Who decides data types size in Java

Question

Who decides the size of data types such as int in Java? JVM or OS or Processor?

int size is 4 bytes..Will it be always 4 bytes irrespective of OS or processor?

Answer 1

The Java Language Specification decides them. They're the same size on all VMs, on all OSes, on all processors. If they're not, it's not Java anymore.

Answer 2

It is the JVM specification that drives JVM implementations to decide the size of data-types. Refer http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.3

Answer 3

While the Java spec decides how many bits each type actually uses, the default 32 bit JVM actually pads some types in memory, using 32 bits of space to store values, even ones that don't need that much space. They still all behave (regarding the program), as if they took up their real amount of storage, but the amount of space used is still much larger.

Other JVMs can do this differently, for instance they could store an array of booleans with only one bit per value, rather than 32 bits per value.

So while the size of an int will always appear to be 32 bits, because it will always wrap around at 32 bits and can only hold 32 bit values, the JVM does have the option of internally storing it using 64 bits of space, or some other number depending on the hardware and implementation.

Also, floating point values are actually much less restricted in this regard - while the spec for float and double does indeed require they use a particular format, the presence of the two different libraries java.lang.Math and java.lang.StrictMath (see What's the difference between java.lang.Math and java.lang.StrictMath? for an explanation) is a clear example. Using java.lang.Math does not require that the intermediate calculations for those functions be stored in any particular way, or that it use a particular number of bits to compute everything.