SimpleSinglyLinkedList.java --- Implementing singly linked link-list in java

This shows three programs.

• A simple singly-linked list. This shows the basics.
• A doubly-linked list. This is almost as simple as the singly-linked list, but makes some operations easier.
• Use of the java.util.LinkedList class, which is easy to use because it hides the details.

import java.util.Scanner;

public class SimpleSinglyLinkedList {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        
        Elem front = null;    // First element of list.
        Elem back  = null;    // Last element of list.
        
        //... Read a list of words.
        while (in.hasNext()) {
            String word = in.next();
            
            Elem e = new Elem();     // Create a new list element.
            e.data = word;           // Set the data field.
            
            //... Two cases must be handled differently
            if (front == null) {
                //... When the list is empty, we have to set the front pointer.
                front = e;            // Back element will be set below.
            } else {
                //... When we already have elements, we need to link to it.
                back.next = e;       // Link last elem to new element.
            }
            back = e;                // Update back to link to new element.
        }
        
        //... While loop to print list in forward order.
        System.out.println("*** Print words in order of entry");
        Elem curr = front;
        while (curr != null) {
            System.out.println(curr.data);
            curr = curr.next;
        }
        
        System.out.println("*** Print words in order of entry");
        for (Elem e = front; e != null; e = e.next) {
            System.out.println(e.data);
        }
        
        //... Printing list in backward order is an interesting exercise.
        //    But too much for here.
    }
}

////////////////////////////////////////////////////////////////////////// Elem
// Simple class to hold data are sometimes defined with public fields.
// This practice isn't good, but was done here for simplicity.
class Elem {
    public Elem next;    // Link to next element in the list.
    public String data;  // Reference to the data.
}