|Java Object Size|
|type||size in bytes|
|String||length * 2 + 4|
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 (Operating System) have multilayer caches, on chip-cache, SRAM (Static Random Access Memory) cache, DRAM (Dynamic RAM) cache, ordinary RAM (Random Access Memory) 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 Java version 1.5 or later you can use java.lang.instrument. Instrumentation. getObjectSize()
Other methods you may find useful in estimating RAM use:
|char||unsigned Unicode||16||2||4:5||'\u0000'  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 aka -2 gig, roughly -2 billion||+2,147,483,647 [231-1] aka Integer. MAX_VALUE. aka 2 gig, roughly 2 billion||2 gig|
|long||signed||64||8||1:19||-9,223,372,036,854,775,807 [-263] aka Long. MIN_VALUE about -9×1018||9,223,372,036,854,775,808 [+263-1] aka Long. MAX_VALUE about 9×1018||9 exabytes, or 9 billion gig|
|float||signed exponent and mantissa||32||4||7||±1.40129846432481707e-45 aka Float. MIN_VALUE||±3.40282346638528860e+38 aka Float.
or roughly ±2127
with about 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||16||±4.94065645841246544e-324 aka Double. MIN_VALUE||±1.79769313486231570e+308 aka Double.
or roughly ±21023
with 15 significant digits of accuracy, almost 16 with 15.95 significant digits.
A double can exactly represent integers
in the range -253 to +253.
|high precision float|
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