How do I implement abstraction and generics in java?

Question

I currently have a project that requires the use of abstraction and generics, but I don't even know where I should start for this. The abstract class is as follows.

public abstract class Links<AType> {
    abstract AType getElem(); //returns the head of the list
    abstract Links<AType> getNext(); //return the next link
}

This is the class that extends the abstract class

public class Cons<AType> extends Links<AType> {
    AType elem;
    Links<AType> next;

    Cons(AType elem, Links<AType> next) {
        this.elem = elem;
        this.next = next;
    }

    @Override
    AType getElem() {
        return elem;
    }

    @Override
    Links<AType> getNext() {
        return next;
    }
}

Here's another class that extends the abstract class

public class Nil<AType> extends Links<AType> {
    Nil(){}

    @Override
    AType getElem() {
        return null;
    }

    @Override
    Links<AType> getNext() {
        return null;
    }
}

And here is the class that is supposed to implement everything

public class LList<AType> {
    Links<AType> list;
    LList() {
        list = new Cons<>();
    }

    Links<AType> getList() {
        return list;
    }

    AType get(int n, AType a) {
        Cons<AType> aTypeCons = new Cons<>(a, list);
        return null;
    }

    void add(AType elem) {
        //add to head of list
    }

    void remove(int i) {
        //remove ith element
        //do nothing if i is invalid
    }

    void print() {
        //prints the list
    }


}

I just need some help figuring out where to actually start in making the LList class. I can't figure out the constructor because Links is abstract, so I can't make that an object, and I can't make a new Cons<> because there are no elements that are passed into the constructor. However, the constructor is supposed to instantiate a new list. I also can't figure out how I'm supposed to be able to access an individual element of that list. If I can just have a bit of understanding of what needs to happen in the constructor, I should be able to figure out how to implement the rest of the methods.

Answer 1

Your LList is a singly-linked list, where each element has a value and a link to the list that follows. The final element of the list will always be a Nil object, which represents an empty list. When you first initialize an empty list, you can just assign list = new Nil<>(); . When you add an element to the list, you can reassign it as list = new Cons<>(elem, list); .

To access an an element in the list by index, just use a while loop that calls getNext() until it's either reached the desired index or found the end of the list.

AType get(int n) {
    Links<AType> current = list;
    while (n > 0 && current instanceof Cons) {
        current = current.getNext();
        n--;
    }
    return current.getElem();
}