Compare space used by three different options to store a matrix of 1000x1000 double values in Java

OccupancyComparer.java

package demo;

import java.util.ArrayList;

public class OccupancyComparer {
    static int  S = 1000;

    static double[][] array_of_double;
    static Double[][] array_of_double_obj;
    static ArrayList<Double>    array_list_of_double_obj;
    
    interface NullaryFunction {
        void doIt();
    }
    
    public static void howMuchSpaceUsed(String useCase, NullaryFunction fct) {
        Runtime rt = Runtime.getRuntime();
        long usedMemBefore = rt.totalMemory() - rt.freeMemory();
        
        fct.doIt();
        
        long usedMB = (rt.totalMemory() - rt.freeMemory() - usedMemBefore) / 1024 / 1024;
        System.err.println(useCase + " memory usage " + usedMB + " MB");
    }
        
    public static void main(String[] args) {
        howMuchSpaceUsed("array_of_double", () -> {
            array_of_double = new double[S][S];
        });
        howMuchSpaceUsed("array_of_double_obj", () -> {
            array_of_double_obj = new Double[S][S];
            for(int i = 0; i < S; ++i)
                for(int j = 0; j < S; ++j)
                    array_of_double_obj[i][j] = new Double(0.0);
        });
        howMuchSpaceUsed("array_list_of_double_obj", () -> {
            array_list_of_double_obj = new ArrayList<>(S*S);
            for(int i = 0; i < S*S; ++i)
                array_list_of_double_obj.add(new Double(0.0));
        });
    }

}