Comparison of two battery elements

Good, I'm doing a job for my university, this consists among other things in inserting caracateres in 3 batteries, 1 of them has a sequence which we want to find in the other 2 batteries.

The problem is that in the method Comparar that I will leave next I miss an error that also has to do with the method pop of the stack, I will leave the code and the error that jumps to see if I could help.

(the code is not complete, I only have the comparison of the first stack with the sequence stack).

Class of the stack:

public class Pila {
    char elementos[];
    int tope;

    public Pila(int n) {
        elementos= new char[n];
        tope = -1;
    }



    public boolean empty(){
        if(tope == -1){
            return true;
        }else{
            return false;
        }
    }

    public boolean full(){
        if(elementos.length-1 == tope){
            return true;
        }else{
            return false;
        }
    }

    public void push(char a){
        tope++;
        elementos[tope]=a;
    }

    char aux;
    public char pop(){
       aux= elementos[tope];
       tope--;
       return aux;
    }

    public char peek(){
        return elementos[tope];
    }


    public void mostrar(){
       for(int i=0;i<elementos.length;i++){
           System.out.print(elementos[i]+", ");
       }

    }

}

/*metodos de comparacion, copia(ya que tengo que comparar con el pop y no quiero que se e elimine la pila original) y  el main*/

public class Tarea2 {

    //cmetodod para comparar 
    public static boolean Comparar(Pila x,Pila primera){
        // se copian las pilas a comparar
        Pila copia1=copia(x); // pila con secuencia
        Pila copiaprimera= copia(primera);// pila a comparar con la secuencia



       for(int i =0;i<=x.tope;i++){
           copia1.elementos[i]=x.elementos[i];
       }
        for(int i =0;i<=x.tope;i++){
           copiaprimera.elementos[i]=primera.elementos[i];
       }
        int contador=0;

        char c= copia1.pop();
        char b= copiaprimera.pop();

        System.out.println(c);
        // a continuacion se comparan los caracteres de la pila
        while(contador < 3){
        if(c==b){
             c=copia1.pop();
             b=copiaprimera.pop();
             contador++;
         }else{
            b=copiaprimera.pop();
        }
        }
        if(contador==3){
            return true;
        }
        return false;
    }



    public static Pila copia(Pila original){
        Pila copia = new Pila(original.elementos.length);

        for(int i=0; i<copia.elementos.length;i++){
            copia.elementos[i]=original.elementos[i];
        }
        return copia;
    }
    public static void main(String[] args) {

        Scanner teclado = new Scanner(System.in);

        System.out.println("Ingrese una serie de caracteres para la primera pila: ");
        String x = teclado.nextLine();

        Pila primera = new Pila(x.length());

        for( int i=0; i<x.length();i++){
            primera.elementos[i]= x.charAt(i);
        }

        primera.mostrar();
        System.out.println();

        System.out.println("Ingrese una serie de caracteres para la segunda pila: ");
        String y = teclado.nextLine();

        Pila segunda = new Pila(y.length());

        for( int i=0; i<y.length();i++){
            segunda.elementos[i]= y.charAt(i);
        }

        segunda.mostrar();
        System.out.println();

        System.out.println("Ingrese una serie de caracteres para la pila secuencia: ");
        String z = teclado.nextLine();

        Pila secuencia = new Pila(z.length());

        for( int i=0; i<z.length();i++){
            secuencia.elementos[i]= z.charAt(i);
        }

        secuencia.mostrar();
        System.out.println();
        // llama a a la funcion para comparar
        System.out.println(Comparar(secuencia,primera));


    }

}

This is the error you sent me:

  

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException:   -1 at tarea2.Pila.pop (Pila.java:46) at tarea2.Tarea2.Comparar (Tarea2.java:32) at   task2.Tarea2.main (Task2.java:102)   C: \ Users \ User \ AppData \ Local \ NetBeans \ Cache \ 8.2 \ executor-snippets \ run.xml: 53:   Java returned: 1 BUILD FAILED (total time: 22 seconds)