Java Pattern Matching in Practice

June 28, 2016

Implementing Scala style Structural Pattern Matching in Java

When switching from Scala back to Java one of the language features I find myself missing is pattern matching. Luckily I stumbled across a gem of a blog post that explains how you can emulate structural pattern matching in Java. (If pattern matching is new for you then there is a good primer over on stack overflow).

After reading that post I was curious as to how this looks in practice when using something a little more complicated than a tree. To see how it looked I thought I would implement Json as an Algebraic Data Type as this is a commonly used example so would provide an easy comparison between Scala and Java pattern matching.

Here is a Java implementation of Json as an ADT. A the top level we declare Json as an abstract class with a match method that accepts a function taking each possible case to T.

Then we subclass Json for each case with the subclass implementation of Match being to call the relevant function with itself as the argument.

Once we have defined our ADT we can pattern match away. Here is an example of using match to recursively build up a String representation of a Json Object.

This comes out looking pretty good, Lambdas allow match statments to look pretty natural and we have the compiler ensuring our matches are complete.

Admittedly there is quite a bit of upfront boilerplant when defining your ADT as you can see when you look at the full implementation. Using an IDE like like Intellij IDEA mitigates this somewhat as it can do most of the heavy lifting. Alternatively you could look derive4j which will generate the code for you. (I was recommended it on Twitter but have not yet had a chance to use it yet)