How do I set a variable from a loop in to my array and then show the array in a different class?

Good evening, im struggeling again...
I want a loop from my first class to calculate the value x and then put the value x in my Array and then display the array in my main class. Whats my mistake? I would be so thankfull if any advise could solve this issue.

import java.util.Arrays;

public class Data {
	public double for1() {

		double x = 2;
		for (int i = 0; i < 8; i++) {
			x = x + x;
			
		}

		return x;
	}

	double[] arr = { x, 3, 4, 4, -5, 4, 6, 2 }; // x cannot be resolved to a variable
}
//-------------
import java.util.Arrays;

public class Javaapp {
	public static void main(String[] args) {

		Data a = new Data();
		a.for1();

		System.out.println(Arrays.toString(a.arr));
	}
}

One possibility would be to call the for1 function in your array assignment. If you do that, you wouldn't need to call it in the main routine:

import java.util.Arrays;

class Data {
    public double for1() {

        double x = 2;
        for (int i = 0; i < 8; i++) {
            x = x + x;
            
        }

        return x;
    }

    double[] arr = { for1(), 3, 4, 4, -5, 4, 6, 2 };
}

public class Main
{  
    public static void main(String args[])
    {
        Data a = new Data();

        System.out.println(Arrays.toString(a.arr));
    }
}

The issue is, the variable x is only accessible from within the method for1() . A solution for this would be to move the variable x outside of the method, or put xinto the array after the method is called.

what I would recommend is the code below, it will work as long as for1() is called first.

import java.util.Arrays;

class Data {
    public double for1() {
        double x = 2;
        for (int i = 0; i < 8; i++) {
            x = x + x;
            
        }
     //we set the first index in the array to our new x variable
        arr[0] = x;
        return x;
    }
   //                v  place holder value for now
    double[] arr = { 2, 3, 4, 4, -5, 4, 6, 2 };
}

public class Main {
    public static void main(String[] args) {

        Data a = new Data();
        a.for1();

        System.out.println(Arrays.toString(a.arr));
    }
}

The main problem here is that the variable x is not accessible to the arr array. This is because x only exists in the scope of the for1() function. The simplest ways to get around this are:

1. Make x a global variable

private double x = 2;
public void for1(){
    for (int i = 0; i < 8; i++) {
        x = x + x;        
    }
}
double[] arr = { x, 3, 4, 4, -5, 4, 6, 2 };

2. Make for1() return x like most people have already suggested

public double for1(){
    double x = 2;
    for (int i = 0; i < 8; i++) {
        x = x + x;        
    }
    return x;
}
double[] arr = { for1(), 3, 4, 4, -5, 4, 6, 2 };