Pila de Nodos

Interfaz a Utilizar:

package Pilas_de_Nodos;

/**
 * @author Daniel Cataño Restrepo <dasniel199410@gmail.com>
 */
public interface INodeStack<E>{
    public boolean isEmpty();
    public E peek();
    public E pop();
    public void print();
    public void push(E target);
}

La clase NodeStack

package Pilas_de_Nodos;

/**
 * @author Daniel Cataño Restrepo <dasniel199410@gmail.com>
 */
public class NodeStack<E> implements INodeStack<E>{
    
    private NodeStack<E> next;
    private E item;
    private NodeStack<E> raiz;
    
    public NodeStack(E item){
        this.item=item;
    }
    
    public NodeStack(NodeStack<E> next){
        this.next=next;
    }
    
    public NodeStack(E item, NodeStack<E> next){
        this.item=item;
        this.next=next;
    }
    
    public NodeStack<E> getNext() {
        return next;
    }

    public void setNext(NodeStack<E> next) {
        this.next = next;
    }

    public E getItem() {
        return item;
    }

    public void setItem(E item) {
        this.item = item;
    }

    public NodeStack getRaiz() {
        return raiz;
    }

    public void setRaiz(NodeStack raiz) {
        this.raiz = raiz;
    }
    
    public boolean isEmpty() {
        return raiz==null;
    }

    public E peek() {
        E firstElement=raiz.item;
        if(!isEmpty()){
            if(raiz.next==null)
                raiz=null;
            else
                raiz=raiz.next;
            return firstElement;
        }
        return null;
    }

    public E pop() {
        return raiz.item;
    }

    public void print() {
        System.out.print("[");       
        for(NodeStack temp=raiz; temp!=null; temp=temp.next){
            if(temp.next==null)
                System.out.print(temp.item);
            else
                System.out.print(temp.item+",");
        }
        System.out.println("]");
    }

    public void push(E target) {
        NodeStack<E> nuevo=new NodeStack<>(target);
        if(isEmpty()){
            nuevo.next=null;
            raiz=nuevo;
        }else{
            nuevo.next=raiz;
            raiz=nuevo;
        }       
    }   
}

La clase principal

package Pilas_de_Nodos;
/**
 * @author Daniel Cataño Restrepo <dasniel199410@gmail.com>
 */
public class Main {

    public static void main(String[] args) {
        NodeStack pila=new NodeStack(1);
        pila.push(1);
        pila.print();
        pila.push(2);
        pila.print();
        pila.push(3);
        pila.print();
        pila.push(4);
        pila.print();
        pila.push(5);
        pila.print();
        pila.push(6);
        pila.print();
        pila.peek();
        pila.print();
        pila.peek();
        pila.print();
        pila.peek();
        pila.print();
        pila.peek();
        pila.print();
        pila.peek();
        pila.print();
        System.out.println(pila.pop());
    }
}

Lo Que imprime en pantalla: