How to get amount of serialized bytes representing a Java object?

Question

What syntax would I use to get the number of bytes representing a string and compare them to the number of bytes representing an ArrayList holding that string, for

Answer 1

You can convert your object into a byte array using ObjectOutputStream and ByteArrayOutputStream:

public static int sizeof(Object obj) throws IOException {

    ByteArrayOutputStream byteOutputStream = new ByteArrayOutputStream();
    ObjectOutputStream objectOutputStream = new ObjectOutputStream(byteOutputStream);

    objectOutputStream.writeObject(obj);
    objectOutputStream.flush();
    objectOutputStream.close();

    return byteOutputStream.toByteArray().length;
}

I just tested this out. The object who's size you're trying to calculate, needs to implement Serializable (which means you may have to mark every object as such simply to get its size. Might not be desirable). I wrote a quick and dirty program to test this out:

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class Sizeof {

    public static class Person implements Serializable {
        private String name;
        private String age;

        public Person(String name, String age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getAge() {
            return age;
        }

        public void setAge(String age) {
            this.age = age;
        }
    }

    public static void main(String[] args) {
        Person p1 = new Person("Alby", "20");
        Person p2 = new Person("VeryLongName", "100");
        String s1 = "This is it";
        String s2 = "This";

        try {
            System.out.println("p1 " + sizeof(p1));
            System.out.println("p2 " + sizeof(p2));
            System.out.println("s1 " + sizeof(s1));
            System.out.println("s2 " + sizeof(s2));                                 
        }

        catch(Exception e) {
            e.printStackTrace();
        }
    }

    public static int sizeof(Object obj) throws IOException {

        ByteArrayOutputStream byteOutputStream = new ByteArrayOutputStream();
        ObjectOutputStream objectOutputStream = new ObjectOutputStream(byteOutputStream);

        objectOutputStream.writeObject(obj);
        objectOutputStream.flush();
        objectOutputStream.close();

        return byteOutputStream.toByteArray().length;
    }
}

Which gave me:

p1 85
p2 94
s1 17
s2 11

EDIT

Stephen C's answer highlights some caveats with this method.