What does immutable mean?

Question

If a string is immutable, does that mean that.... (let\'s assume JavaScript)

var str = \'foo\';

alert(str.substr(1)); // oo

alert(str); // foo

Answer 1

I'm not certain about JavaScript, but in Java, strings take an additional step to immutability, with the "String Constant Pool". Strings can be constructed with string literals ("foo") or with a String class constructor. Strings constructed with string literals are a part of the String Constant Pool, and the same string literal will always be the same memory address from the pool.

Example:

    String lit1 = "foo";
    String lit2 = "foo";
    String cons = new String("foo");

    System.out.println(lit1 == lit2);      // true
    System.out.println(lit1 == cons);      // false

    System.out.println(lit1.equals(cons)); // true

In the above, both lit1 and lit2 are constructed using the same string literal, so they're pointing at the same memory address; lit1 == lit2 results in true, because they are exactly the same object.

However, cons is constructed using the class constructor. Although the parameter is the same string constant, the constructor allocates new memory for cons, meaning cons is not the same object as lit1 and lit2, despite containing the same data.

Of course, since the three strings all contain the same character data, using the equals method will return true.

(Both types of string construction are immutable, of course)