一、类定义
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
二、属性
// 序列化id
private static final long serialVersionUID = 8683452581122892189L;
// 默认初始的容量
private static final int DEFAULT_CAPACITY = 10;
// 一个空对象
private static final Object[] EMPTY_ELEMENTDATA = new Object[0];
// 一个空对象，如果使用默认构造函数创建，则默认对象内容默认是该值
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = new Object[0];
// 当前数据对象存放地方，当前对象不参与序列化
transient Object[] elementData;
// 当前数组长度
private int size;
// 数组最大长度
private static final int MAX_ARRAY_SIZE = 2147483639;
三、构造方法
/**
* 注意：此时我们创建的ArrayList对象中的elementData中的长度是1，size是0,当进行第一次add的时候，elementData将会变成默认的长度：10.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
} /**
* 如果传入参数，则代表指定ArrayList的初始数组长度，传入参数如果是大于等于0，则使用用户的参数初始化，如果用户传入的参数小于0，则抛出异常，构造方法如下：
*/
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);
}
} /**
* 1）将collection对象转换成数组，然后将数组的地址的赋给elementData。
* 2）更新size的值，同时判断size的大小，如果是size等于0，直接将空对象EMPTY_ELEMENTDATA的地址赋给elementData
* 3）如果size的值大于0，则执行Arrays.copy方法，把collection对象的内容（可以理解为深拷贝）copy到elementData中。
* 注意：this.elementData = arg0.toArray(); 这里执行的简单赋值时浅拷贝，所以要执行Arrays,copy 做深拷贝
*/
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;
}
}
四、主要方法
// 扩容：
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
if (minCapacity - elementData.length > 0)
grow(minCapacity);
} private void grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);// 扩1.5倍
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;// 或者要求的容量
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
} // get
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
E elementData(int index) {
return (E) elementData[index];
} // set
public E set(int index, E element) {
rangeCheck(index); E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
} // add
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
} 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 boolean addAll(Collection<? extends E> c) {
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;
} public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index); Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
} // remove
public E remove(int index) {
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) {
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;
} protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
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;
} 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;
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) {
// 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;
}
五、内部类
private class Itr implements Iterator<E> // 迭代器 只能从头到尾 只有remove
private class ListItr extends Itr implements ListIterator // 加强迭代器 可以指定开始位置、倒序、set、add
static final class ArrayListSpliterator<E> implements Spliterator<E> // 并行迭代器 // 截取部分 还是原数组的引用，只是记录了开始结束下标
private class SubList extends AbstractList<E> implements RandomAcces {
SubList(AbstractList<E> parent, int offset, int fromIndex, int toIndex) {
this.parent = parent;
this.parentOffset = fromIndex;
this.offset = offset + fromIndex;
this.size = toIndex - fromIndex;
this.modCount = ArrayList.this.modCount;
}
}
关于并行迭代器Spliterator：jdk8中Spliterator的作用
六、其他
1.Fail-Fast机制：java中的fail-fast(快速失败)机制
2.ArrayList与Vector区别：实现原理一样主要区别
①线程安全：Vector增删改查的方法都有synchronized，线程安全，但性能较ArrayList差
②扩容：ArrayList每次扩1.5倍(int newCapacity = oldCapacity + (oldCapacity >> 1))
Vector默认扩成原来的2倍(int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity))
参考资料：