Why using a class as a variable member inside that said class

Question

I am new to Java and been studying it. So I am having trouble to understand what are these and how it being processed by Java also why are we declaring these variables like that. I mean can you educate me on this?

Public abstract class ListItem {
    protected Listitem leftLink = null;
    protected Listitem rightLink = null;
    protected Object value;

    some code here
}

Thanks in advance!

Answer 1

Why declare a Class field which has the ClassName as variable type instead of a int, string... ?

Because the developer needs to. Sometimes, an instance of a class must reference another instance of the same class. A typical example is the LinkedList.

Consider a linked list as a sequence of nodes. Each node knows the next one to be linked. Here would be a naive implementation of a node:

class Node<T> {
    private Node<T> next;
    private T value;

    Node(T value) {
        this.value = value;
    }

    public T getValue() {
        return value;
    }

    void setNext(Node<T> next) {
        this.next = next;
    }
}

As you can see, the class Node contains a variable member of type Node, to reference the next element of the linked list. Finally, a simplistic implementation of the linked list would be:

class LinkedList<T> {

    private Node<T> first;
    private Node<T> last;
    private int length = 0;

    public void add(T value) {
        Node<T> node = new Node<T>(value);

        if(length != 0) {
            last.setNext(node);
            last = node;
        }
        else {
            first = node;
            last = node;
        }

        length++;
    }

    public Node<T> getFirst() {
        return first;
    }
}

When a new node is added to the collection, the previous last node references it and therefore, becomes the new last node.