How to know the size of an object in memory?

If your object is Serializable , you can use java.io.ObjectOutputStream associated with java.io.ByteArrayOutputStream to measure the size of your object when it is serialized:

class Tamanho {
    public static int tamanho(Serializable obj) {
        try (
            java.io.ByteArrayOutputStream baos = new java.io.ByteArrayOutputStream();
            java.io.ObjectOutputStream oos = new java.io.ObjectOutputStream(baos)
        ) {
            oos.writeObject(obj);
            oos.flush();
            return baos.toByteArray().length;
        } catch (java.io.IOException ex) {
            // ignore a IOException que é levantada quando se fecha o oos
    }
}

Here I am using the "try-with-resources" construction of Java 7, which closes the streams for me. If you are using Java 6 or earlier, you have to transform this construction into a common try fault, but since Java 7 is already very old, here's the answer ...

If you use eclipse, this plugin might be useful link . It gives you a complete report and even lets you find out if some memory leak is occurring at some point in your application.