacit原本长度+1---》因为新添加一个元素,肯定要默认长度+1
private void ensureCapacityInternal(int minCapacity) {
//如果这个数组等于空,返回最大的容量,否则,还是原来的容量+1
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
确认扩展容量
private void ensureExplicitCapacity(int minCapacity) {
//修改次数+1
modCount++;
// 如果容量不够,调用增加容量方法
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
扩展方法
private void grow(int minCapacity) {
// 获取原来数组容量的长度,老的数组长度
int oldCapacity = elementData.length;
//新增加的容量长度为原来容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
//新的容量小于需要的容量;那么新的容量=需要的容量
if (newCapacity - minCapacity < 0)//只有之前该list为空的情况,才创建list长度为10
newCapacity = minCapacity;
//新创建的容量(1.5)超过数组的最大值。按照最大的长度创建list
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);//为数组的最大值
// 调用复制方法,在原来元素上增加容量,这就是传说中的可变集合。用新长度复制原数组
elementData = Arrays.copyOf(elementData, newCapacity);
}
//获取数组的最大值
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
具体的复制方法
public static
Class newType) {
@SuppressWarnings("unchecked")
获得者数组对象(创建一个长度为1.5oldLength的新数组)
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
调用系统的方法增加数组容量;(将老的数组复制到新数组中。)
System.arraycopy(original, 0, copy, 0,Math.min(original.length, newLength));
return copy;
}
3.当ArrayList中有空闲的空间时,缩减ArrayList的容量;应用程序可以使用这个方法最小化ArrayList实例
public void trimToSize() {
modCount++;//记录修改次数
//判断真实数组的长度是否小于扩容缓存的长度
if (size < elementData.length) {
//如果数组的大小为0,则给该ArrayList定义为空数组,否则按照真实的数组长度定义ArrayList容量长度
elementData = (size == 0)? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size);
}
}
4.确定容量
public void ensureCapacity(int minCapacity) {
//最小扩容长度;如果该ArrayList不为空;可以不进行扩容0;如果为空就要进行默认长度10
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
5.ArrayList的克隆;因为实现了Cloneable接口,所以支持克隆
//可以一个完全一样的ArrayList
public Object clone() {
try {
ArrayList v = (ArrayList) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
}
6.删除操作
/**
* 移除此列表中指定位置上的元素
* @param index 需被移除的元素的索引
* @return the element 被移除的元素值
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
rangeCheck(index); //判断index是否 <= size
modCount++;
E oldValue = elementData(index);
//将数组elementData中index位置之后的所有元素向前移一位
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; //将原数组最后一个位置置为null,由GC清理
return oldValue;
}
//移除ArrayList中首次出现的指定元素(如果存在),ArrayList中允许存放重复的元素
public boolean remove(Object o) {
// 由于ArrayList中允许存放null,因此下面通过两种情况来分别处理。
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index); //私有的移除方法,跳过index参数的边界检查以及不返回任何值
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
//私有的删除指定位置元素的方法,跳过index参数的边界检查以及不返回任何值
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
}
//清空ArrayList,将全部的元素设为null
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
//删除ArrayList中从fromIndex(包含)到toIndex(不包含)之间所有的元素,共移除了toIndex-fromIndex个元素
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex; //需向前移动的元素的个数
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;
}
//删除ArrayList中包含在指定容器c中的所有元素
public boolean removeAll(Collection c) {
Objects.requireNonNull(c); //检查指定的对象c是否为空
return batchRemove(c, false);
}
//移除ArrayList中不包含在指定容器c中的所有元素,与removeAll(Collection c)正好相反
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; //读写双指针
boolean modified = false;
try {
for (; r < size; r++)
if (c.contains(elementData[r]) == complement) //判断指定容器c中是否含有elementData[r]元素
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;
}
6.修改操作
//将指定索引上的值替换为新值,并返回旧值
public E set(int index, E element) {
//检查是否index越界
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
7.查找元素
//判断ArrayList中是否包含Object(o)
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
//正向查找,返回ArrayList中元素Object o第一次出现的位置,如果元素不存在,则返回-1
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;
}
//逆向查找,返回ArrayList中元素Object o最后一次出现的位置,如果元素不存在,则返回-1
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;
}
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
//返回指定索引处的值
public E get(int index) {
rangeCheck(index);
return elementData(index); //实质上return (E) elementData[index]
}
8.序列化和反序列化
//序列化:将ArrayList的“大小,所有的元素值”都写入到输出流中
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 s.writeObject(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } //反序列化:先将ArrayList的“大小”读出,然后将“所有的元素值”读出 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 a[i] = s.readObject(); } } } 9.其他方法: //返回ArrayList的大小(元素个数) public int size() { return size; } //判断ArrayList是否为空 public boolean isEmpty() { return size == 0; } //返回此 ArrayList实例的浅拷贝 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); } } //返回一个包含ArrayList中所有元素的数组 public Object[] toArray() { return Arrays.copyOf(elementData, size); } //如果给定的参数数组长度足够,则将ArrayList中所有元素按序存放于参数数组中,并返回 //如果给定的参数数组长度小于ArrayList的长度,则返回一个新分配的、长度等于ArrayList长度的、包含ArrayList中所有元素的新数组 @SuppressWarnings("unchecked") public 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; }