Java Learning(5) -- ArrayList

標籤：

如何動態查看ArrayList的容量？

在ArrayList提供的方法中，有三個方法：

size() -- 返回當前列表中的元素數量；

trimToSize() -- 將當前ArrayList容量調整為列表的當前大小；

ensureCapacity(int minCapacity) -- 如果需要擴充列表的容量以確保能夠裝下minCapacity個元素；

那如何動態監控整個列表的容量變化？

請參考如下代碼：

　　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();        }    }}

結果：

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

從代碼中可以看出：

ArrayList每次在追加item之後都在重新建立一個ArrayList對象，直到運用ensureCapacity方法。

雖然我們設定了最大長度為6，但是當追加的內容數目超出6之後，隨著size的增大，ArrayList的容量在變大，具體變大多少從本例中看到的是2，我也嘗試過追加內容，增大原則大致是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的容量後對ArrayList修改，下次再要進行容量計算時需要重新做Object Get。

Java Learning(5) -- ArrayList