[TOC]

ArrayList （数组，非线程安全）

• get直接获取O(1)
• reemove都要移动元素O(n)
• add可能扩容(new，遍历拷贝)，或者是插入(遍历移动)O(n)

add(E e)

/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

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);
}

• 如有扩容，则需new新数组，然后遍历拷贝

add(int index, E element)

/**
* Inserts the specified element at the specified position in this
* list. Shifts the element currently at that position (if any) and
* any subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
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++;
}

• 插入要System.arraycopy移动元素

remove(int index)

/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
*
* @param index the index of the element to be removed
* @return the element that was removed from the list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
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;
}

迭代器remove报错ConcurrentModificationException

mubi@mubideMacBook-Pro hotload $ vim Solution.java
mubi@mubideMacBook-Pro hotload $ javac Solution.java
注: Solution.java使用了未经检查或不安全的操作。
注: 有关详细信息, 请使用 -Xlint:unchecked 重新编译。
mubi@mubideMacBook-Pro hotload $ java Solution
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
    at java.util.ArrayList$Itr.next(ArrayList.java:859)
    at Solution.main(Solution.java:13)
mubi@mubideMacBook-Pro hotload $ cat Solution
cat: Solution: No such file or directory
mubi@mubideMacBook-Pro hotload $ cat Solution.java
import java.util.*;

class Solution {

    public static void main(String[] args) {
        List<String> list = new ArrayList() {{
            add("Java");
            add("Scalar");
            add("Kotlin");
            add("Groovy");
        }};

        for (String item : list) {
            list.remove(item);
        }
    }

}

• System.arraycopy移动元素

LinkedList（链表）

• addLast O(1), 插入要遍历前面的链表元素O(n)
• remove O(1)

• get 遍历O(n)

• 双向链表

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0;

    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node<E> first;

    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node<E> last;

    /**
     * Constructs an empty list.
     */
    public LinkedList() {
    }

add(E e)

/**
    * Appends the specified element to the end of this list.
    *
    * <p>This method is equivalent to {@link #addLast}.
    *
    * @param e element to be appended to this list
    * @return {@code true} (as specified by {@link Collection#add})
    */
public boolean add(E e) {
    linkLast(e);
    return true;
}

 /**
    * Links e as last element.
    */
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++;
}

add(int index, E element)

/**
    * Inserts the specified element at the specified position in this list.
    * Shifts the element currently at that position (if any) and any
    * subsequent elements to the right (adds one to their indices).
    *
    * @param index index at which the specified element is to be inserted
    * @param element element to be inserted
    * @throws IndexOutOfBoundsException {@inheritDoc}
    */
public void add(int index, E element) {
    checkPositionIndex(index);

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

remove(int index)

/**
    * Removes the element at the specified position in this list.  Shifts any
    * subsequent elements to the left (subtracts one from their indices).
    * Returns the element that was removed from the list.
    *
    * @param index the index of the element to be removed
    * @return the element previously at the specified position
    * @throws IndexOutOfBoundsException {@inheritDoc}
    */
public E remove(int index) {
    checkElementIndex(index);
    return unlink(node(index));
}

/**
    * Unlinks non-null node x.
    */
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;
}

get 索引

/**
    * Returns the (non-null) Node at the specified element index.
    */
Node<E> node(int index) {
    // assert isElementIndex(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;
    }
}

• 需要遍历链表

Vector（线程安全）

• Vector使用同步方法实现(synchronized),线程安全

• Vector和ArrayList底层是数组，ArrayList不可以设置扩展的容量, 默认1.5倍; Vector可以设置, 默认2倍

• ArrayList无参构造函数中初始量为0; Vector的无参构造函数初始容量为10