/**
* 默认初始化容量
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 如果自定义容量为0，则会默认用它来初始化ArrayList。或者用于空数组替换。
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 如果没有自定义容量，则会使用它来初始化ArrayList。或者用于空数组比对。
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 这就是ArrayList底层用到的数组
* 非私有，以简化嵌套类访问
* transient 在已经实现序列化的类中，不允许某变量序列化
*/
transient Object[] elementData;
/**
* 实际ArrayList集合大小
*/
private int size;
/**
* 可分配的最大容量
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* 根据initialCapacity 初始化一个空数组
*/
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);
}
}
/**
* 不带参数初始化，默认容量为10
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 通过集合做参数的形式初始化
*/
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;
}
}
/**
* 这个方法用来最小化实例存储。
*/
public void trimToSize() {
modCount ;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
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);
}
}
/**
* 在数组末尾添加元素
*/
public boolean add(E e) {
ensureCapacityInternal(size 1); // Increments modCount!!
elementData[size ] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
private void ensureExplicitCapacity(int minCapacity) {
modCount ;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity (oldCapacity >> 1);
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 static void main(String[] args) {
ArrayList arrayList = new ArrayList();
System.out.println(arrayList.size());
}
输出:0
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 static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
E elementData(int index) {
return (E) elementData[index];
}
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;
}
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
public E remove(int index) {
// 检测index是否合法
rangeCheck(index);
// 数据结构修改次数
modCount ;
E oldValue = elementData(index);
// 记住这个算法
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 class MyArrayList {
// 非私有，以简化嵌套类访问
// transient 在已经实现序列化的类中，不允许某变量序列化
transient Object[] elementData;
//默认容量
private static final int DEFAULT_CAPACITY = 10;
// 用于空实例的 空数组实例
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
// 实际ArrayList集合大小
private int size;
/**
* 构造方法
*/
public MyArrayList(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);
}
}
public MyArrayList(){
this(DEFAULT_CAPACITY);
}
public void add(Object o){
//1. 判断数据容量是否大于 elementData
ensureExplicitCapacity(size 1);
//2. 使用下标进行赋值
elementData[size ] = o;
}
private void ensureExplicitCapacity(int minCapacity){
if (size == elementData.length){
// 需要扩容,扩容1.5倍(ArrayList默认扩容1.5倍)
// 注意：如果oldCapacity值为1
int oldCapacity = elementData.length;
int newCapacity = oldCapacity (oldCapacity >> 1);
// 如果新容量 < 最小容量， 则将最小容量赋值给新容量
// 如果 oldCapacity=1, 则 minCapacity=1 1=2 newCapacity=1 (1>>1)=1
if (newCapacity - minCapacity < 0){
newCapacity = minCapacity;
}
// 创建新数组
Object[] objects = new Object[newCapacity];
// 将数据复制给新数组
System.arraycopy(elementData, 0, objects, 0, elementData.length);
// 修改引用
elementData = objects;
}
}
public Object get(int index) {
rangeCheck(index);
return elementData[index];
}
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException("下标越界");
}
/**
* 通过下标删除
* @param index
* @return
*/
public Object remove(int index) {
rangeCheck(index);
// modCount ;
// 先查出元素
Object oldValue = elementData[index];
// 找出置换结束位置
int numMoved = size - index - 1;
if (numMoved > 0)
// 从 index 1 开始 将值覆盖为 index-numMoved 的值
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) {
for (int index = 0; index < size; index ){
if (o.equals(elementData[index])) {
remove(index);
return true;
}
}
return false;
}
}