What is the amount of memory required to store a single byte primitive in an ArrayList?

To store a single primitive byte in an ArrayList, you need:

1. Byte Object: Since ArrayList stores objects, the primitive byte will be boxed into a java.lang.Byte object. The size of this object in Java depends on the JVM, but typically includes:

   • Object header (Object Header): Contains metadata about the object (class, lock flags, hash code), usually 8 or 12 bytes (for 64-bit JVM with compressed oops) or 16 bytes.
   • Value field: One byte field (1 byte).
   • Additional padding for alignment: Objects are aligned to 8-byte boundaries. The size of Byte (header + 1 byte) will be padded to the nearest multiple of 8.

   Roughly, the size of a Byte object can be about 16 bytes (12 bytes header + 1 byte value + 3 bytes padding to reach 16).

2. ArrayList element: ArrayList uses an internal Object[] array to store elements. The array holds references to Byte objects. The size of each reference depends on the JVM: 4 bytes (32-bit JVM or 64-bit with compressed oops) or 8 bytes (64-bit JVM without compressed oops).

In total, the memory needed to store a single primitive byte in an ArrayList is the sum of the memory occupied by the Byte object and the memory occupied by the reference to this object in the internal ArrayList array.

On a 64-bit JVM with compressed oops, this is approximately:

• Size of Byte object: ~16 bytes
• Size of reference in the internal array: 4 bytes

Total size: ~16 bytes (Byte object) + 4 bytes (reference) = ~20 bytes.

Note that this is an estimate. The exact size may vary depending on the specific JVM implementation. Also, overheads of the ArrayList object itself (distributed across all elements) and the internal array are not included.

For more efficient storage of byte primitives with less overhead, it is better to use specialized collections like byte[] or libraries such as Trove (TByteArrayList).