String如何实现不变性？

Question

In one of the interview I was asked 'How String is made immutable?' As i wasnot sure about the answer, i didnot reply. I asked the interviewer later regarding the same. Answer was String class is final that's how immutability is achieved.

Is that the correct answer? if yes, even StringBuffer is also marked as final class. Then why not StringBuffer is immutable?

Answer 1

It is a combination of:

1. Fields are private - so you cannot change them directly.
2. No set methods provided - so they cannot be changed indirectly either.
3. String is final - so you cannot add mutability (ie setters etc.) to it.

Answer 2

No that's not the correct answer. String achieves immutability because it doesn't provide you any method to change its internal contents. Thus you can instantiate a String object, assign a reference to it but cannot change its contents once initialized.

Answer 3

String is immutable object.

Make a class immutable by following these guidelines :

• ensure the class cannot be overridden
• make the class final , or use static factories and keep constructors private
• make fields private and final
• do not provide any methods which can change the state of the object in any way - not just setXXX methods, but any method which can change state if the class has any mutable object fields, then they must be defensively copied when passed between the class and its caller
• force callers to construct an object completely in a single step, instead of using a no- argument constructor combined with subsequent calls to setXXX methods (that is, avoid the Java Beans convention)

Answer 4

The final keyword is not the same as immutability. String is immutable as it does not define any methods that allow a user to change its content and it is final, removing the possibility to change things in a subclass.

Making something like a List instance variable final will still allow you to change its contents, making it mutable.

Answer 5

Being final means it can't be derived from. That doesn't confer immutability

Immutability is achieved by encapsulation and not providing any means to amend the internally held character array. Ie no methods exist to modify the internal fields.

Answer 6

A pool of strings, initially empty, is maintained privately by the class String.

You should look at the JavaDoc of String: public native String intern();

See:

• http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#intern%28%29
• http://en.wikipedia.org/wiki/String_interning

Answer 7

The usual way to make a class immutable is to ensure that:

• all the fields are private;
• there is no way to modify the fields after construction;
• it is final (so that extensions cannot break immutability).

String is a bit special, at least in the Sun/Oracle implementation in that it does not actually follow this procedure. The implementation has a mutable field in which it caches the hash code of the object. So while there is a method that changes the internal state of the object ( hashCode ), this state change does not change the behaviour of the object in any way. Any subsequent calls to hashCode will run faster, but the result won't be any different.