我想要看ArrayList的真正容积，但是正真容积不能从他的size得到，

所以这里我使用反射得到他的私有域“elementData”，elementData的大小就是ArrayList的实际大小了。

 1 public static void testCapasity(){
 2         //看看我先把ArrayList增大，看看如果我把元素减少会不会他也会自动将空间缩小
 3         try{
 4             ArrayList<Integer> array = new ArrayList<Integer>();
 5             Class clazz = array.getClass();
 6             Field field =clazz.getDeclaredField("elementData");
 7             field.setAccessible(true);
 8             Object obj ;
 9 
10             obj = field.get(array);
11             System.out.println("before init "+Array.getLength(obj));
12 
13             for(int i = 0;i<100;i++){
14                 array.add(i);
15             }
16 
17             System.out.println("\nafter added size is "+array.size());
18 
19             obj = field.get(array);
20             System.out.println("after added "+Array.getLength(obj));
21 
22             array.clear();
23 
24             System.out.println("\nafter removed size is "+array.size());
25             obj = field.get(array);
26             System.out.println("after removed "+Array.getLength(obj));
27 
28         }catch(NoSuchFieldException e){
29             e.printStackTrace();
30         }catch (IllegalAccessException e){
31             e.printStackTrace();
32         }
33 
34     }

输出的结果是：

before init 0

after added size is 100
after added 109

after removed size is 0
after removed 109

Process finished with exit code 0

所以由这里看出来ArrayList不会自动缩小容积，那么如果我们需要缩小容积怎么办呢？

其实ArrayList里面有一个方法可以缩小容积

 /**
     * 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.
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = Arrays.copyOf(elementData, size);
        }
    }

所以我们在增加了容积之后可以使用这个方法来缩小不需要的容积。