I'm getting a nullpointerexception and I can't find why

Question

The program is supposed to make a non-deterministic automaton in the form of a tree holding information of both keys and the next pieces of automaton. The automaton is to find patterns on a text (more comlpicated than that, but that should suffice for my question as I haven't gotten to the next part yet).

I don't understand how but when I do a simple pattern for the automaton to build up ("AA" is my pattern) and I get that the "sgte"(next) is becoming null while the array's length (saved as N [kN]) is not 0. And I cannot find why :(

Here's the code:

public class AFND {

    private boolean estado; // true o false dependiendo de si es final o inicial respectivamente.
    private AFND sgte[];  //arreglo con todos los posibles estados siguientes
    private int N;  //cantidad de posibles estados siguientes
    private char key[]; //key[i] es el caracter con el que se accede a sgte[i] { '-' = e  }
    private int q; //denominador de estado
    private String alfa = "aaabcdefghijklmnopqrstuvwxyzABCDEFGHIJLKMNOPQRSTUVWXYZZZ"; //para ahorrarnos errores revisamos indexOf desde la posicion 2 donde sea necesario

    public AFND() {
        estado = true;
        sgte = null;
        N = 0;
        key = null;
        q = 0;
    }

    public AFND(int q) {
        estado = true;
        sgte = null;
        N = 0;
        key = null;
        this.q = q;
    }

    public AFND(String s) {
        if (check(s) == false) {
            U.println("Patrón Invalido.");
            System.exit(0);
        }
        AFND k = Construccion(s, 0);
        estado = k.estado;
        int i = 0;
        sgte = new AFND[k.N];
        key = new char[k.N];
        while (i < k.N) {
            sgte[i] = k.sgte[i];
            key[i] = k.key[i];
            i++;
        }
        N = k.N;
        q = k.q;
    }

    public AFND Construccion(String s, int l) {
        if (s.length() == 0) {
            return new AFND();
        }
        AFND k = new AFND(l);
        k.estado = false;
        k.q = l;
        if (s.charAt(0) == '[') {
            AFND sgte[] = new AFND[5];
            char key[] = new char[5];
            String h = s.substring(1, s.indexOf(']'));
            int i = 0;
            int j = 0;
            int x;
            String L[] = new String[5];
            while (i < 5) {
                L[i] = "";
                while (j < h.length()) {
                    x = alfa.substring(2).indexOf(h.charAt(j));
                    L[i] += alfa.charAt(x + i - 2);
                    j++;
                }
                j = 0;
                L[i] += s.substring(s.indexOf(']') + 1);
                sgte[i] = Construccion(L[i].substring(1), l);
                l++;
                key[i] = L[i].charAt(0);
                i++;
            }
            k.N = 5;
        } else {
            AFND sgte[] = new AFND[1];
            char key[] = new char[1];
            key[0] = s.charAt(0);
            if (s.length() > 1) {
                sgte[0] = Construccion(s.substring(1), l);
            } else {
                sgte[0] = new AFND(l);
            }
            k.N = 1;
            l++;
        }
        int o = 0;
        k.sgte = new AFND[k.N];
        k.key = new char[k.N];
        while (o < k.N) {
            k.sgte[o] = sgte[o];
            k.key[o] = key[o];
            o++;
        }
        return k;
    }

    public boolean estado() {
        return estado;
    }

    public AFND[] sgte() {
        return sgte;
    }

    public int ancho() {
        return N;
    }

    public char[] key() {
        return key;
    }

    public int num() {
        return q;
    }

    public boolean check(String s) {
        int i = 0;
        int j = 0;
        while (i < s.length()) {
            if (j == 0) {
                if (s.charAt(i) == '[') {
                    j = 1;
                } else if (s.charAt(i) == ']') {
                    return false;
                } else if (!Character.isLetter(s.charAt(i))) {
                    return false;
                }
            } else {
                if (s.charAt(i) == ']') {
                    j = 0;
                } else if (s.charAt(i) == '[') {
                    return false;
                } else if (!Character.isLetter(s.charAt(i))) {
                    return false;
                }
            }
            i++;
        }
        return true;
    }
}

And here's the running program:

import java.io.IOException;

public class Tarea2 {

static public void main(String[] args) throws IOException{
    String m=U.readLine("Ingresar Patrón: ");
    AFND patron=new AFND(m);
    U.println("AFND Patron Desplazado: ");
    U.println("");
    U.println("<!--Deus ex Machina-->");
    U.println("<structure>");
    U.println("<type>");
    U.println("fa");
    U.println("</type>");
    U.println("<automaton>");
    imprimirEstados(patron);
    imprimirTransiciones(patron);
    U.println("</automaton>");
    U.println("</structure>");
}

static public void imprimirEstados(AFND m){
    U.println("<state id="+m.num()+" name=q"+m.num()+">");
    U.println("<x>");
    U.println("0.0");
    U.println("</x>");
    U.println("<y>");
    U.println("0.0");
    U.println("</y>");
    U.println("</state>");
    int i=0;
    if(m.ancho()!=0){
        AFND[] s=m.sgte();
        while(i<m.ancho()){
            imprimirEstados(s[i]);
            i++;
            }
        }
    }

static public void imprimirTransiciones(AFND m){
    if(m.ancho()!=0){
        int i=0;
        while(i<m.ancho()){
            U.println("<transition>");
            U.println("<from>");
            U.println(m.num());
            U.println("</from>");
            U.println("<to>");
            U.println(m.sgte()[i].num());
            U.println("</to>");
            U.println("<read>");
            U.println(m.key()[i]);
            U.println("</read>");
            imprimirTransiciones(m.sgte()[i]);
            i++;
        }
    }
}

}

Please help :(

Here's the exception:

Exception in thread "main" java.lang.NullPointerException
at tarea2cs.AFND.Construccion(AFND.java:104)
at tarea2cs.AFND.Construccion(AFND.java:95)
at tarea2cs.AFND.<init>(AFND.java:48)
at tarea2cs.Tarea2.main(Tarea2.java:9)

The 104 is this part:

        while(o<k.N){ 
        k.sgte[o]=sgte[o];    <=
        k.key[o]=key[o];
        o++;
    }

I could just add the "if(sgte!=null)" but that would not solve the issue that it's becoming a null when it shouldnt :(

Answer 1

I think it is a shadowing problem. You have an instance variable sgte but in a couple of places you declare local variables with the same name; eg

   AFND sgte[] = new AFND[5];

This looks like a mistake ... and my guess it should be:

   sgte = new AFND[5];

(You make the same mistake in at least one other place.)

---

I should also comment that the code as written has a serious maintainability issue. Your pervasive use of one letter variable names and abbreviations (like sgte and AFND ) without any explanatory comments will make it hard for someone else to figure out what this application is about, let alone how it is supposed to work.

Answer 2

You've created an array but you've not created objects.

 sgte = new AFND[k.N];
 for(int i=0;i<k.N;i++){
   sgte[i]=new AFND();
 }

Answer 3

you only created sgte array but never initialized it.

private AFND sgte[];  //arreglo con todos los posibles estados siguientes


   public AFND(int q) {
    estado = true;
    sgte = null;
    N = 0;
    key = null;
    this.q = q;
}

and you are trying to get the element at 0 index here

   while (o < k.N) {
        k.sgte[o] = sgte[o];   //NPE
        k.key[o] = key[o];
        o++;
    }

and you are also creating a local variable with the same name(its not an issue anyway). but, to diffrentiate use this.sgte for instance variable.