Reference Materials

Java Data Type Size Chart

1. Primitive Data Types

Data Type	Size (bits)	Size (bytes)	Default Value	Range	Notes
byte	8	1	0	-128 to 127	Smallest integer type
short	16	2	0	-32,768 to 32,767	Compact integer
int	32	4	0	-2^31 to 2^31-1 (~±2.14B)	Common for integers
long	64	8	0L	-2^63 to 2^63-1	Use L suffix
float	32	4	0.0f	±3.4E+38 (~7 digit precision)	Use f suffix
double	64	8	0.0d	±1.7E+308 (~15 digit precision)	Default for decimals
char	16	2	\u0000	0 to 65,535	Represents Unicode characters
boolean	1 (JVM dependent)	JVM-defined (1 byte min)	false	true or false	Actual size varies (not directly defined in spec)

JVM may pad primitives and objects for alignment — actual memory usage may exceed declared size.

2. Reference Types (Object Overhead)

Component	64-bit JVM (compressed oops)	Notes
Object header	12 bytes	Includes mark word & class pointer
Object reference	4 bytes (compressed), 8 bytes (uncompressed)	Pointers to other objects
Array header	16 bytes (object header + length)	All arrays are objects
Alignment padding	Align to 8-byte boundary	JVM aligns for performance

3. Common Object Sizes (Estimated)

Object Type	Approx Size (bytes)	Comments
Integer	~16 bytes	Object wrapping a primitive int
Long	~24 bytes	Larger due to 64-bit value
String (empty)	~40 bytes	Includes char array reference
UUID	~32 bytes	2 long values internally
ArrayList (empty)	~24 bytes	Plus internal array (default size 10)

Tips for Memory-Efficient Code

• Prefer primitives (int, long) over wrappers (Integer, Long) in large collections.

• Use record instead of class for data holders (Java 14+).

• Use final where possible to help the JVM with optimization.

• Avoid deep object graphs in caches — flatten where feasible.

• Use off-heap caching (e.g., Redis, Ehcache) for large datasets.