Mutable strings in Java

Question

As almost everybody knows strings in Java are immutable. Recently I discovered something that might suggest that it\'s not always true. Let\'s try out this code:

Answer 1

String literals are interned into a pool. This means that when you write

String s1 = "Foo";
String s2 = "Foo";
String s3 = new String("Foo");

s1 and s2 refer to the same String object, and s3 refers to another one, backed by another char array.

In your code, you violate String's invariants by modifying the private char array holding the characters of the "Original" String literal instance. But since beforeTest refers to another String instance, it's not modified.

Immutability is achieved by keeping fields private into an object, and not providing any method to modify this private state. By using reflection, you break all the rules of encapsulation, and you can thus violate immutability.