Immutable object in java

Immutable objects are the  one whose state can't be changed once they are constructed. Example objects of class String and Integer. Immutable objects are considered useful in concurrent applications because threads can't change their state.






How to create an immutable object ?

1. Ensure that the class can't be overridden . The simplest way is to make the class final.   Another way is to make the construstor private and use factory methods to construct instances.

2. Make all the fields private and final.

3. Do not provide any methods that can change object's state . For example  don't provide any setter methods to change object's state.

4. If the instance members refer to any mutable objects, don't allow those objects to get changed. This can be done in following ways :

• Don't provide methods that can modify the mutable objects.
• Don't share references to the mutable objects.
• Never store references to external, mutable objects passed to the constructor. if necessary, create copies, and store references to the copies.
• Never return the original mutable object in the methods, if necessary create copies of mutable objects and return them.

Example 
To demonstrate this behavior , we can take example of class java.lang.String

String oldString = new String("Old String");
System.out.println("Before : " + oldString);
oldString.replace("Old", "new");
System.out.println("After  : " + oldString);

The output will be:
Before : Old String
After     : Old String

Here we can see that the contents of the string oldString did not change. It is because the objects of class String are immutable.