README.md

Shallow size

Jol$Patient1 object internals:
OFF  SZ                  TYPE DESCRIPTION                VALUE
  0   8                       (object header: mark)      N/A
  8   4                       (object header: class)     N/A
 12   4                   int Patient1.patientID         N/A
 16   1               boolean Patient1.exitusIndicator   N/A
 17   3                       (alignment/padding gap)
 20   4      java.lang.String Patient1.givenName         N/A
 24   4   java.time.LocalDate Patient1.birthDate         N/A
 28   4                       (object alignment gap)
Instance size: 32 bytes
Space losses: 3 bytes internal + 4 bytes external = 7 bytes total

Jol$Patient2 object internals:
OFF  SZ                  TYPE DESCRIPTION                  VALUE
  0   8                       (object header: mark)        N/A
  8   4                       (object header: class)       N/A
 12   4                   int Patient2.patientID           N/A
 16   2                  char Patient2.administrativeSex   N/A
 18   1               boolean Patient2.exitusIndicator     N/A
 19   1                       (alignment/padding gap)
 20   4      java.lang.String Patient2.givenName           N/A
 24   4   java.time.LocalDate Patient2.birthDate           N/A
 28   4                       (object alignment gap)
Instance size: 32 bytes
Space losses: 1 bytes internal + 4 bytes external = 5 bytes total

We can see there is some padding applied, so adding or removing fields may not increase or decrease object shallow size.

Deep size

Jol$Patient1@d6da883d footprint:
     COUNT       AVG       SUM   DESCRIPTION
         1        32        32   Jol$Patient1
         1        32        32   [B
         1        24        24   java.lang.String
         1        24        24   java.time.LocalDate
         4                 112   (total)


Jol$Patient1@61d47554d footprint:
     COUNT       AVG       SUM   DESCRIPTION
         1        32        32   Jol$Patient1
         1        40        40   [B
         1        24        24   java.lang.String
         3                  96   (total)

references set to Null do not increase deep size

JolPoc.java

///usr/bin/env jbang "$0" "$@" ; exit $?

//DEPS org.openjdk.jol:jol-core:0.16

import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.info.GraphLayout;

public class JolTest {
    
    static class Patient1{
        int patientID;
        String givenName;
        boolean exitusIndicator;
        java.time.LocalDate birthDate;
    }

    static class Patient2{
        int patientID;
        String givenName;
        boolean exitusIndicator;
        java.time.LocalDate birthDate;
        char administrativeSex;
    }

    public static void main(String... args) {
        System.out.println(ClassLayout.parseClass(Patient1.class).toPrintable());
        System.out.println(ClassLayout.parseClass(Patient2.class).toPrintable());

        Patient1 p1 = new Patient1();
        p1.givenName = "Art Vandelay";
        p1.birthDate = java.time.LocalDate.now();
        System.out.println(GraphLayout.parseInstance(p1).toFootprint());

        Patient1 p2 = new Patient1();
        p2.givenName = "My friend Bob Sacamano";
        System.out.println(GraphLayout.parseInstance(p2).toFootprint());

    }

}