immutable : Java Glossary
Not changeable. All Java variables are by default mutable. An Object is immutable if there is no way you can change its fields after
it has been constructed. Tools for creating making objects of a class immutable:
- Using the final keyword.
- Make variables private and provide no public accessors to the outside world to change them.
- The wrapper classes, Byte, Character, Short, Integer, Long, Float and Double are all immutable.
Strings are immutable, though the references to them are
mutable. StringBuffers are mutable. The only way to
change the value of the number inside the object wrapper is to create a new object
and point to that instead.
An object can be immutable even if some of its primitive fields are not. There
must be no public fields and no public methods that let you change fields. The simplest tool to
ensure immutability is to use the keyword final on all of
the fields and initialise them all in the constructor. You must also ensure that any
objects your immutable object points are in turn immutable. Normally you would
declare references to all such objects final.
Advantages of Immutability
- You can share immutable objects between threads without danger of changes
confusing the other thread. You don’t need any locking. Two threads can both
work on an immutable object at the same time without any possibility of
- Once you check the value, you know it has to stay safe. No one
can pass you a value, then behind your back swap it to an unsafe one using a
background thread. This is particularly important in high security situations where
allowing an invalid value to sneak in could compromise system integrity, e.g. a
filename. This is probably the main reason that Strings
- You can share duplicates by pointing them to a single instance. You need only
one copy, e.g. String interning. This saves RAM (Random Access Memory).
With mutable StringBuilders that were temporarily
identical, you could not throw one away, and replace it with a reference to the
other, since at any moment their values could diverge again.
- You can create substrings without copying. You just create a pointer into an
existing base String guaranteed never to change.
Immutability is the secret behind Java’s very fast substring implementation.
- Immutable objects are much better suited to be Hashtable keys. If you change the value of an object that is used as
a hash table key without removing it and re-adding it you lose the mapping.
- Since String is immutable, inside each String is a char exactly the correct
length. Unlike a StringBuilder there is no need for
padding to allow for growth.
Returning an Immutable Result
Let us say you wanted to return some data from your method, but you did not want
the user to muck with the original. Here are six approaches to the problem:
- Wrap the reference in an immutable wrapper class and return that.
- Give the caller his own private copy of the data he can muck with to his
heart’s content. There is high CPU (Central Processing Unit)
overhead in the array copying.
- Beg or threaten the user not to modify the data in the reference you return to
- Return an immutable interface to the original data. You can then change fields
in the object, but the caller cannot unless he cheats by casting. You expose only
the methods you want the user to have. Doing the same with classes is trickier
since a subclass must expose everything its superclass does.
- You can turn a Collection into an immutable
Collection with methods like
Collections. unmodifiableSet, Collections.
unmodifiableSortedMap or Collections. unmodifiableSortedSet.
- You can return an Iterator with a dummy
Of course same techniques can be used if you want to pass an Object to a method and you don’t want the method messing with
it. You declare the method with an interface parameter and pass it an interface
referenece or a reference to an object of some class implementing that interface.
Essay on double vs