Java集合（二）：ArrayList

ArrayList底层是用数组来实现存储及容量大小变化

内部参数

public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
//默认大小
private static final int DEFAULT_CAPACITY = 10;
//有参构造且传入的大小为0，或者传入的Collection大小为0时，会将此参数赋值给elementData
private static final Object[] EMPTY_ELEMENTDATA = {};
//无参构造会将此参数赋值给elementData
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//ArrayList类中用来存放数据的数组
transient Object[] elementData; // non-private to simplify nested class access
//表示的是elementData实际有多少元素
private int size;
//此变量定义在AbstractList中，用于记录List被操作的次数。主要使用在iterator，防止在遍历过程中被修改
protected transient int modCount = 0;
}

构造方法

//无参构造，构造时只是给elementData赋值了一个空数组，是在第一次添加元素的时候扩大至10的
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
//有初始大小的构造
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);
     }
 }
//传入一个集合，复制到elementData
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;
    }
}

add方法

public boolean add(E e) {
ensureCapacityInternal(size + 1);  // Increments modCount!!
elementData[size++] = e;
return true;
}
//确认集合大小
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }

ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// 扩容
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

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;
}

grow（扩容）方法

//默认将容量扩容为原来的1.5倍
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);
}

remove方法

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;
}

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
}

迭代器iterator

//直接返回一个内部类对象
public Iterator<E> iterator() {
return new Itr();
}
//实现了iterator接口
private class Itr implements Iterator<E> {
int cursor;       // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;

public boolean hasNext() {
return cursor != size;
  }

@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
  }

public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
    } catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
    }
  }
//注意这个方法，是必须用迭代器遍历的原因
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
  }
}

总结

ArrayList 底层基于数组实现容量大小动态可变。
扩容机制为首先扩容为原始容量的 1.5 倍。如果1.5倍太小的话，则将我们所需的容量大小赋值给 newCapacity，如果1.5倍太大或者我们需要的容量太大，那就直接拿 newCapacity = (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE 来扩容。
扩容之后是通过数组的拷贝来确保元素的准确性的，所以尽可能减少扩容操作。
ArrayList 的最大存储能力：Integer.MAX_VALUE。
size 为集合中存储的元素的个数。elementData.length 为数组长度，表示最多可以存储多少个元素。
如果需要边遍历边 remove ，必须使用 iterator。且 remove 之前必须先 next，next 之后只能用一次 remove。