NullPointerException

[geenie]

i`m developind my second program using java which is supposed to solve a system of ordinary differential equations(S'=-beta*S*I-alfa*I;I'=beta*S*I+alfa*I) using runge kutta method;my first program was supposde to solve a single eq and worked just great(after a initial nullpointer exception error ,which reading some things on the internet i managed to eliminate).now i get the same error and i figure it has something to do with dereferencing ,but no matter how hard i tried i haven`t solved the problem;
the code source looks like that(actually it`s now the final implementation of my algorithm,i`m just trying to test some things):
-------------------------------------------------------------------------------------------------------------------

import java.io.*;
import java.util.ArrayList;
 
 
 
class SolutiiDeTe{
 
double timp;
double[] solutie;
 
public static double[] inmultireVectorCuScalar(double a, double[] b){
 
    double[] x=new double[b.length];
    for(int i=0;i<b.length;i++)
        x[i]=b[i]*a;
    return x;
}
 
public static double[] AdunareVectori(double[] a, double[] b){
 
    double[] x=new double[b.length];
    for(int i=0;i<b.length;i++)
        x[i]=b[i]+a[i];
    return x;
}    
 
 
 
public static double[] definireSIS(double[] y,double xi,double alfa,double beta){
 
    xi=(double)0;
    double[] dy=new double[2];
    dy[0]=-beta*y[0]*y[1]+alfa*y[1];
    dy[1]=beta*y[0]*y[1]-alfa*y[1];
    return dy;
}
 
//public double[] definireSIRS(double[] y,double alfa,double beta){
//    double[] dy=new double[3];
//    dy[0]=-beta*y[0]*y[1];
//    dy[1]=beta*y[0]*y[1]-alfa*y[1];
//    dy[2]=alfa*y[1];
//    return dy;
//}
 
public static SolutiiDeTe[] RK_SIS(int an1,int an2,int N,double pas,double a,double b,int inf) {
 
double[] k1,k2,k3,k4,k;
 
int nr_pasi=(int)((an2-an1)/pas);
 
SolutiiDeTe[] y=new SolutiiDeTe[nr_pasi+1];
 
for (int i=0;i<nr_pasi;i++)
    {y[i]=new SolutiiDeTe();
    y[i].solutie=new double[2];
    }
 
double[] dy=new double[nr_pasi+1];
 
 
y[0].timp=an1;
y[0].solutie[0]=(double)(N-inf);
y[0].solutie[1]=(double)inf;
 
for (int i=0;i<nr_pasi;i++){
    k1=new double[2];
    k2=new double[2];
    k3=new double[2];
    k4=new double[2];
    k=new double[2];
    k1=SolutiiDeTe.inmultireVectorCuScalar(pas,SolutiiDeTe.definireSIS(y[i].solutie,y[i].timp,a,b));
 
    k=SolutiiDeTe.inmultireVectorCuScalar(0.5,k1);
    k=SolutiiDeTe.AdunareVectori(y[i].solutie,k);
 
    k2=SolutiiDeTe.inmultireVectorCuScalar(pas,SolutiiDeTe.definireSIS(k,y[i].timp+0.5*pas,a,b));
 
    k=SolutiiDeTe.inmultireVectorCuScalar(0.5,k2);
    k=SolutiiDeTe.AdunareVectori(y[i].solutie,k);
    k3=SolutiiDeTe.inmultireVectorCuScalar(pas,SolutiiDeTe.definireSIS(k,y[i].timp+0.5*pas,a,b));
    k=SolutiiDeTe.AdunareVectori(y[i].solutie,k3);
    k4=SolutiiDeTe.inmultireVectorCuScalar(pas,SolutiiDeTe.definireSIS(k,y[i].timp+pas,a,b));
    k=SolutiiDeTe.inmultireVectorCuScalar(2,k2);
    dy=SolutiiDeTe.AdunareVectori(k1,k);
    k=SolutiiDeTe.inmultireVectorCuScalar(2,k2);    
    dy=SolutiiDeTe.AdunareVectori(dy,k);
    dy=SolutiiDeTe.AdunareVectori(dy,k4);
    dy=SolutiiDeTe.inmultireVectorCuScalar(1/6,dy);
 
 
    y[i+1].solutie=SolutiiDeTe.AdunareVectori(dy,y[i].solutie);   //line 92
 
    y[i+1].timp=((double)(Math.round((y[0].timp+pas*(i+1))*10)))/10;
}
 
return y;}
 
public static void main(String[] args) {
 
SolutiiDeTe[] x=new SolutiiDeTe[21];
 
for (int i=0;i<21;i++)
 
