String vs char[]

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.

In the JVM, a character variable is stored in a single 16-bit memory allocation and changes to that Java variable overwrite that same memory location.This makes creating or updating character variables very fast and memory-cheap, but increases the JVM's overhead compared to the static allocation as used in Strings.

The JVM stores Java Strings in a variable size memory space (essentially, an array), which is exactly the same size (plus 1, for the string termination character) of the string when the String object is created or first assigned a value. Thus, an object with initial value "HELP!" would be allocated 96 bits of storage ( 6 characters, each 16-bits in size). This value is considered immutable, allowing the JVM to inline references to that variable, making static string assignments very fast, and very compact, plus very efficient from the JVM point of view.

Reference