Re: what the benefit is by using annotation, like "@Immutable" ?

Lew <>
Wed, 21 Jul 2010 15:25:48 -0700 (PDT)
On Jul 21, 4:09 pm, markspace <> wrote:

Lew wrote:

The field in question is private, and does not change the externally
observable state. So the String instance acts like it's immutable, b=


has mutable internal state.

So in a real sense String is mutable, and just as real, it's immutable.

To expound on this a bit, final fields in a constructor are treated
specially by the JVM and JLS. Even if an entire class is not strictly
immutable, the values and objects referred to by final fields will be
treated as immutable by the compiler & runtime.

Thus, if you have your own class which follows the rules for immutablity
for some of its fields, those fields will also be treated as immutable &
safe for publication with a data race.

public class Stooges {
   private final List<String> stooges;
   private int count;

   public Stooges {
     stooges = new ArrayList<String>();

   public boolean isStooge( String name ) {
     return stooges.contains( name );

   public int size() {
     if( count == 0 ) {
       count = stooges.size();
     return count;


The field "stooges" is immutable and treated as such by the JVM. The
fact that "count" is kind of lazily-immutable (*) still is thread safe;
this whole class can be treated as immutable, not as effectively
immutable, with no repercussions or harm done.

Still, it's better most times to make something like 'count' final.
The reason 'hash' wasn't final in 'String' was the ubiquity of
'String' measured against its likely usage, making the possible
speedup likely to be worthwhile. Nearly no custom classes will have
the dynamic to justify lazy-loading for the attributes one considers
loading lazily.

 public class Stooges {
   private final Set <String> stooges;
   private final int count;

   public Stooges {
     HashSet <String> sts = new HashSet <String> ();
     sts.add( "Lary" );
     sts.add( "Curie" );
     sts.add( "Moo" );
     stooges = Collections.unmodifiableSet( sts );
     count = stooges.size();

   public boolean isStooge( String name ) {
     return stooges.contains( name );

   public int size() {
     return count;


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