List集合源码解读

一：总述：

　　主要讲解3个集合

　　1.ArrayList:

　　　底层是数组，线程不安全；

　　2.LinkedList:

　　　底层是链表，线程不安全；

　　3.Vector

　　   底层数据结构是数组。线程安全；

二：ArrayList解析

　　

　　首先，我们来看一下ArrayList的属性：

　　　　/**
　　　　* Default initial capacity.
　　　　*/
　　　　private static final int DEFAULT_CAPACITY = 10;//初始化容量值

  /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};//指定ArrayList的容量为0时，返回该空数组
 
    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};//与上个属性的区别是：该数组是默认返回的，而上个属性是指定容量为0时返回
 
    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access//第一次保存元素时，数组将会扩容
 
    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;//ArrayList的实际大小

　　根据上面我们可以清晰的发现：ArrayList底层其实就是一个数组，ArrayList中有扩容这么一个概念，正因为它扩容，所以它能够实现“动态”增长

2.2构造方法

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
　　//指定初始化长度initCapacity
    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);
        }
    }
 
    /**
     * Constructs an empty list with an initial capacity of ten.
     */
　　//否则返回的是：DEFAULTCAPACITY_EMPTY_ELEMENTDATA
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
 
    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    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;
        }
    }

2.3 Add()方法

源码如下：

    /**
     * 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;
    }
 
    /**
     * 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++;
    }

2.3.1 Add(E e)

步骤：

• 检查是否需要扩容
• 插入元素

首先，我们来看看这个方法：

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

该方法很短，我们可以根据方法名就猜到他是干了什么：

• 确认list容量，尝试容量加1，看看有无必要
• 添加元素

接下来我们来看看这个小容量(+1)是否满足我们的需求：

    private void ensureCapacityInternal(int minCapacity) {
　　　　  //想要得到的最小的容量
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
　　　　 //确定明确的容量
        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
　　　　  //如果最小容量比数组长度大，则用用grow扩容
        // overflow-conscious code
        if (minCapacity - elementData.length > )
            grow(minCapacity);
    }

接下来看grow是如何扩容的

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);//扩容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);//扩容完后调用copyOf方法把原数组的值存入新数组
    }

再来看是怎么把原数组的值放入新数组

    /**
     * Copies the specified array, truncating or padding with nulls (if necessary)
     * so the copy has the specified length.  For all indices that are
     * valid in both the original array and the copy, the two arrays will
     * contain identical values.  For any indices that are valid in the
     * copy but not the original, the copy will contain <tt>null</tt>.
     * Such indices will exist if and only if the specified length
     * is greater than that of the original array.
     * The resulting array is of the class <tt>newType</tt>.
     *
     * @param <U> the class of the objects in the original array
     * @param <T> the class of the objects in the returned array
     * @param original the array to be copied
     * @param newLength the length of the copy to be returned
     * @param newType the class of the copy to be returned
     * @return a copy of the original array, truncated or padded with nulls
     *     to obtain the specified length
     * @throws NegativeArraySizeException if <tt>newLength</tt> is negative
     * @throws NullPointerException if <tt>original</tt> is null
     * @throws ArrayStoreException if an element copied from
     *     <tt>original</tt> is not of a runtime type that can be stored in
     *     an array of class <tt>newType</tt>
     * @since 1.6
     */
    public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
        @SuppressWarnings("unchecked")
        T[] copy = ((Object)newType == (Object)Object[].class)
            ? (T[]) new Object[newLength]
            : (T[]) Array.newInstance(newType.getComponentType(), newLength);
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

到目前为止，我们就可以知道add(E e)的基本实现了：

• 首先去检查一下数组的容量是否足够
  • 足够：直接添加
  • 不足够：扩容
    • 扩容到原来的1.5倍
    • 第一次扩容后，如果容量还是小于minCapacity，就将容量扩充为minCapacity。

2.3.2：add(int index, E element)

步骤：

• 检查角标
• 空间检查，如果有需要进行扩容
• 插入元素

我们来看看插入的实现：

    /**
     * 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);//检查是否越界
 
        ensureCapacityInternal(size + 1);  // Increments modCount!!//扩容
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);//调用arraycopy进行插入
        elementData[index] = element;
        size++;
    }

注：arraycopy是用c++来编写的

2.4：get()

• 检查角标
• 返回元素

    /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        rangeCheck(index);
 
        return elementData(index);
    }

   // 检查角标
   private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
 
    // 返回元素
    E elementData(int index) {
        return (E) elementData[index];
    }

2.5:set()方法

步骤：

• 检查角标
• 替代元素
• 返回旧值

    /**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        rangeCheck(index);
　　　　　//将值进行替代，返回旧值
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

2.6:remove()方法

步骤：

• 检查角标
• 删除元素
• 计算出需要移动的个数，并移动
• 设置为null，让Gc回收

    /**
     * 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);
　　　　
　　　　 //左移的个数
        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;
    }

2.7:总述

• ArrayList是基于动态数组实现的，在增删时候，需要数组的拷贝复制（使用的是System.arrayCopy()效率最高的数组拷贝方法）。
• ArrayList的默认初始化容量是10，每次扩容时候增加原先容量的一半，也就是变为原来的1.5倍
• 删除元素时不会减少容量，若希望减少容量则调用trimToSize()
• 它不是线程安全的。它能存放null值。

三：Vector与ArrayList的区别

1.Vector底层也是数组，与ArrayList最大的区别就是：同步(线程安全),Vector的每个方法都是同步的 （相对效率较低）

2.在要求非同步的情况下，我们一般都是使用ArrayList来替代Vector的了,如果想要ArrayList实现同步，可以使用Collections的方法：List list =Collections.synchronizedList(new ArrayList(...));，就可以实现同步了

3.ArrayList是以1.5倍扩容，Vector是以2倍扩容

以上的结论可以在源码中得到验证

四：LinkedList解析

此处放一张全家桶

LinkedList底层是双向链表

    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;
        }
    }

　　4.1：构造方法

    /**
     * Constructs an empty list.
     */
    public LinkedList() {
    }
 
    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param  c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

4.2： add()方法

　　public boolean add(E e) {
        linkLast(e);
        return true;
    }
　　//往链表的最后添加元素
    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++;
    }

4.3：remove()方法

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If this list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns {@code true} if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if this list contained the specified element
     */
    public boolean remove(Object o) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
　　　　　　　　　　　　//删除元素
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
　　　　　　　　   //判断元素是否存在里面
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * 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;
    }

4.4：get()方法

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

node（）方法

    /**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);
　　　　  //下标小于长度的一半，从头部开始遍历
        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
　　　　 //否则从尾部开始遍历
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

4.5:set方法

set方法和get方法其实差不多，根据下标来判断是从头遍历还是从尾遍历

    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

具体请参考源码

五：总结

ArrayList：

• 底层实现是数组
• ArrayList的默认初始化容量是10，每次扩容时候增加原先容量的一半，也就是变为原来的1.5倍
• 在增删时候，需要数组的拷贝复制(C++实现)

LinkedList：

• 底层实现是双向链表[双向链表方便实现往前遍历]

Vector：

• 底层是数组，现在已少用，被ArrayList替代，原因有两个：
  • Vector所有方法都是同步，有性能损失。
  • Vector初始length是10 超过length时 以100%比率增长，相比于ArrayList更多消耗内存。

总的来说：查询多用ArrayList，增删多用LinkedList。

ArrayList增删慢不是绝对的(在数量大的情况下，已测试)：

• 如果增加元素一直是使用add()(增加到末尾)的话，那是ArrayList要快
• 一直删除末尾的元素也是ArrayList要快【不用复制移动位置】
• 至于如果删除的是中间的位置的话，还是ArrayList要快！

但一般来说：增删多还是用LinkedList，因为上面的情况是极端的~