Java/Memory

Rules of Thumb

Memory usage and space in Java

Primitive types:

• boolean - 1 byte
• byte - 1 byte
• char - 2 bytes
• int - 4 bytes
• float - 4 bytes
• long - 8 bytes
• double - 8 bytes

An integer has 2^32 different values, with 32 bits or 4 bytes (since there are 8 bits per byte)

Objects:

• Object overhead is 16 bytes of object overhead for each object
• References to objects typically use 8 bytes of memory.
• Data types can contain references to objects. The data type automatically incurs 16 bytes overhead for each object instance, plus 8 more bytes for each object's reference to another object.

Arrays:

• Arrays are implemented in Java as objects, with two instance variables - a pointer to the memory location of the first array element, and the length of the array.
• Primitive types - arrays use 24 bytes of header information, plus n times number of bytes to store each element

Example: int array

• int = 4 bytes
• int[] = 4n + 24 ~ 4n bytes

Example: double array 1D

• double = 8 bytes
• double[] = 8n + 24 ~ 8n bytes
• double[][] = 8n^2 + 32n + 24 ~ 8n^2

Computing Max Memory

Max memory for a java application :

[-Xmx] + [-XX:MaxPermSize] + number_of_threads * [-Xss]

VM memory overhead is not a fixed %.

Flags

See also:

• Template:DataStructuresFlag
• Template:AlgorithmsFlag