概述
ArrayList是一个以动态数组为基础实现的非线程安全的集合,ArrayList的元素可以为空、可以重复,同时又是有序的(读取和存放的顺序一致 )。
ArrayList继承AbstractList,实现了List
、RandomAccess
(可以快速访问)、Cloneable
(可以被克隆)、java.io.Serializable
(支持序列化)
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ArrayList的初始化方式有三种:
1、无参构造,默认长度为10,是我们使用的最多的一种初始化方式:
/** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }
这个时候,我们从源码中可以看到,里面只有一行代码:this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA,那么定义的DEFAULTCAPACITY_EMPTY_ELEMENTDATA可以在源码中找到:
/** * 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 = {};
通过注释可以得知,源码中定义了一个空的数组作为默认的大小,并且在第一个元素添加进来的时候再确定把数组扩充多少,这段逻辑会在接下来添加元素部分作出解释。
2、指定初始化长度:
/** * 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); } }
3、用一个Collection对象来构造
/** * 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; } }
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