    {x[i]=new SolutiiDeTe();
     x[i].solutie=new double[2];     
    }
 
 
 
x=SolutiiDeTe.RK_SIS(1,3,50,0.1,0.2,0.1,2);  //line 111
for (int i=0;i<21;i++)
    System.out.println(x[i].timp+" "+x[i].solutie);
 
}
}

---------------------------------------------------------------------------------------------------------------

the exact error i get is

Exception in thread "main" java.lang.NullPointerException
at SolutiiDeTe.RK_SIS(SolutiiDeTe.java:92)
at SolutiiDeTe.main(SolutiiDeTe.java:111)
---------------------------------------------------------------------------------------------------------------
i applologise for how my code looks like(i copied from jcreator but i doesn`t look in here how i expected to);i`ve marked marked the lines 92 and 111 for you
i would extremely apreciate any help;looking forward for reply;
thank you

[blazedaces]

First of all, use [ code ] brackets [ /code ] (without the spaces) to show code the way you want (and to show line numbers), like this:

import java.io.*;
import java.util.ArrayList;
 
 
 
class SolutiiDeTe{
 
double timp;
double[] solutie;
 
public static double[] inmultireVectorCuScalar(double a, double[] b){
 
    double[] x=new double[b.length];
    for(int i=0;i<b.length;i++)
        x[i]=b[i]*a;
    return x;
}
 
public static double[] AdunareVectori(double[] a, double[] b){
 
    double[] x=new double[b.length];
    for(int i=0;i<b.length;i++)
        x[i]=b[i]+a[i];
    return x;
}    
 
 
 
public static double[] definireSIS(double[] y,double xi,double alfa,double beta){
 
    xi=(double)0;
    double[] dy=new double[2];
    dy[0]=-beta*y[0]*y[1]+alfa*y[1];
    dy[1]=beta*y[0]*y[1]-alfa*y[1];
    return dy;
}
 
//public double[] definireSIRS(double[] y,double alfa,double beta){
//    double[] dy=new double[3];
//    dy[0]=-beta*y[0]*y[1];
//    dy[1]=beta*y[0]*y[1]-alfa*y[1];
//    dy[2]=alfa*y[1];
//    return dy;
//}
 
public static SolutiiDeTe[] RK_SIS(int an1,int an2,int N,double pas,double a,double b,int inf) {
 
double[] k1,k2,k3,k4,k;
 
int nr_pasi=(int)((an2-an1)/pas);
 
SolutiiDeTe[] y=new SolutiiDeTe[nr_pasi+1];
 
for (int i=0;i<nr_pasi;i++)
    {y[i]=new SolutiiDeTe();
    y[i].solutie=new double[2];
    }
 
double[] dy=new double[nr_pasi+1];
 
 
y[0].timp=an1;
y[0].solutie[0]=(double)(N-inf);
y[0].solutie[1]=(double)inf;
 
for (int i=0;i<nr_pasi;i++){
    k1=new double[2];
    k2=new double[2];
    k3=new double[2];
    k4=new double[2];
    k=new double[2];
    k1=SolutiiDeTe.inmultireVectorCuScalar(pas,SolutiiDeTe.definireSIS(y[i].solutie,y[i].timp,a,b));
 
    k=SolutiiDeTe.inmultireVectorCuScalar(0.5,k1);
    k=SolutiiDeTe.AdunareVectori(y[i].solutie,k);
 
