Question

I am newly learning java.I am trying to understand the various data structures in util package.

/*create an ArrayList object*/
List<String> arrayList = new ArrayList<String>();

In the decleration of ArrayList class I have seen

public class ArrayList
extends AbstractList
implements List, RandomAccess, Cloneable, Serializable

My question is if I use an object of type List<> to store the object returned by constructor of ArrayList<>

will it work.You may assume that I need to use only the methods specified in the List interface,such as add() or size().

I have tried this on my machine and it does not work.I found this in the java tutorial below

http://www.javamex.com/tutorials/collections/using_1.shtml

The complete example code that I tried is as below.

package arraylist;
import java.util.ArrayList;
import java.util.List;

public class ArrayListExample {

        public static void main(String[] args)
        {

        /*create an ArrayList object*/
         List<String> arrayList = new ArrayList<String>();

                /*
                Add elements to Arraylist using
                boolean add(Object o) method. It returns true as a general behavior
                of Collection.add method. The specified object is appended at the end
                of the ArrayList.
                 */
        arrayList.add("1");
        arrayList.add("2");
        arrayList.add("3");

        /*
        Use get method of Java ArrayList class to display elements of ArrayList.
        Object get(int index) returns and element at the specified index in
        the ArrayList
         */
        System.out.println("Getting elements of ArrayList");
        System.out.println(arrayList.get(0));
        System.out.println(arrayList.get(1));
        System.out.println(arrayList.get(2));
    }
}

I forgot to import List.that is why it did not work.However my next question is we use a variable of type List to store on object of type ArrayList.

Comparing the methods of List and ArrayList ArrayList has a method called trimToSize() that is not there in list.

In the above case will I be able to call that method?

If yes,what that generally means is that a base class pointer can be used to store a derived class object as the method list of derived class will always be a super-set of base class methods?

Is my above conclusion correct? It my sound stupid question but I am very new to java.

Answer 1

Since ArrayList implements the List interface you should be able to use the object declared as a List that is initialized to an ArrayList. This is pretty common practice. It allows you to switch the implementation of the list without code changes.

Make sure your importing the list interface:

package arraylist;
import java.util.ArrayList;
import java.util.List;

public class ArrayListExample {

        public static void main(String[] args)
        {

        /*create an ArrayList object*/
         List<String> arrayList = new ArrayList<String>();

                /*
                Add elements to Arraylist using
                boolean add(Object o) method. It returns true as a general behavior
                of Collection.add method. The specified object is appended at the end
                of the ArrayList.
                 */
        arrayList.add("1");
        arrayList.add("2");
        arrayList.add("3");

        /*
        Use get method of Java ArrayList class to display elements of ArrayList.
        Object get(int index) returns and element at the specified index in
        the ArrayList
         */
        System.out.println("Getting elements of ArrayList");
        System.out.println(arrayList.get(0));
        System.out.println(arrayList.get(1));
        System.out.println(arrayList.get(2));
    }
}

When referencing the List you will not be able to use the trimToSize function since it is not on the interface for List