What is the best way to determine the size of an object in Java?

Firstly “the size of an object” isn’t a well-defined concept in Java. You could mean the object itself, with just its members, the Object and all objects it refers to (the reference graph). You could mean the size in memory or the size on disk. And the JVM is allowed to optimise things like Strings.

So the only correct way is to ask the JVM

Serialization method will probably be a good approximation on most JVMs. The easiest way to do this is as follows:

 Serializable ser;
 ByteArrayOutputStream baos = new ByteArrayOutputStream();
 ObjectOutputStream oos = new ObjectOutputStream(baos);
 oos.writeObject(ser);
 oos.close();
 return baos.size();

Remember that if you have objects with common references this will not give the correct result, and size of serialization will not always match size in memory, but it is a good approximation. The code will be a bit more efficient if you initialise the ByteArrayOutputStream size to a sensible value.

Uday Ogra

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