Java性能 -- ArrayList + LinkedList

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

// 默认初始化容量
private static final int DEFAULT_CAPACITY = 10;
// 对象数组
transient Object[] elementData;
// 数组长度
private int size;

private void writeObject(java.io.ObjectOutputStream s)
private void readObject(java.io.ObjectInputStream s)

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

// 指定合理的初始大小，有助于减少数组的扩容次数，提供系统性能
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);
    }
}

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;
    }
}

// 直接将元素添加到数组的末尾
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    // 如果一开始就指定了合理的初始大小，不会发生动态扩容，添加元素，只需要在数组末尾添加元素，性能也会很好
    // ArrayList在大量新增元素的场景下，效率一定会变慢？ -- 不一定，看场景
    elementData[size++] = e;
    return true;
}

// 添加元素到任意位置
public void add(int index, E element) {
    rangeCheckForAdd(index);
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    // 数组拷贝
    System.arraycopy(elementData, index, elementData, index + 1, size - index);
    elementData[index] = element;
    size++;
}

// 如果容量不够大，会按原来数组的1.5倍大小进行扩容
// 在扩容之后需要将数组复制到新分配的内存地址
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;
    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1); // 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);
}

public E remove(int index) {
    rangeCheck(index);
    modCount++;
    E oldValue = elementData(index);
    int numMoved = size - index - 1;
    if (numMoved > 0)
        // 数组重组，删除的元素位置越靠前，数组重组的开销就越大
        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 {
        // 遍历数组
        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)
        // 同样也要数组重组
        System.arraycopy(elementData, index+1, elementData, index, numMoved);
    elementData[--size] = null; // clear to let GC do its work
}

// ArrayList是基于数组实现的，所以在获取元素的时候非常快
public E get(int index) {
    rangeCheck(index);
    return elementData(index);
}

E elementData(int index) {
    return (E) elementData[index];
}

// 1. 清晰地表达了链表中链头和链尾概念
// 2. 在链头和链尾的插入删除操作更加快捷
transient Node<E> first;
transient Node<E> last;

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;
    }
}

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable {
}

// LinkedList也实现了自定义的序列化和反序列化
transient int size = 0;
transient Node<E> first;
transient Node<E> last;

private void writeObject(java.io.ObjectOutputStream s)
private void readObject(java.io.ObjectInputStream s)

// 添加到队尾
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++;
}

public void add(int index, E element) {
    checkPositionIndex(index);

    if (index == size)
        linkLast(element);
    else
        linkBefore(element, node(index));
}

// 从链头或者链尾查找元素
Node<E> node(int index) {
    if (index < (size >> 1)) {
        Node<E> x = first;
        for (int i = 0; i < index; i++)
            x = x.next;
        return x;
    } else {
        Node<E> x = last;
        for (int i = size - 1; i > index; i--)
            x = x.prev;
        return x;
    }
}

void linkBefore(E e, Node<E> succ) {
    // assert succ != null;
    final Node<E> pred = succ.prev;
    final Node<E> newNode = new Node<>(pred, e, succ);
    succ.prev = newNode;
    if (pred == null)
        first = newNode;
    else
        pred.next = newNode;
    size++;
    modCount++;
}

// 从链头或者链尾查找元素
public boolean remove(Object o) {
    if (o == null) {
        for (Node<E> x = first; x != null; x = x.next) {
            if (x.item == null) {
                unlink(x);
                return true;
            }
        }
    } else {
        for (Node<E> x = first; x != null; x = x.next) {
            if (o.equals(x.item)) {
                unlink(x);
                return true;
            }
        }
    }
    return false;
}

public E remove(int index) {
    checkElementIndex(index);
    return unlink(node(index));
}

public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
}