We know that elementData is private data member of ArraryList class, but here we are getting value of that data member runtime in our program, here we used elementData member variable to see change in capacity of ArrayList, capacity of ArrayList is by default 10, if we will try to store more data than capacity then capacity will increase by 50%.
import java.lang.reflect.Field;
import java.util.ArrayList;
public class PrivateDataMembersValue {
static Field
field;
public static
void main(String[] args) {
ArrayList<Integer>
arrList = new ArrayList<Integer>();
try {
field
= ArrayList.class.getDeclaredField("elementData");
field.setAccessible(true);
}
catch (NoSuchFieldException e) {
e.printStackTrace();
}
catch (SecurityException e) {
e.printStackTrace();
}
for(int
i=0; i<=11;i++)
{
arrList.add(i);
}
Object[]
elementData;
try {
elementData
= (Object[]) field.get(arrList);
System.out.println("Actual
Size of array list "+arrList.size());
System.out.println("Capacity
of elementData---A internal array of ArrayList: " + elementData.length );
}
catch (IllegalArgumentException e) {
e.printStackTrace();
}
catch (IllegalAccessException e) {
e.printStackTrace();
}
}
}
Output:-
Actual Size of array list 12
Capacity of elementData---A internal array of ArrayList 15
Similarly use private methods using below code
Method m=ArrayList.class.getDeclaredMethod("size");
//m.invoke(d); //exception
java.lang.IllegalAccessException
m.setAccessible(true);
m.invoke(d); //now
its ok
This terminology is called as "Reflection".
