Thomas Abeel

We all know that Java is a memory hog. This page aims to give you an idea how much it actually hogs without using a profiler.
Primitive types
A reference to an object is in this listing also considered to be a primitive type.

• boolean: 1 byte
• byte: 1 byte
• char: 2 bytes
• short: 2 bytes
• int: 4 bytes
• long: 8 bytes
• float: 4 bytes
• double: 8 bytes
• reference: 4 bytes

Objects
Each Java object uses the following:

• 8 byte Object overhead
• size of all primitives and references combined
• The total size must be a multiple of 8, so add bytes till you have a multiple of 8.

Arrays

• 12 byte overhead
• size of the array times the size of the primitive type

Some examples

• Integer = 8 overhead + 4 data + 4 padding = 16 bytes
• String = 8 bytes overhead + 2 * the number of characters in the char[] + 12 bytes overhead for the char array + 8 bytes for two int fields = 32 bytes for an empty String + 2 bytes for each character.

Using auto-boxed primitives or Strings as keys in a large Map will give you very large amounts of overhead.