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I just can't get my code right how I want to. (Java Beginner)

So the way it should work is this:
When running the program I want to get a window and fill in a number(X) how large(Z) the array should be. Next I get X windows where I have to type in the numbers I want to put in the array. Finally the program should calculate the sum of all numbers and divide by the size(Z) of the array, to get the average of all numbers in the array.

I'm almost finished but all I get are errors regarding Double to String conversion. I tried everything I know out but couldn't make it work. Please excuse there may be spelling mistakes, I had to translate it to english.

Error:
Below //Initialization and Output line, either it's 'void' type not allowed here or incompatible types: void cannot be converted to String

import javax.swing.*;
import java.util.*;

public class Aufgabe42

 public static void main(String[] args)
 
 //Declare variable
 String input, output, requestNumber;
 double[] arrayNumber;
 int size;

 //Input
 input = JOptionPane.showInputDialog(null, "Type in the size of the array.");

 //Variable Initialization
 size = Integer.parseInt(input);
 arrayNumbers = new double[size];

 //Request numbers
 for(int i = 0; i < size; i++)
 
 requestNumber = JOptionPane.showInputDialog(null, "Tell me a number:");
 arrayNumber[i] = Double.parseDouble(requestNumber);
 

 String returnAverage;

 //Initialization and Output
 output = Double.toString(JOptionPane.showMessageDialog(null, "The Average is: " + getAverage(arrayNumber)));
 System.exit(0);
 

 public static double getAverage(double[] arrayNumber)
 
 //Declare variable

 double arraySum,average;

 //Initialization
 arraySum = 0;



 returnAverage = Double.toString(average);

 //Sum array values
 for(int i = 0; i < arrayZNumber.length; i++)
 
 arraySum += arrayNumber[i];
 

 //Calculate Average
 average = arraySum / arrayNumber.length;

 return average;
 

JOptionPane.showMessageDialog()

Has a void return type. You either want

JOptionPane.showInputDialog()

Or to remove

Double output = Double.valueOf(JOptionPane.showMessageDialog...)

near the end

https://docs.oracle.com/javase/7/docs/api/javax/swing/JOptionPane.html