Using variable from another method in Java

Question

I want to calculate the position of two particles that will be constantly reacting between each other. My problem is that I need to bring my particle listParticle array from my readData() to my main(String[] args) method.

I have tried making the array of particles a global variable, but it always forces me to make it static and then my code does not work.

My program reads from a file, with the data as such:

2 

1 1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

2 1 1.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

The first number is count, how many particles are in the file. Each particle has an ID, type, and x, y, and z for position, velocity, and force.

Here is my code:

import java.io.File;
import java.io.IOException;
import java.util.*;
import java.io.FileReader;
import java.io.FileNotFoundException;

public class Particle 
{
    int particleID;
    int particleType;
    double particlePosition[] = new double[3];
    double particleVelocity[] = new double[3];
    double particleForce[] = new double[3];
    static int count;
    static int current = 0;
    static Scanner readData;

    public static void main(String[] args) 
    {
        int k = 100; // in [N/m] or [kg*m/s^2]
        int m = 1; // in [kg]
        double x0 = 1; // in [m]
        double t;  // in [s]
        double dt;  //  in [s]
        double oldForce1;
        double oldForce2;
        double curTime = 0;
        double finTime;

        t = 1/((1/(2*(Math.PI))) * Math.sqrt(2*k/m));
        System.out.println(t);

        dt = t/150;

        readfile();

        //System.out.println("First:  [ " + listParticles[0].particlePosition[0] + " , " + 0 + " ]"); <-- does not work
        //System.out.println("Second:  [ " + listParticles[1].particlePosition[0] + " , " + 0 + " ]"); <-- does not work
    }

    public static void readfile()
    {
        try
        {
            readData = new Scanner(new File("src/2particle-initial.data"));
        }
        catch(Exception e)
        {
            System.out.println("Could not find file");
        }
        count = readData.nextInt();

        Particle [] listParticles = new Particle[count];

        while (current < count)
        {
            listParticles[current] = new Particle();
            listParticles[current].particleID = readData.nextInt();
            listParticles[current].particleType = readData.nextInt();

            listParticles[current].particlePosition[0] = readData.nextDouble();
            listParticles[current].particlePosition[1] = readData.nextDouble();
            listParticles[current].particlePosition[2] = readData.nextDouble();

            listParticles[current].particleVelocity[0] = readData.nextDouble();
            listParticles[current].particleVelocity[1] = readData.nextDouble();
            listParticles[current].particleVelocity[2] = readData.nextDouble();

            listParticles[current].particleForce[0] = readData.nextDouble();
            listParticles[current].particleForce[1] = readData.nextDouble();
            listParticles[current].particleForce[2] = readData.nextDouble();

            current++;
        }
        current = 0;

        System.out.println("First:  [ " + listParticles[0].particlePosition[0] + " , " + 0 + " ]");
        System.out.println("Second:  [ " + listParticles[1].particlePosition[0] + " , " + 0 + " ]");

        readData.close();
    }
}

How can I send my local readData() listParticle array to my main?

Each particle has an ID, type, and x, y, and z for position, velocity, and force. That is seven variables, but I see 11 variables in your file? — Riley Carney
– Riley Carney, Commented Jan 31, 2016 at 19:23
So declare listParticles as a data member of the class, then expose a getter method. — OldProgrammer
– OldProgrammer, Commented Jan 31, 2016 at 19:27
@Riley Carney the x y z are for each position velocity and force, not just position — StealthVice7
– StealthVice7, Commented Jan 31, 2016 at 20:51

Idos · Accepted Answer · 2016-01-31 19:23:56Z

2

You can define your method to return the listParticle array. Like so:

Change its signature to:

public static Particle[] readfile()

And add a return statement after the close:

return listParticles;

In your main you can call this function and assign its return to a variable:

myParticleArray = Particle.readfile();

answered Jan 31, 2016 at 19:23

Idos

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3 Comments

StealthVice7 Over a year ago

Doesn't work for me when I change the signature of the method, I get an exception: java.lang.ArrayIndexOutOfBoundsException: 0 I added the return and the main call.

Idos Over a year ago

This is a whole other error you should handle... Maybe consider posting a new question with that complete error and stack trace. Send a link to it and I'll try and help

StealthVice7 Over a year ago

Thank you for helping out, here is my new question stackoverflow.com/questions/35120747/…

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