java源码分析(1)---------ArrayList的扩容机制

package source;

import java.util.AbstractList;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.RandomAccess;

public class ArrayListNyx<E> extends AbstractList<E> implements List<E>,
RandomAccess, Cloneable, java.io.Serializable {

private static final long serialVersionUID = 2024755979807710831L;

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

private transient Object[] elementData;

private int size;

public ArrayListNyx(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);
this.elementData = new Object[initialCapacity];
}

public ArrayListNyx() {
this(10);
}

public ArrayListNyx(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
// 有可能返回的数组类型发生改变，不是Object类型，这里是返回对应的类型
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}

public boolean isEmpty() {
return size == 0;
}

public boolean add(E e){
ensureCapacityInternal(size+1);
elementData[size++] = e;
return true;
}

private void ensureCapacityInternal(int minCapacity) {
modCount++;
if(minCapacity>elementData.length){
grow(minCapacity);
}
}

private void grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//如果创建的数据大小大于MAX_ARRAY_SIZE则使用Integer.MAX_VALUE否则使用MAX_ARRAY_SIZE
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}

private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}

public boolean contains(Object o) {
return indexOf(o) >= 0;
}

public int indexOf(Object o) {
if (o == null) {
// 如果查询为null，那么找到第一个数据中的null值，返回位置
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
// 匹配找到的第一个数据
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}

public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}

public void clear() {
modCount++;

// Let gc do its work
for (int i = 0; i < size; i++)
elementData[i] = null;

size = 0;
}

private void fastRemove(int index) {
//modCount记录ArrayList结构变化次数
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // Let gc do its work
}

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;
}

public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size+numNew);
System.arraycopy(a, 0, elementData, size, numNew);
size = size+numNew;
return numNew!=0;
}

public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}

public boolean retainAll(Collection<?> c) {
return batchRemove(c, true);
}

public boolean batchRemove(Collection<?> c,boolean complement){
Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
for (; r < size; r++)
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) {
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}

@Override
public E get(int index) {
return null;
}

@Override
public int size() {
return size;
}

}

从源码上看ArrayList的底层存储来自于数组，Object[] element = new Object[10];其初始化长度为10，它的扩容机制是当前长度的二进制右移一位加上当前的长度。

（相当于*1.5）

10的二进制：1010-------------------------右移一位------------------------>101(机位数字5，为原数字的一半)========>第一次扩容大小为15

源码详见：

private void grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//如果创建的数据大小大于MAX_ARRAY_SIZE则使用Integer.MAX_VALUE否则使用MAX_ARRAY_SIZE
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}