StackAsList.java

/**
 * This class takes a Stack and converts it to a Linked List.
 * 
 * @author	robinwettstaedt
 * @author	n-c0de-r
 * @version	21-06-03 update 21-06-07
 */

//Assignment 2
public class StackAsList implements Stack{
	
    private Node first;
    
    /**
     * constructor for the stack as a linked list.
     */
    public StackAsList() {
    }
    
    class Node {
        String data;
        Node next;

        //Constructor of the Node class
        public Node() {
        }

    }

    /**
     * Insert an element in the beginning of the list,
     * as the top-most element of a stack.
     * @return The String just stored in the Node.
     */
    @Override
    public String push(String element) {

    	Node node = new Node();
    	node.data = element;
    	node.next = first;
    	first 	  = node;
    	return element;
    }

    /**
     * Gets the first/ top-most element of the list,
     * and removes it from the order and it's linking,
     * @return String stored in the Node's data field.
     */
    @Override
    public String pop() {
    	if (!isEmpty()) {
    		String s = first.data;
    		first	 = first.next;
    		return s;
    	} else {
    		return null;
    	}
    }
    
    /**
     * Just look into the data field of the top-most Node.
     * * @return String stored in the Node's data field.
     */
	@Override
    public String peek() {
		if (isEmpty())
			return null;
		else 
			return first.data;
    }
    
	/**
	 * Check if the list/stack is empty.
	 * @return Boolean, true if there are no Nodes in the list.
	 */
    @Override
    public boolean isEmpty() {
        if (first == null)
            return true;
        return false;
    }

    //not sure why we would need this.
    @Override
    public String toString() {
	String s = "";
        if (isEmpty()) {
            return s;
        }

        Node current = first;

        while (true) {
            s = s + "[ " + current.data + " ] ";
            if (current.next == null) {
                break;
            }
            current = current.next;
        }
        return first.data;
    }
}