Functional list in Java - working proof of concept

f_list.java

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

interface List {
  boolean is_empty();
  int head();
  List tail();
}

class Cons implements List {
  
  int head;
  List rest;
  
  Cons (int _head, List _tail) {
  	head = _head;
  	rest = _tail;
  }
  
  public boolean is_empty() {
  	return false;
  }
  
  public int head() {
  	return this.head;
  }
  
  public List tail() {
  	return this.rest;
  }
  
  public String toString() {
  	if (this.tail().toString() == "()") return String.valueOf(this.head()) + ")";
  	else {
  		return "(" + String.valueOf(this.head()) + " " + this.tail().toString();
  	}
  }
  
}

class Nil implements List{
	private static Nil single = null;
	
	public boolean is_empty() {
		return true;
	}
	
	public int head() {
		throw new IndexOutOfBoundsException();
	}
	
	public List tail() {
		throw new IndexOutOfBoundsException();
	}
	
	public String toString() {
		return "()";
	}
	
	
	public static Nil Nil() {
		if (single == null) {
			single = new Nil();
		}
		return single;
	}
}

class Main {
	static int length(List xs) {
		if (xs.is_empty()) return 0;
		else return 1 + length(xs.tail());
	}
	public static void main (String [] args) {
			List list = new Cons(1, new Cons(2, new Nil()));
			System.out.println(list);
			System.out.println(length(list));
	}
}