Immutable vs Mutable Objects in Java - the question answer approach

1. What is an immutable object ? 

An object is considered immutable if its state cannot change after it is constructed. 


     2.  What is happenning to immutable object myString in the example below ? 

String myString = new String( "old String" ); String myCache = myString; System.out.println( "equal: " + myString.equals( myCache ) ); System.out.println( "same: " + ( myString == myCache ) ); myString = "not " + myString; System.out.println( "equal: " + myString.equals( myCache ) ); System.out.println( "same: " + ( myString == myCache ) );

Result : 

        equal: true
	same:  true
	equal: false
	same:  false

The contents of myString is not changing here. We are discarding the instance and changed our reference to a new one with new contents. 

     3.  How can you change the values of a variable ? 

• You can always change the value of a variable by getting your variable to reference a new object. 
• Sometimes you can change the value of a variable by keeping a reference to the same instance, but change the contents of the instance.

    4.  What are the uses of immutable objects ?

• They can promote thread safety in your code 
• You can share them around without being afraid that they will change without your knowledge 
• They are great for caching and constants

    5.  How can we create a mutable class ? 

• 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.

References : 

• http://www.javaranch.com/journal/2003/04/immutable.htm
• http://www.ibm.com/developerworks/java/library/j-jtp02183/index.html
• http://stackoverflow.com/questions/214714/mutable-vs-immutable-objects