    k2=SolutiiDeTe.inmultireVectorCuScalar(pas,SolutiiDeTe.definireSIS(k,y[i].timp+0.5*pas,a,b));
 
    k=SolutiiDeTe.inmultireVectorCuScalar(0.5,k2);
    k=SolutiiDeTe.AdunareVectori(y[i].solutie,k);
    k3=SolutiiDeTe.inmultireVectorCuScalar(pas,SolutiiDeTe.definireSIS(k,y[i].timp+0.5*pas,a,b));
    k=SolutiiDeTe.AdunareVectori(y[i].solutie,k3);
    k4=SolutiiDeTe.inmultireVectorCuScalar(pas,SolutiiDeTe.definireSIS(k,y[i].timp+pas,a,b));
    k=SolutiiDeTe.inmultireVectorCuScalar(2,k2);
    dy=SolutiiDeTe.AdunareVectori(k1,k);
    k=SolutiiDeTe.inmultireVectorCuScalar(2,k2);    
    dy=SolutiiDeTe.AdunareVectori(dy,k);
    dy=SolutiiDeTe.AdunareVectori(dy,k4);
    dy=SolutiiDeTe.inmultireVectorCuScalar(1/6,dy);
 
 
    y[i+1].solutie=SolutiiDeTe.AdunareVectori(dy,y[i].solutie);   //line 92
 
    y[i+1].timp=((double)(Math.round((y[0].timp+pas*(i+1))*10)))/10;
}
 
return y;}
 
public static void main(String[] args) {
 
SolutiiDeTe[] x=new SolutiiDeTe[21];
 
for (int i=0;i<21;i++)
 
    {x[i]=new SolutiiDeTe();
     x[i].solutie=new double[2];     
    }
 
 
 
x=SolutiiDeTe.RK_SIS(1,3,50,0.1,0.2,0.1,2);  //line 111
for (int i=0;i<21;i++)
    System.out.println(x[i].timp+" "+x[i].solutie);
 
}
}

Now, NullPointerException happens basically when you try to do something on an object that does exist at that point in time. For example, Object objectname; (the object still doesn't exist) until you do objectname = new Object();

I remember also in C++ whenever I divided by 0 it would give me a similar problem. Did this information help?

-blazed

P.S.

Ok, this might be something:

take a look at the method called at line 92:

public static double[] AdunareVectori(double[] a, double[] b){
 
    double[] x=new double[b.length];
    for(int i=0;i<b.length;i++)
        x[i]=b[i]+a[i];
    return x;
}    
 

If a[i] has fewer elements then b[i] (you should test for this in the code anyway) then perhaps at some point a[i] does not exist anymore. Although, I don't believe that would be a nullPointerException, it would be arrayIndexOutOfBounds.

[geenie]

i am sure it`s not about the fact that those 2 arrays would have different lenghts; and,yeah i`ve read a thousand times that an object doesn`t exist unless you make exist by using " new something" but i still don`t understand what i should in my case;what i would like to know is wheather the error has to something with this:

------------------------------------------------------------------------------------------------------------------
class SolutiiDeTe{

double timp;
double[] solutie;

//code lines that don`t make my interest right now//

public static SolutiiDeTe[] RK_SIS(int an1,int an2,int N,double pas,double a,double b,int inf) {

//other lines//

SolutiiDeTe[] y=new SolutiiDeTe[nr_pasi+1];

for (int i=0;i<nr_pasi;i++)
{y[i]=new SolutiiDeTe();
y[i].solutie=new double[2];
}
//other lines

y[i+1].solutie=SolutiiDeTe.AdunareVectori(dy,y[i].solutie); //the problem line

---------------------------------------------------------------------------------------------------------------
doesn`t y[i].solutie exist?
and how exactly should i call the j-th element of the y[i].solutie array ;all the ways i`ve tried give errors;
sorry for bothering you too much

[r035198x]

i am sure it`s not about the fact that those 2 arrays would have different lenghts; and,yeah i`ve read a thousand times that an object doesn`t exist unless you make exist by using " new something" but i still don`t understand what i should in my case;what i would like to know is wheather the error has to something with this:

