Java 集合系列（二）—— ArrayList

ArrayList

　　ArrayList 是经过一个数组来实现的，所以它是在连续的存储位置存放对象的引用，只不过它比 Array 更智能，可以根据集合长度进行自动扩容。

 

　　假设让咱们来实现一个简单的可以自动扩容的数组，咱们最容易想到的点就是：

1. add()的时候须要判断当前数组size+1是否等于此时定义的数组大小；
2. 若小于直接添加便可；不然，须要先扩容再进行添加。

实际上，ArrayList的内部实现原理也是这样子，咱们能够来研究分析一下ArrayList的源码

 add(E e) 源码分析

 1   /**
 2      * Appends the specified element to the end of this list.
 3      *
 4      * @param e element to be appended to this list
 5      * @return <tt>true</tt> (as specified by {@link Collection#add})
 6      */
 7     public boolean add(E e) {
 8         ensureCapacityInternal(size + 1);   // 进行扩容校验
 9         elementData[size++] = e;            // 将值添加到数组后面，并将 size+1
10         return true;
11     }
12 
13 
14 
15     /**
16      * The array buffer into which the elements of the ArrayList are stored.
17      * The capacity of the ArrayList is the length of this array buffer. Any
18      * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
19      * will be expanded to DEFAULT_CAPACITY when the first element is added.
20      */
21     transient Object[] elementData; // non-private to simplify nested class access
22     
23     private void ensureCapacityInternal(int minCapacity) {
24         ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));    // elementData 数组
25     }
26 
27 
28 
29     /**
30      * Default initial capacity.
31      */
32     private static final int DEFAULT_CAPACITY = 10;
33     
34     /**
35      * Shared empty array instance used for default sized empty instances. We
36      * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
37      * first element is added.
38      */
39     private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
40 
41     // 返回最大的 index
42     private static int calculateCapacity(Object[] elementData, int minCapacity) {
43         if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {   //  与空数组实例对比
44             return Math.max(DEFAULT_CAPACITY, minCapacity);
45         }
46         return minCapacity;
47     }
48 
49 
50 
51     private void ensureExplicitCapacity(int minCapacity) {
52         modCount++;
53 
54         // overflow-conscious code
55         if (minCapacity - elementData.length > 0)
56             grow(minCapacity);
57     }

扩容调用方法，实际也就是数组复制的过程

 1     /**
 2      * Increases the capacity to ensure that it can hold at least the
 3      * number of elements specified by the minimum capacity argument.
 4      *
 5      * @param minCapacity the desired minimum capacity
 6      */
 7     private void grow(int minCapacity) {
 8         // overflow-conscious code
 9         int oldCapacity = elementData.length;
10         int newCapacity = oldCapacity + (oldCapacity >> 1);
11         if (newCapacity - minCapacity < 0)
12             newCapacity = minCapacity;
13         if (newCapacity - MAX_ARRAY_SIZE > 0)
14             newCapacity = hugeCapacity(minCapacity);
15         // minCapacity is usually close to size, so this is a win:
16         elementData = Arrays.copyOf(elementData, newCapacity);
17     }

 add(int index, E element) 源码分析

 1   /**
 2      * Inserts the specified element at the specified position in this
 3      * list. Shifts the element currently at that position (if any) and
 4      * any subsequent elements to the right (adds one to their indices).
 5      *
 6      * @param index index at which the specified element is to be inserted
 7      * @param element element to be inserted
 8      * @throws IndexOutOfBoundsException {@inheritDoc}
 9      */
10     public void add(int index, E element) {
11         rangeCheckForAdd(index);    // 校验index是否超过当前定义的数组大小范围，超过则抛出 IndexOutOfBoundsException
12 
13         ensureCapacityInternal(size + 1);  // Increments modCount!!
14         System.arraycopy(elementData, index, elementData, index + 1,
15                          size - index);     // 复制，向后移动
16         elementData[index] = element;
17         size++;
18     }
19     
20 
21     /**
22      * A version of rangeCheck used by add and addAll.
23      */
24     private void rangeCheckForAdd(int index) {
25         if (index > size || index < 0)
26             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
27     }

从上面的源码分析可知，扩容和随机插入元素的消耗比较大，所以在实际开发中，应尽可能指定ArrayList大小，减小在随机插入操做。

 

优缺点

优势

• 封装了一个动态再分配的对象数组
• 使用索引进行随机访问效率高

缺陷

• 在数组中增删一个元素，全部元素都要日后往前移动，效率低下

 

知识脑图

 

在 github 上建了一个 repository ，Java Core Knowledge Tree，各位看官如果喜欢请给个star，以示鼓励，谢谢。
https://github.com/suifeng412/JCKTree

 

（以上是本身的一些看法，如有不足或者错误的地方请各位指出）

 做者：那一叶随风   http://www.cnblogs.com/phpstudy2015-6/

 原文地址： http://www.javashuo.com/article/p-wpafiemz-co.html

 声明：本博客文章为原创，只表明本人在工做学习中某一时间内总结的观点或结论。转载时请在文章页面明显位置给出原文连接