接收空指针异常

Question

I have a class that sets up an array. Another class extends this class and uses the mentioned array in a method to return another array. I am receiving a null exception when I do this however when I set up the original array, it works just fine. Not sure what I'm doing wrong here. Still pretty new at this.

The class that sets up the array (part of it):

public class InitialGrids { 

int iic,ioc,iman ;
int nic, noc;
double masIC, masOC, masMAN;
double volIC, volOC, volMAN, volP;
double radIC, radOC, radP;

public int iMat[ ];
public double rad[ ];
public double vol[ ];
public double mas[ ];
public double dmas[ ];

public double [ ] radius(int nr)
{
    rad = new double [nr +1];

        for( int k = 0 ; k <= nic ; k++ )
        {
            rad[ k ] = radIC * ((double) k / nic);
            System.out.println("the radius at shell " + k + " is " + rad[k]);

        }

        for( int k = nic + 1 ; k <= noc ; k++ )
        {       
            rad[ k ] = radIC + ( (radOC - radIC) * ((double) (k - nic) / (noc - nic)) );
            System.out.println("the radius at shell " + k + " is " + rad[k]);

        }

        for( int k = noc + 1 ; k <= nr ; k++ )
        {
            rad[ k ] = radOC + ( (radP - radOC) * ((double) (k - noc) / (nr - noc)) );
            System.out.println("the radius at shell " + k + " is " + rad[k]);

        }   

        return(rad);
}

This is the class that extends InitialGrids(part):

public class Compression extends InitialGrids {

double G;
double PI;
double Pave;
double x, z, dy, dz, dydx; // this is for the iteration that will update the density
double delta;
double mix;

public double grav[ ];
public double P[ ];
public double rhon[ ];
public double radn[ ];

public double [ ] gravity( int nr ) 
{
    G = Constants.PI;
    PI = Constants.PI;
    grav = new double [ nr + 1 ];

    for(int k = 1; k <= nr; k++)
    {
        grav[ 0 ] = 0;
        grav[ k ] = ( (G*mas[k]) / (Math.pow(rad[k], 2)) ); 
    }
    return(grav);
}

I am calling these in the main class like:

    InitialGrids ig = new InitialGrids();
ig.setup(icMat, ocMat, manMat, nr, masP, fic, foc, fman, fSi, fSi, fO);
 ig.material(nr);
 ig.radius(nr);
 ig.density(nr,fSi, fSi, foc);
 ig.volume(nr);
 ig.dmass(nr);
 ig.mass(nr);

   Compression com = new Compression();
   com.gravity(nr);
   com.pressure(nr);
   com.newDensity(nr);
   com.radn(nr);

Answer 1

"mas[]" is not initialized anywhere in your code. Unless you are initializing it elsewhere, that's your issue.

Answer 2

So, you're breaking nearly every rule here. For one thing, the field rad is not initialized until you call a method radius, so newly created instances of InitialGrids are invalid until this method is called.

So your null pointer is happening because you create an instance of InitialGrids , ig , and call radius , and then a separate instance of Compression and somehow expect the rad in that instance to be related to the one in the ig instance? I don't think so.

Why are these classes in an inheritance relationship anyway?

Answer 3

Most likely, instance variable mas and rad should be initialized. May be you need to call like this.

Compression com = new Compression();
com.setup(icMat, ocMat, manMat, nr, masP, fic, foc, fman, fSi, fSi, fO);
com.material(nr);
com.radius(nr);
com.gravity(nr);

The instance named "com" and "ig" is totally different.