Is it correct to call java.lang.String immutable?

Question

This Java tutorial says that an immutable object cannot change its state after creation.

java.lang.String has a field

/** Cache the hash code for the string */
private int hash; // Default to 0

which is initialized on the first call of the hashCode() method, so it changes after creation:

    String s = new String(new char[] {' '});
    Field hash = s.getClass().getDeclaredField("hash");
    hash.setAccessible(true);
    System.out.println(hash.get(s));
    s.hashCode();
    System.out.println(hash.get(s));

output

0
32

Is it correct to call String immutable?

Answer 1

A better definition would be not that the object does not change, but that it cannot be observed to have been changed. It's behavior will never change: .substring(x,y) will always return the same thing for that string ditto for equals and all the other methods.

That variable is calculated the first time you call .hashcode() and is cached for further calls. This is basically what they call "memoization" in functional programming languages.

Reflection isn't really a tool for "programming" but rather for meta-programming (ie programming programs for generating programs) so it doesn't really count. It's the equivalent of changing a constant's value using a memory debugger.