Error when trying to use variable in different method

问题

After following the advice on my last question HERE , I ran into the problem with an exception: java.lang.ArrayIndexOutOfBoundsException

My program reads data from a file about (currently) two particles and their current position, velocity, and force (with an x,y, and z for all three attributes). Here is my file:

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 there are. Each particle has an ID, type, and position, velocity, and force.

What I am trying to do is to use the local variable of readfile() in main, and attempted to fix my previous problem.

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();
    Particle [] mainParticles = Particle.readfile();
    System.out.println("First:  [ " + mainParticles[0].particlePosition[0] + " , " + 0 + " ]");
    System.out.println("Second:  [ " + mainParticles[1].particlePosition[0] + " , " + 0 + " ]");

}

public static Particle[] readfile(){

        Particle [] listParticles = new Particle[count];

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

        count = readData.nextInt();

        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();
        return listParticles;
    }
}

回答1:

In your readFile() method, you are trying to create an array of size count before count has been read from the file, so it always creates an array of size 0 (Default value of type int)

Change below part

  Particle [] listParticles = new Particle[count];

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

    count = readData.nextInt();

to

    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];

In the 2nd code block variable count is set before creating array, and so it creates array of appropriate size

回答2:

Shot in the dark but an obvious error is how you're initializing the Particle[] array list. You should do this after you set count, otherwise the arraysize will be 0.

    count = readData.nextInt();
    Particle [] listParticles = new Particle[count];

来源：https://stackoverflow.com/questions/35120747/error-when-trying-to-use-variable-in-different-method