Question

Today while I was reading some documentation regarding the BigDecimal class,
I’ve stumbled upon an fundamental property, the BigDecimal class is Immutable.

How could I explain to my grandmother the immutability concept ?

What are the pro and cons of the immutability of a class ?

Can an extended class become mutable ?

Considering that I want to extend the BigDecimal with my class:

`MyBigDecimal extends BigDecimal` 

Does the extending violate the basic Object Oriented design principles ?

Answer 1

How could I explain to my grandmother the immutability concept ?

Check this question: What is meant by immutable

Or from Effective Java:

An immutable class is a class whose instances cannot be modified. All of the information contained in each instance is provided when it is created and is fixed for the lifetime of the object.

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What are the pro and cons of the immutability of a class ?

Pros:

• it’s easier to reason about the state of the object, because there is only one, the state that was built upon initialisation
• corollary: immutable objects are easier to use in concurrent programming, where state is everything

Cons:

• when you want to change a property of the object, you need to create a new one = expensive
• construction can be more complicated (cf builder pattern)

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Can an extended class become mutable ?

Yes – which is why immutable class should be made final (or alternatively, make all constructors private and provide factories to create new objects).

BigDecimal is a good example of what should not be done when creating an immutable class (it can be extended which can cause issues as you mentioned in your question).