09. Recursive Classes - Linked List

We can use recursive class initialisation to define a linked list:

class ListNode {
	int value;
	ListNode next;
}

We can then create two constructors:

class ListNode {
	int value;
	ListNode next;
	
	public ListNode(int value) {
		this.value = value;
		// This means next is null as ListNode 
	}
	
	public ListNode(int value, ListNode next) {
		this(value); // Calls the other constructor ListNode(int)
		this.next = next;
	}
}

We then have recursive initialisation:

ListNode last = new ListNode(30);
ListNode middle = new ListNode(20, last);
ListNode first = new ListNode(10, middle);
 
// Is equivalent to
 
ListNode first = new ListNode(10, new ListNode(20, new ListNode(30)));

Operations on LL

We have LinkedList LL = ....

To insert at the beginning: list = new LinkedList(10, list); to insert 10 at the beginning.

To insert at the end: list.next.next...next = new ListNode(10); appends at the back.

Nested Classes

We can nest the class definitions to make the ListNode unavailable outside of the LinkedList implementation.

public class LinkedList {
	private class ListNode {
		private int value;
		private ListNode next;
	}
	
	private ListNode front;
	
	public LinkedList() {
		front = null;
	}
}

We are not allowed to initialise an object of type ListNode in a static method of LinkedList!