ArrayList是一个有序列表。
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
-
RandomAccess标识接口，无内部方法，表示可以随机访问内部元素 -
Cloneable标识接口，无内部方法，表示可以进行负责 -
java.io.Serializable标识接口，无内部方法，表示支持序列化
底层数据结构是一个对象数组
transient Object[] elementData;
ArrayList对象有存储上限
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
缩容，当buffer分配的空间容量大于实际存储的元素数量时，通过缩容释放对于的资源。
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
扩容，步长为原容量的 50%， 扩容后将原buffer复制到新创建的buffer中。所以ArrayList如果存在频繁扩容的情况，会引起程序性能下降。
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);
}
插入或删除指定位置的Element，都会引起底层数据的批量复制
插入
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++;
}
删除
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;
}
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
}
清空数据时，并不释放底层数组分配的空间，只是将数组中的对象持有复制成null,方便虚拟机在GC时，直接将对象内存释放。
public void clear() {
modCount++;
// clear to let GC do its work
// 释放句柄，告诉GC对象可以被回收，但并不是立即回收
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
在数据计算中存在集合求交集运算，在ArrayList中同样提供了该方法。
求交集
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;
}
ArrayList实现了Serializable接口，表示它可以被序列化，在其内部提供了jdk标准的序列化和反序列号的方法。通过序列化和发序列化方法可以发现，JDK并不是把ArrayList所有属性都写入文件，只是将size，以及数组中的每一个对象进行了序列化。
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<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
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<size; i++) {
a[i] = s.readObject();
}
}
}
