sizeof : Java Glossary

2008-06-24 by Roedy Green ©1996-2009 Canadian Mind Products
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sizeof

Java does not have a sizeof operator like that in C/C++. All primitive types have a standard size (or at least appear to have a standard size), so you don’t really need one. There are typically no pad or alignment bytes.

Java Object Size
type	size in bytes
overhead	~8 bytes/object
boolean	1
byte	1
char	2
short	2
int	4
long	8
float	4
double	8
reference	4/8†
String	length * 2 + 4

† addresses are 4 bytes in a 32-bit JVM and 8-bytes in a 64-bit JVM.

A JVM is free to store data any way it pleases internally, big or little endian, with any amount of padding or overhead, though primitives must behave as if they had the official sizes. For example, the JVM or native compiler might decide to store a boolean[] in 64-bit long chunks like a BitSet. It does not have to tell you, so long as the program gives the same answers. It might allocate some temporary Objects on the stack. It may optimise some variables or method calls totally out of existence replacing them with constants. It might version methods or loops, i.e. compile two versions of a method, each optimised for a certain situation, then decide up front which one to call. Then of course the hardware and OS have multilayer caches, on chip-cache, SRAM cache, DRAM cache, ordinary RAM working set and backing store on disk. Your data may be duplicated at every cache level. All this complexity means you can only very roughly predict RAM consumption.

There is a header on each object, typically 8 bytes for objects and 12 bytes for arrays, and 16 bytes in JET.

In JDK 1.5+ you can use java.lang.instrument.Instrumentation. getObjectSize()

Other methods you may find useful in estimating RAM use:

Minimum and Maximum Possible Primitive Values

Primitive variables include boolean, char, byte, short, int, long, float and double. Strings, arrays and Objects are not considered primitives.

Java Primitives
Type	Signed?	Bits	Bytes	Digits	Lowest	Highest	Mnemonic
boolean	n/a	1	1	1	false	true	zero/one
char	unsigned Unicode	16	2	4:5	'\u0000' [0] aka Character.MIN_VALUE	'\uffff' [2¹⁶-1] aka Character.MAX_VALUE	Unicode chars are twice as big as C’s.
byte	signed	8	1	2:3	-128 [-2⁷] aka Byte.MIN_VALUE	+127 [2⁷-1]aka Byte.MAX_VALUE	Bytes are signed, so half the usual 255 range.
short	signed	16	2	4:5	-32,768 [-2¹⁵] aka Short.MIN_VALUE	+32,767 [2¹⁵-1] aka Short.MAX_VALUE	32K
int	signed	32	4	9:10	-2,147,483,648 [-2³¹] aka Integer.MIN_VALUE	+2,147,483,647 [2³¹-1] aka Integer.MAX_VALUE	2 gig
long	signed	64	8	18:19	-9,223,372,036,854,775,808 [-2⁶³] aka Long. MIN_VALUE	+9,223,372,036,854,775,807 [2⁶³-1] aka Long. MAX_VALUE	9 exabytes, or 9 billion gig
float	signed exponent and mantissa	32	4	6:7	±1.40129846432481707e-45 aka Float.MIN_VALUE	±3.40282346638528860e+38 aka Float.MAX_VALUE or roughly ±2¹²⁷ with 6 to 7 significant digits of accuracy. A float can exactly represent integers in the range -2²⁴ to +2²⁴.	rough, compact float
double	signed exponent and mantissa	64	8	14:15	±4.94065645841246544e-324 aka Double.MIN_VALUE	±1.79769313486231570e+308 aka Double.MAX_VALUE or roughly ±2¹⁰²³ with 14 to 15 significant digits of accuracy. A double can exactly represent integers in the range -2⁵³ to +2⁵³.	high precision float

binary formats
power
primitive
println file format

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