Memory Usage in Java

[Dallman, John]

Hi, Kenneth,

> From my C/C++ days I learned that the actual amount of RAM used was related to the word size of a CPU
> so that when a byte is read you are really reading a word. This meant to me that if I had 10 variables of
> type byte, not in an array, it would consume 80 bytes of RAM.

I'm afraid you are confused about that, too. There have been computers that would do that, but they have been obsolete for decades. I'm pretty sure Java has never run on a "word addressed" machine.

On any remotely modern processor architecture, ten byte-sized variables will only occupy ten bytes of space. The way they get fetched into the processor is somewhat complicated but ensures that several of them will normally be fetched in one read.

With best regards,

--
John Dallman (he/him)

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From: discuss <discuss-retn at openjdk.org> On Behalf Of Shipilev, Aleksey
Sent: 08 August 2025 17:35
To: Kenneth Fogel <kfogel at dawsoncollege.qc.ca>; discuss at openjdk.org
Subject: Re: Memory Usage in Java

Hi Kenneth,

The answers to most of these questions are, "No, this is not how it works in any modern JVM".

I would really recommend playing with JOL, especially look through its Samples:
https://github.com/openjdk/jol
https://github.com/openjdk/jol/tree/master/jol-samples/src/main/java/org/openjdk/jol/samples

And maybe read this for even more discussion:
https://shipilev.net/jvm/objects-inside-out/

Thanks,
-Aleksey

From: discuss <discuss-retn at openjdk.org<mailto:discuss-retn at openjdk.org>> on behalf of Kenneth Fogel <kfogel at dawsoncollege.qc.ca<mailto:kfogel at dawsoncollege.qc.ca>>
Date: Friday, 8. August 2025 at 18:11
To: "discuss at openjdk.org<mailto:discuss at openjdk.org>" <discuss at openjdk.org<mailto:discuss at openjdk.org>>
Subject: [EXTERNAL] Memory Usage in Java


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Please let me know where I might ask this question or find the answer if this is not the right place.

In a book I am working on I am writing about how Java deals with memory allocation for the primitive types on a 64 bit CPU. I know the size of primitive variable types such as byte is 1, int is 4, etc. and that the size is related to the range of allowable values. From my C/C++ days I learned that the actual amount of RAM used was related to the word size of a CPU so that when a byte is read you are really reading a word. This meant to me that if I had 10 variables of type byte, not in an array, it would consume 80 bytes of RAM. From my reading it appears to be the same in Java except for arrays. Is this accurate? For example, an int is 4 bytes but when Java runs on a 64 bit CPU is it really reading and writing 8 bytes? If it is, can we say that the 4 bytes of padding is RAM dead space that cannot be used for anything else?

Thank you for your time,

Ken Fogel





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