Java Glossary : immutable
Last updated 2004-06-28 by Roedy
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Java definitions: 0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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Glossary : I words : immutable.
- immutable
- Not changeable. All Java variables are by default mutable. You can make them
immutable by using the final keyword. You can also
make them private and provide no 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. 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.
Advantages of Immutablitiy
-
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 conflict.
-
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. This is
particularly important in high security situations where allowing an invalid
value to sneak in could compromise system integrity, e.g. a filename.
-
You can share duplicates by pointing them to a single instance. You need only
one copy, e.g. String interning.
-
You can create substrings without copying. 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.
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 four 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 and RAM overhead in the array copying.
-
Beg or threaten the user not to modify the data in the reference you return to
him.
-
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.
Essay on double vs Double.