------------------------------------------------------------------------------------------------------------------
class SolutiiDeTe{

double timp;
double[] solutie;

//code lines that don`t make my interest right now//

public static SolutiiDeTe[] RK_SIS(int an1,int an2,int N,double pas,double a,double b,int inf) {

//other lines//

SolutiiDeTe[] y=new SolutiiDeTe[nr_pasi+1];

for (int i=0;i<nr_pasi;i++)
{y[i]=new SolutiiDeTe();
y[i].solutie=new double[2];
}
//other lines

y[i+1].solutie=SolutiiDeTe.AdunareVectori(dy,y[i].solutie); //the problem line

---------------------------------------------------------------------------------------------------------------
doesn`t y[i].solutie exist?
and how exactly should i call the j-th element of the y[i].solutie array ;all the ways i`ve tried give errors;
sorry for bothering you too much

which i are you refering to outside the for in

 
y[i+1].solutie=SolutiiDeTe.AdunareVectori(dy,y[i].solutie); //the problem 
 

Oh and please use code tags if you have to post code.

[blazedaces]

I think I may have found your problem!!! It's exactly what you were complaining about, you missed one "= new SolutiiDete();"

See, look at just these two for loops down here (again, use code brackets next time):

SolutiiDeTe[] y=new SolutiiDeTe[nr_pasi+1];
 
for (int i=0;i<nr_pasi;i++)
    {y[i]=new SolutiiDeTe();
    y[i].solutie=new double[2];
    }
 
// Not important lines...
 
for (int i=0;i<nr_pasi;i++){
    //Not important
 
    y[i+1].solutie=SolutiiDeTe.AdunareVectori(dy,y[i].solutie);   //line 92
 
    y[i+1].timp=((double)(Math.round((y[0].timp+pas*(i+1))*10)))/10;
}
 
 
}
}

See, both your for loops are i < nr_pasi but when you declare all the "new new SolutiiDete();" stuff it's all y[i]. Yet, in the next for loop you do this:

y[i+1].solutie=SolutiiDeTe.AdunareVectori(dy,y[i].solutie);

So you get to y[i+1]. Try changing your first for loop to i<nr_pasi+1 and see if that fixes the problem. I'm pretty sure it should...

Good luck,
-blazed

[geenie]

blazedaces ,you were quite right; i in the second loop should have values between 0 and (nr_pasi-1);now i got another problem,which i suppose has to do exactly with what i have asked in my second post :about the j-th element of the y[i].solutie array;
what i get is:
Error in thread "main" NullPointerException
at SolutiiDeTe.main(SolutiiDeTe.java:115)
meaning here

    System.out.println(x[i].timp+" "+x[i].solutie);

[blazedaces]

blazedaces ,you were quite right; i in the second loop should have values between 0 and (nr_pasi-1);now i got another problem,which i suppose has to do exactly with what i have asked in my second post :about the j-th element of the y[i].solutie array;
what i get is:
Error in thread "main" NullPointerException
at SolutiiDeTe.main(SolutiiDeTe.java:115)
meaning here

    System.out.println(x[i].timp+" "+x[i].solutie);

I may not be understanding what the problem is, but if I'm right this is the solution to your problem:

for (int i=0;i<21;i++)
        for(int j = 0; j < 2; j++) {
            System.out.println(x[i].timp+" "+x[i].solutie[j]);
        }
 
}
 

Tell me if this helped or if I was off-track,
-blazed

[geenie]

during the time between my last post and yours j-th managed to call the j-th element of x[i].solutie using the "clone" method but i was still getting the NullPointerException,that after printed me the (length(x[i].solutie)-1)th element and i figured it out;you were not quite right but absolutely right :in the first loop i should take values between 0 and nr_pasi(meaning i should have right

for (int i=0;i<nr_pasi+1;i++)

and not as did with

for (int i=0;i<nr_pasi;i++)

in the first loop and

for (int i=0;i<nr_pasi-1;i++)

three times thank you;i own you