String vs char[]

java, memory, memory-management, string

Solution

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.

Problem

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 :

Original source