美文网首页Java基础Android开发经验谈Java源码
【ArrayList源码】ArrayList构造方法

【ArrayList源码】ArrayList构造方法

作者: 秀叶寒冬 | 来源:发表于2019-07-16 09:55 被阅读20次

    ArrayList简介

    ArrayList称为数组链表,它是继承AbstractList,并实现了List、RandomAccess、Cloneable和Serializable接口

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

    ArrayList一共有三个构造方法,三个构造方法如下:

    /**
         * 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
         */
        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.
         */
        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;
            }
        }
    

    下面逐步分析ArrayList的构造方法中做了哪些事情。

    1 无参构造方法

    /**
         * 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的缓冲数组,里面存储的是ArrayList数据。ArrayList的容量长度就是这个数组的长度,任何空的使用elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA初始化的ArrayList在添加第一个元素时都会被扩容为DEFAULT_CAPACITY (10)
         */
        transient Object[] 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.
         * 用于默认大小的空的共享空数组实例。我们将此与EMPTY_ELEMENTDATA区分开来,以便在添加第一个元素时知道需要扩张多少。
         */
        private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
     /**
         * 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;
        }
    

    看了上面的源码的注释,才知道,ArrayList的无参构造函数里ArrayList的初始容量为空,真正初始化容量的地方是在添加第一个元素的时候。

    2 一个参数的构造方法,参数为初始的容量

    /**
         * Shared empty array instance used for empty instances.
         * 空的共享数组实例
         */
        private static final Object[] EMPTY_ELEMENTDATA = {};
        /**
         * Constructs an empty list with the specified initial capacity.
         * 构造一个指定容量的空List
         *
         * @param  initialCapacity  the initial capacity of the list
         * initialCapacity是list的初始容量
         * @throws IllegalArgumentException if the specified initial capacity is negative
         * 如果指定容量为负数,则抛出异常
         */
        public ArrayList(int initialCapacity) {
            if (initialCapacity > 0) {//#1
                this.elementData = new Object[initialCapacity];//#2
            } else if (initialCapacity == 0) {//#3
                this.elementData = EMPTY_ELEMENTDATA;//#4
            } else {
                throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);//#5
            }
        }
    
    • 第#1行如果指定容量大于0,则进行#2操作
    • 第#2行使用指定容量初始化ArrayList的数组。
    • 第#3行如果指定容量为0,则进行#4
    • 第#4行初始化elementData为空数组
    • 第#5行如果指定容量小于0,则抛出异常

    3 参数为集合的构造方法

    /**
         * Constructs a list containing the elements of the specified
         * collection, in the order they are returned by the collection's
         * iterator.
         *按照集合的迭代器返回的顺序构造一个包含指定集合元素的列表。
         * @param c the collection whose elements are to be placed into this list
         * 参数是要放入列表中的集合
         * @throws NullPointerException if the specified collection is null
         * 如果指定集合为空则报空指针异常
         */
        public ArrayList(Collection<? extends E> c) {
            elementData = c.toArray();//#1
            if ((size = elementData.length) != 0) {//#2
                // c.toArray might (incorrectly) not return Object[] (see 6260652)
                if (elementData.getClass() != Object[].class)//#3
                    elementData = Arrays.copyOf(elementData, size, Object[].class);//#4
            } else {
                // replace with empty array.
                this.elementData = EMPTY_ELEMENTDATA;//#5
            }
        }
    
    • 第#1行将参数集合转化为数组并赋值给elementData
    • 第#2行如果elementData的长度(size指的是数组中数据的个数)不等于0,则进行#3,否则进行#5
    • 第#3行如果elementData不是Object数组类型,则执行#4
    • 第#4行将数据赋值到Object数组类型的elementData
    • 第#5行如果elementData长度为0,初始化elementData为空数组

    4 总结

    以下针对构造函数进行总结

    • ArrayList就是一个数组,在ArrayList里是通过Object[] elementData来存储数据的。
    • ArrayList初始化时容量为0(使用空构造方法初始化),它在添加第一个元素(调用add方法)时如果ArrayList容量为0,则会扩容默认值10。

    相关文章

      网友评论

        本文标题:【ArrayList源码】ArrayList构造方法

        本文链接:https://www.haomeiwen.com/subject/qiqzkctx.html