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

1.2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;
        }
    }

1.3 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++;
    }

1.3.1 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：

未完待续