List小结

　　List接口继承自Collection接口，直接或者间接实现List接口的容器类有ArrayList、LinkedList、Vctor等，下面从源码看下这些容器的差别。

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

private transient Object[] elementData;

ArrayList主要依靠Object数组来存储元素，但它能够灵活地改变尺寸，先看下add方法

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

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);
}

容量不足时先扩展一半的容量，超过限度直接到hugeCapacity().

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;
}

删除元素后会调整原先容器元素位置。

LinkedList：

元素通过双向链表来存储，节点：

private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;

Node(Node<E> prev, E element, Node<E> next) {
this.item = element;
this.next = next;
this.prev = prev;
}
}

添加元素：

public boolean add(E e) {
linkLast(e);
return true;
}

void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}

E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;

if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}

if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}

x.item = null;
size--;
modCount++;
return element;
}

可见增删元素LinkedList会比较有优势。

Vector：Vector里的方法都是加锁的，理论上实现了线程安全，但已经不推荐用了。