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Immutable class:How to create Immutable Class and Object in Java

asked Sep 13, 2013 in Java Interview Questions by rajesh
edited Sep 12, 2013
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How to create Immutable Class and Object in Java
Writing or creating immutable classes in Java is becoming popular day by day, because of concurrency and multithreading advantage provided by immutable objects. Immutable objects offers several benefits over conventional mutable object, especially while creating concurrent Java application. Immutable object not only guarantees safe publication of object’s state, but also can be shared among other threads without any external synchronization. In fact JDK itself contains several immutable classes like String, Integer and other wrapper classes. For those, who doesn’t know what is immutable class or object, Immutable objects are those, whose state can not be changed once created e.g. java.lang.String, once created can not be modified e.g. trim, uppercase, lowercase. All modification in String result in new object, see why String is immutable in Java for more details. In this Java programming tutorial, we will learn, how to write immutable class in Java or how to make a class immutable. By the way making a class immutable is not difficult on code level, but its the decision to make, which class mutable or immutable which makes difference. I also suggest reading, Java Concurrency in Practice to learn more about concurrency benefit offered by Immutable object.
What is immutable class in Java
As said earlier, Immutable classes are those class, whose object can not be modified once created, it means any modification on immutable object will result in another immutable object. best example to understand immutable and mutable objects are, String and StringBuffer. Since String is immutable class, any change on existing string object will result in another string e.g. replacing a character into String, creating substring from String, all result in a new objects. While in case of mutable object like StringBuffer, any modification is done on object itself and no new objects are created. Some times this immutability of String can also cause security hole, and that the reason why password should be stored on char array instead of String.
How to write immutable class in Java
Despite of few disadvantages, Immutable object still offers several benefits in multi-threaded programming and it’s a great choice to achieve thread safety in Java code. here are few rules, which helps to make a class immutable in Java :
1. State of immutable object can not be modified after construction, any modification should result in new immutable object.
2. All fields of Immutable class should be final.
3. Object must be properly constructed i.e. object reference must not leak during construction process.
4. Object should be final in order to restrict sub-class for altering immutability of parent class.
By the way, you can still create immutable object by violating few rules, like String has its hashcode in non final field, but its always guaranteed to be same. No matter how many times you calculate it, because it’s calculated from final fields, which is guaranteed to be same. This required a deep knowledge of Java memory model, and can create subtle race conditions if not addressed properly. In next section we will see simple example of writing immutable class in Java. By the way, if your Immutable class has lots of optional and mandatory fields, then you can also use Builder design pattern to make a class Immutable in Java.
answered Sep 13, 2013 by rajesh
edited Sep 12, 2013