Java Learning(5) -- ArrayList

如何动态的查看ArrayList的容量？

在ArrayList提供的方法中，有三个方法：

size() -- 返回当前列表中的元素数量；

trimToSize() -- 将当前ArrayList容量调整为列表的当前大小；

ensureCapacity(int minCapacity) -- 如果需要扩展列表的容量以确保能够装下minCapacity个元素；

那如何动态的监控整个列表的容量变化？

请参考如下代码：

　　import java.lang.reflect.Field;

import java.util.ArrayList;

public class ArraySizeTest {

    @SuppressWarnings("rawtypes")
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ArrayList<String> testArray = new ArrayList<String>(5);

        Field AL_Field;
        Field AL_Field_1;
        Field AL_Field_2;
        Field AL_Field_3;
        try {
            //check the initial capacity of testArray
            Class AL_Class = testArray.getClass();
            AL_Field = AL_Class.getDeclaredField("elementData");
            AL_Field.setAccessible(true);
            Object[] objects = (Object[]) AL_Field.get(testArray);
            System.out.println(objects.length);
            System.out.println(testArray.size());
            //result: capacity = 5, size = 0;

            //add 4 items in the testArray
            System.out.println("1::>    " + testArray.hashCode());
            for(int i=0; i<4; i++){　　　　　　　　　　testArray.add(i, Integer.toString(i));　　　　　　　　　　System.out.println("1."+i+"::> " + testArray.hashCode());
            }
            System.out.println(objects.length);
            System.out.println(testArray.size());
            System.out.println("2::>    " + testArray.hashCode());
            //result: capacity = 5, size = 4;
            //result: new a ArrayList

            //after trimToSize
            testArray.trimToSize();
            //re-get the objects
            Class AL_Class_1 = testArray.getClass();
            AL_Field_1 = AL_Class_1.getDeclaredField("elementData");
            AL_Field_1.setAccessible(true);
            Object[] objects_1 = (Object[]) AL_Field_1.get(testArray);
            System.out.println(objects_1.length);
            System.out.println(testArray.size());
            //result: capacity = 4, size = 4;

            //larger the capacity to 6
            System.out.println("3::>    " + testArray.hashCode());
            testArray.ensureCapacity(6);
            testArray.add("5");
            testArray.add("6");
            testArray.add("7");
            System.out.println("4::>    " + testArray.hashCode());
            //after adding item the testArray is re-created
            //re-get the objects
            Class AL_Class_2 = testArray.getClass();
            AL_Field_2 = AL_Class_2.getDeclaredField("elementData");
            AL_Field_2.setAccessible(true);
            Object[] objects_2 = (Object[]) AL_Field_2.get(testArray);
            System.out.println(objects_2.length);
            System.out.println(testArray.size());
            System.out.println("5::>    " + testArray.hashCode());
            //result: capacity = 9, size = 7;
            //do not create a new ArrayList

            testArray.trimToSize();
            //re-get objects
            Class AL_Class_3 = testArray.getClass();
            AL_Field_3 = AL_Class_3.getDeclaredField("elementData");
            AL_Field_3.setAccessible(true);
            Object[] objects_3 = (Object[]) AL_Field_3.get(testArray);
            System.out.println(objects_3.length);
            System.out.println(testArray.size());
            System.out.println("6::>    " + testArray.hashCode());
            //result: capacity = 7, size = 7;
            //result: do not create a new ArrayList

        } catch (NoSuchFieldException | SecurityException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IllegalArgumentException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

结果：

5
0
1::> 1
1.0::> 79
1.1::> 2498
1.2::> 77488
1.3::> 2402179
5
4
2::> 2402179
4
4
3::> 2402179
4::> -1451076781
9
7
5::> -1451076781
7
7
6::> -1451076781

从代码中可以看出：

ArrayList每次在追加item之后都在重新创建一个ArrayList对象，直到运用ensureCapacity方法。

虽然我们设定了最大长度为6，但是当追加的内容数目超出6之后，随着size的增大，ArrayList的容量在变大，具体变大多少从本例中看到的是2，我也尝试过追加内容，增大原则大致是2,3,5,8,13...(i = (i-1)+(i-2))。

另外需要注意的是在每次利用反射找出ArrayList的容量后对ArrayList修改，下次再要进行容量计算时需要重新做Object Get。