What is the size of an object in java

Question

As we all know, java uses the following data types

byte Occupy 8 bits in memory short Occupy 16 bits in memory int Occupy 32 bits in memory long Occupy 64 bits in memory 

If I create a class like

 class Demo{ byte b; int i; long l; } Demo obj = new Demo(); 

Now my question is: obj size is < or > or = size b+i+l , which is 104 bytes . Please give me an explanation for a reasonable reason.

Thanks,
Anil Kumar C

Answer 1

The size of the memory in memory depends on the architecture, mainly on whether the virtual machine is 32 or 64-bit. The actual implementation of VM also matters.

For each object, you need space for the object header (usually 2 * 8 bytes on 64-bit virtual machines), its fields (additional space for alignment depending on the implementation of the virtual machine). The final space is rounded to the nearest multiple word size.

Answer 2

From http://www.javamex.com/tutorials/memory/object_memory_usage.shtml

• The bare object takes up 8 bytes;
• an instance of a class with one Boolean field takes 16 bytes: 8 bytes of the header, 1 byte for Boolean and 7 bytes of "padding", so that the size is a multiple of up to 8;
• an instance with eight Boolean fields will also occupy 16 bytes: 8 for the header, 8 for Boolean; since this is already a few of 8, a gasket is not required;
• an object with two long fields, three int fields and a boolean value:
  • 8 bytes for the header;
  • 16 bytes for two lengths (8 each);
  • 12 bytes for 3 integers (4 pieces);
  • 1 byte for boolean;
  • 3 more bytes of padding to round the total number from 37 to 40, a multiple of 8.

Answer 3

It’s hard to say that it will be the size of obj in memory, the font size indicator will help developers, but in reality it is slightly different in memory. I advise you to read this article , it is really interesting.

Answer 4

First you confuse bits and bytes.

Secondly, he will also need a pointer to "vtable", where information about his class is stored. This will most likely be 4 bytes (32 bits) on 32-bit systems and 8 bytes on 64-bit systems.

Finally, note that due to memory fragmentation, the total memory of the program may be higher than the sum of all objects.

Answer 5

An object header can accept 8 bytes in a 32-bit JVM and 12 bytes in a 32-bit JVM.

Each primitive takes a number of bits (without specifying a byte)

The distribution of objects is 8 bytes, so there should be up to 7 bytes at the end of the object. that is, the actual used space is rounded to the next multiple of 8.

 class Demo{ // 8 or 12 bytes byte b; // 1 byte int i; // 4 bytes long l; // 8 bytes } Demo obj = new Demo(); 

Thus, the size of an object can take 24 bytes on a 32-bit JVM and 32 bytes on a 64-bit JVM.

Answer 6

I think this answer will make you deeply understand the size of the object.

What determines the size of a Java object?