Complete List of immutable JDK classes?

Question

is there a list of de-facto immutable classes in the jdk?

technically Immutable classes include the obvious Integer, Double etc..

de-facto immutable will include for example java.lang.String - it might technically be mutable but de-facto it is not.

Also, are there Interfaces/Abstract classes which are required (as stated in the javadoc) to be immutable?

if you cannot provide a complete List, i would already be happy if you know a bunch of classes which state immutability in its javadoc..

Answer 1

I try to compile the list as much as I can:

1. java.lang.String The wrapper classes for the primitive types:
2. java.lang.Integer
3. java.lang.Byte
4. java.lang.Character
5. java.lang.Short
6. java.lang.Boolean
7. java.lang.Long
8. java.lang.Double
9. java.lang.Float
10. java.lang.StackTraceElement (used in building exception stacktraces)
11. Most of the enum classes
12. java.math.BigInteger
13. java.math.BigDecimal
14. java.io.File
15. java.awt.Font
16. java.awt.BasicStroke
17. java.awt.Color
18. java.awt.GradientPaint,
19. java.awt.LinearGradientPaint
20. java.awt.RadialGradientPaint,
21. java.awt.Cursor
22. java.util.Locale
23. java.util.UUID
24. java.util.Collections
25. java.net.URL
26. java.net.URI
27. java.net.Inet4Address
28. java.net.Inet6Address
29. java.net.InetSocketAddress
30. most subclasses of java.security.Permission

Answer 2

You can use MutabilityDetector and feed it JARs from the JDK to enumerate most immutable classes. It's "most" because it's so strict that even the tiny bit of state change in java.lang.String is enough to make it considered mutable, but the goal is to account for this by 1.0 release.

You can download the latest JAR here: https://github.com/MutabilityDetector/MutabilityDetector/releases

Here's an example of it's use. This is what I used to get most of the immutable JDK 1.7 classes on OSX:

java -jar MutabilityDetector-0.9.5.jar -r IMMUTABLE -cp /Library/Java/JavaVirtualMachines/jdk1.7.0_79.jdk/Contents/Home/jre/lib/rt.jar

Here's the output (slightly cleaned up): https://gist.github.com/CalebFenton/85fc87edf64033afe110

I needed to do this for Android framework classes. The tricky part was finding a JAR with Android classes, and not just stubs which is the one included in the SDK. The good people at Robolectric make one, which you can download here: http://search.maven.org/#search%7Cga%7C1%7Ca%3A%22android-all%22

You can see the list of immutable Android classes I use, which includes many JDK classes here: https://github.com/CalebFenton/simplify/blob/master/smalivm/src/main/resources/immutable_classes.cfg

Answer 3

Classes whose object contents cannot be modified is called immutable classes. All primitive data types(Wrapper classes only) are immutable. For any class to be immutable the following needs to be done.

• Make all fields private
• Don't provide mutators
• Ensure that methods can't be overridden by either making the class final (Strong Immutability) or making your methods final (Weak Immutability)
• If a field isn't primitive or immutable, make a deep clone on the way in and the way out.

Thank you