How do I code and compile in the areas that say "IMPLEMENT ME"?

import java.util.Scanner;

import java.util.Stack;

class EvalException extends Exception{ public EvalException(String msg) { super(msg); }

interface Expr {

   public int eval() throws EvalException;

}

class Atom implements Expr {

  String value;

  public Atom(String value) { this.value = value; }

  public String toString() { return value; }

  public int eval() throws EvalException {

  /* IMPLEMENT ME
   * We'll assume Atoms represent integers when we eval them, so try to
   * parse the value as an int.  If this can't be done, we'll want to throw
   * an EvalException with the proper error message.
   */
  }

}

class List<E> implements Expr {

  private Node<E> head;

  public List(E... elements) {
     head = null;

     for(int i = elements.length - 1; i >= 0; i--) {
        head = new Node<E>(elements[i], head);
     }
  }

  public void append(E element) {
     if (head == null) {
        head = new Node<E>(element, null);
     }
     else {
        Node<E> last = head;
        while (last.link != null) last = last.link;
        last.link = new Node<E>(element, null);
     }
  }

  public String toString() {
     String s = "( ";
     Node<E> elements = head;
     while (elements != null) {
        s += elements.data + " ";
        elements = elements.link;
     }
     return s + ")";
  }

  public void evalAndPrintEach() {
     Node<Expr> current = (Node<Expr>) head;
     while (current != null) {
        try {
           System.out.println( ((Expr) current.data).eval() );
        }
        catch (EvalException e) {
           System.out.println(e);
        }
        current = current.link;
     }
  }

  private Node<Integer> evalArgs(Node<Expr> xs) throws EvalException {
     if (xs == null) return null;
        return new Node<Integer>(xs.data.eval(), evalArgs(xs.link));
  }

  public int eval() throws EvalException {
     Node<Expr> elts = (Node<Expr>) head;
     if (elts == null) throw new EvalException("Empty list.");
        String op = ((Atom) elts.data).value;

     /* IMPLEMENT ME
      * To evaluate a (non-empty) List<Expr>, we look at the first element
      * (which we assume is an Atom), evaluate each of the remaining elements
      * (using evalArgs), then apply the appropriate operator to the results.
      * See the examples in the handout.
      *
      * Attempting to evaluate an empty list results in an error.  If op isn't
      * one of "+", "-", "*", "/", then this too is an error.
      */
  }

  private int add(Node<Integer> xs) throws EvalException {
     /* IMPLEMENT ME
      * Find the sum of xs using a right fold.
      */
  }

  private int mul(Node<Integer> xs) throws EvalException {
     /* IMPLEMENT ME
      * Find the product of xs using a left fold.
      */
  }

  private int sub(Node<Integer> xs) throws EvalException {
     /* IMPLEMENT ME
      * Calculate the result by using a tail-recursive helper method.
      */
  }

  private int div(Node<Integer> xs) throws EvalException {
     /* IMPLEMENT ME
      * Calculate the result iteratively.  If xs doesn't have at least 2
      * elements, throw an Exception.
      */
  }

  private class Node<E> {
     E data;
     Node<E> link;

     public Node (E data, Node<E> link) {
        this.data = data;
        this.link = link;
     }
  }

}

class UnclosedListException extends Exception { }

class Lisp {

  public static List<Expr> parse(String[] tokens) throws Exception {
     Stack<List<Expr>> stack = new Stack<List<Expr>>();

     stack.push(new List<Expr>());

     for (String token : tokens) {
        if (token.equals("(")) {
           stack.push(new List<Expr>());
        }

        else if (token.equals(")")) {
           List<Expr> toAppend = stack.pop();
           stack.peek().append(toAppend);
        }

        else {
           stack.peek().append(new Atom(token));
        }
     }

     if (stack.size() > 1)
        throw new UnclosedListException();

     return stack.pop();
  }

  public static void main(String[] args) throws Exception {
     Scanner in = new Scanner(System.in);

     while(true) {
        System.out.print("lisp> ");
        String line = in.nextLine();
        try {
           parse(line.split("\\s+")).evalAndPrintEach();
        }
        catch (Exception e) {
           System.out.println("Parsing error.");
        }
     }
  }

}

How do I code and compile in the areas that say "IMPLEMENT ME"?