I have some slides from IBM named : \"From Java Code to Java Heap: Understanding the Memory Usage of Your Application\", that says, when we use String
instead of <
This figure relates to JDK 6- 32-bit.
In pre-Java-7 world strings which were implemented as a pointer to a region of a char[]
array:
// "8 (4)" reads "8 bytes for x64, 4 bytes for x32"
class String{ //8 (4) house keeping + 8 (4) class pointer
char[] buf; //12 (8) bytes + 2 bytes per char -> 24 (16) aligned
int offset; //4 bytes -> three int
int length; //4 bytes -> fields align to
int hash; //4 bytes -> 16 (12) bytes
}
So I counted:
36 bytes per new String("a") for JDK 6 x32 <-- the overhead from the article
56 bytes per new String("a") for JDK 6 x64.
Just to compare, in JDK 7+ String
is a class which holds a char[]
buffer and a hash
field only.
class String{ //8 (4) + 8 (4) bytes -> 16 (8) aligned
char[] buf; //12 (8) bytes + 2 bytes per char -> 24 (16) aligned
int hash; //4 bytes -> 8 (4) aligned
}
So it's:
28 bytes per String for JDK 7 x32
48 bytes per String for JDK 7 x64.
UPDATE
For 3.75:1
ratio see @Andrey's explanation below. This proportion falls down to 1 as the length of the string grows.
Useful links: