String vs char[]

Question

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 char[], there is

Maximum overhead would be 24:1 for a single character!

but I am not able to understand what overhead is referred here. Can anybody please help?

Source :

Answer 1

This figure relates to JDK 6- 32-bit.

JDK 6

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. 

JDK 7

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:

• Memory usage of Java Strings and string-related objects.
• Calculate memory of a Map Entry - a simple technique to get a size of an object.