目录

• 1.位置
• 2.变量和常量
• 3.构造函数
• 4.trimToSize()方法

1 package java.util;
2 
3 import java.util.function.Consumer;
4 import java.util.function.Predicate;
5 import java.util.function.UnaryOperator;

private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     * ArrayList初始化默认容量大小为10，见无参构造函数。
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     * 空实例共享的空数组实例。
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 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.
     * 默认大小的空实例共享的空数组实例。将它与EMPTY_ELEMENTDATA区分开，以便确定当添加了第一个成员之后扩充多大。
     */

    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; 

  /** 
    * 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. 
    * ArrayList成员存储在这个数组缓冲区中。
    * 容量大小是这个缓冲区的长度，任何elementDate==DEFAULTCAPACITY_EMPTY_ELEMENTDATA的空ArrayList
    * 在添加第一个成员时都会扩充到DEFAULT_CAPACITY大小，即容量为10。
    */ 

    transient Object[] elementData; // non-private to simplify nested class access 

 /** 
   * The size of the ArrayList (the number of elements it contains). 
   * ArrayList的大小，注意是实际包含的成员个数。
   * @serial 
   */ 
    private int size;

    /**
     * Constructs an empty list with an initial capacity of ten.
     * 构造一个初始容量为10的空list。
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

 /**
     * Constructs an empty list with the specified initial capacity.
     * 构造一个指定了初始容量的空list。
     * @param  initialCapacity  the initial capacity of the list
     *         initialCapacity指定list的初始容量
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     *         当指定容量值为负值时，抛出IllegalArgumentException异常
     */
    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 a list containing the elements of the specified
     * collection, in the order they are returned by the collection‘s
     * iterator.
     * 构造一个包含指定Collection成员的list，成员的排列顺序与使用Collection的
     * iterator返回的顺序一致。
     * @param c the collection whose elements are to be placed into this list
     *        c 指定的Collection，它的所有元素都会被放在构造的list中
     * @throws NullPointerException if the specified collection is null
     *         当指定的Collection为null是抛出NullPointerException异常
     */
    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;
        }
    }

/**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list‘s current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     * 将ArrayList实例的容量trim为list的当前size。
     * 应用可以使用这个方法来最小化ArrayList实例的存储空间。
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }