Java Generics Exception

Question

I have two classes, RandomObtainableList<E> and Driver. RandomObtainableList extends ArrayList and implements RandomObtainable (which contains only 2 abstract methods) and implements the methods.

However when I run the Driver class it says that there is a NullPointerException on the second line of the main method in the Diver class (Integer i = ...). I can't work out why it's throwing this exception as both classes look fine to me.

public class RandomObtainableList<E> extends ArrayList<E> implements RandomObtainable<E> {

    private Random random;

    public RandomObtainableList() {
        super();
        random = new Random(); 
    }

    @Override
    public E getRandom() throws NoSuchElementException {
        //returns an element randomly selected from the collection
        E e = this.get(random.nextInt(this.size()));
        return e; 
    }

    @Override
    public boolean removeRandom() throws UnsupportedOperationException {
        //try to remove a random element from the collection
        boolean b = false;
        int size = this.size();

        if (size != 0){
            E e = this.remove(random.nextInt(this.size()));
            b = true;
        }

        return b; 
    }
}

And Driver class

public class Driver {
    public static RandomObtainableList<Integer> list;

    public void Driver(){
        list = new RandomObtainableList<>();
        list.add(1);
        list.add(2);
    }

    public static void main(String[] args){
        Driver d = new Driver();
        Integer i = list.getRandom();
        boolean b = list.removeRandom();

        System.out.println("Element retrieved: " +i);
        System.out.println("Element removed: " +b);
    } 
}

Answer 1

Your Driver must initialize your list in it default constructor which is no defined so it as an empty default constructor :

public Driver() {
   // empty constructor
}

to have the expected result your Driver class would be :

public class Driver {
    private static RandomObtainableList<Integer> list;

    public Driver() {
        list = new RandomObtainableList<>();
        list.add(1);
        list.add(2);
    }

    public RandomObtainableList getList() {
        return this.list;
    } 
}

It's important to encapsulate the access to the properties of class with getters/setters. The properties will remain private and only accessible directly only from the class defining them.

Also your main entrypoint should be declared in a seperate class. Main for example

public class Main {

    public static void main(String[] args){
        Driver d = new Driver();
        Integer i = list.getRandom();
        boolean b = list.removeRandom();

        System.out.println("Element retrieved: " +i);
        System.out.println("Element removed: " +b);
    }
}