The Pragmatic Craftsman :: Simplicity from complexity : by Stanley Kubasek ::

Posts Tagged 'immutability'

Make fields final, objects immutable December 30th, 2009

Make fields final, objects immutable

Just as it is a good practice to make all fields private unless they need greater visibility, it is a good practice to make all fields final unless they need to be mutable.
–Brian Goetz
in Java Concurrency in Practice (page 48)

Are you following these fundamental principles?

Just so we’re on the same page, an immutable object is on whose state cannot be changed after construction.

This view is supported by Joshua Bloch in Effective Java (2nd). Item 13 states: “Minimize the accessibility of classes and members.”

The single most important factor that distinguishes a well-designed module from a poorly designed one is the degree to which the module hides its internal data and other implementation details from other modules.
–Joshua Bloch

He goes on to say some fundamental principles.

A well designed module hides all of its implementation details, cleanly separating its API from its implementation. Modules then communicate only through their APIs and are oblivious to each others’ inner workings.
–Joshua Bloch
Effective Java (2nd), page 67

These are really the basics of information hiding or encapsulation. Basics of OOP programming! It’s a good idea to learn these well.

Think twice before making any fields public. Hold it. No, don’t make it public! Think twice before making it any other than private!

But to make sure you only expose what is absolutely needed requires some thought. You need your own judgment and experience.

As for the second part, item 15 states: “Minimize mutability.”

I think this one is a little harder to justify and not so obvious. But Bloch has some very good arguments. Together with Goetz, they convinced me that I should utilize immutability more often when I’m designing classes.

Immutable classes are easier to design, implement, and use than mutable classes. They are less prone to error and are more secure.
–Joshua Bloch

Convinced!

But how? Follow these 5 steps (as per Bloch).

1. Don’t provide any methods that modify the object’s state.
2. Ensure the class can’t be extended.
3. Make all fields final.
4. Make all fields private.
5. Ensure exclusive access to any mutable components.

Goets says it a little bit differently.

An object is immutable if:
– Its state cannot be modified after construction;
– All its fields are final;
– It is properly constructed (the this reference does not escape during construction)
Immutable objects pack a lot of goodies in them.

One more time. Tell me what are the benefits of immutable objects!

They are simple to understand.
They are thread safe.
They require no synchronization.

Let’s end with a great summary note from Bloch, “Classes should be immutable unless there’s a very good reason to make t hem mutable.”

But.

If…

“If a class cannot be made immutable, limit its mutablity as much as possible.”

Strong statements.

Learn these design principles!

All in all, I recommend reading Item 15 from the Effective Java book. And re-read a few times until it becomes part of your design logic.

It’s all about getting better.

It’s all about improving.

It’s good to learn from the pros. Everybody needs a little guidance. I know I do. (These 2 books are excellent! Recommended.)

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