jdk1.8的arraylist源码解读
今天学了Python的列表，所以想研究下ArrayList的源码：
public class ArrayList
implements List
这是ArrayList的继承以及实现情况。
private static final int DEFAULT_CAPACITY = 10; 默认的空间为10
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData; transient关键字防止序列化。transient关键字详解
private int size;
接下来为构造方法：
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity]; 生成initialCapacity个内存空间
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA; 生成一个内存空间。
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; 还是生成一个内存空间（不明白为什么特意定两个）
}
public void trimToSize() {
modCount++; 其中，这个变量来源于 AbstractList 类中的 protected transient int modCount = 0;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
public void ensureCapacity(int minCapacity) { 感觉这个确保缓冲区的方法包含几乎所有的ArrayList的底层方法
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
// any size if not default element table
? 0
// larger than default for default empty table. It's already
// supposed to be at default size.
: DEFAULT_CAPACITY; 如果数组的大小等于{} 那就赋值为0，否则初始化为默认的10.
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity); 进行扩容。
}
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) { 进行扩容。
// overflow-conscious code
int oldCapacity = elementData.length; 老空间
int newCapacity = oldCapacity + (oldCapacity >> 1); 新空间为老空间的1.5倍
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);
}
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 int size() {
return size; 返回list的大小
}
public boolean isEmpty() {
return size == 0; 判断size和0的关系
}
public boolean contains(Object o) { 判断是否包含某个对象
return indexOf(o) >= 0;
}
public int indexOf(Object o) { 判断对象在集合中的索引位置，
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 int lastIndexOf(Object o) { 从后往前判断对象在集合中的索引位置。
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;
}
public Object clone() { 克隆 返回数组对象
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);
}
}
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
@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;
}
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
public E get(int index) { 返回指定位置的对象
rangeCheck(index);
return elementData(index);
}
private void rangeCheck(int index) { 检查是否超出实际容量
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
public E set(int index, E element) { 在指定索引位置设置值，并返回索引位置上的原值
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
public boolean add(E e) { 增加新值。
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;
}
public E remove(int index) { 移除对象
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
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 boolean remove(Object o) { 是否要移除对象
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;
}
private void fastRemove(int index) { 快速移除指定索引的对象
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
}
public void clear() { 清楚全部对象
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
public boolean addAll(Collection c) { 增加全部
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;
}