Programming with Immutability

For much of the last two years I've delivered a two-part series at NFJS shows entitled “Effective Java Reloaded.” For all pracical purposes, it is an ala carte style rehash of the book Effective Java, written by Josh Bloch. One of my favorite parts of the discussion is of Item #15, which tells us to “Minimize Mutability.” If we turn this inside out, we're actually saying that we want to MAXIMIZE IMMUTABILITY. When we do this, we reap many benefits, such as code that is easier to reason about and that is inherently thread-safe. This can carry us a long way in the direction of program correctness and decreased complexity. However, when we start to program with immutability, several major questions arise.

First, the necessity of using a separate object for each distinct value, never reusing, or “mutating” an object, can quickly cause performance concerns. These concerns are amplified when we're talking about large collections such as lists and maps. These problems are largely solved by what we call “persistent data structures.” Persistent data structures are collections from which we create new values, not by copying the entire data structure and apply changes, but by creating a new structure which contains our changes but points at the previous structure for those elements which have not changed. This allows us to work with data structures in a very performant way with respect to time and resource consumption. We'll examine persistent data structures, their associated algorithms, and implementations on the JVM such as those found in the TotallyLazy library.

Second, because all of an immutable object's state must be provided at the time of construction, the construction of large objects can become very tedious and error prone. We'll examine how the Builder pattern can be applied to ease the construction of large objects, and we'll examine Builder implementations in Java and Groovy.

Third, we run into problems when we start to use frameworks that expect us to program in a mutable style. A prime example is Hibernate, which expects our persistent classes to follow the well-worn JavaBean convention, including a no argument constructor and getters and setters for each property. Such a class can never be mutable! So how do we program with frameworks such as Hibernate and yet still minimize mutability? The key is found in not letting frameworks dictate the way that you design your code. Just because the framework require something, don't let it force you to make the wrong decision. Use the framework as a tool to write your code, don't let your code be a tool of the framework. We'll examine strategies for doing exactly that.

You should come away from this talk better equipped to program in a way that minimizes mutability and maximizes immutability.