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How to program without side effects in Java?

As a longtime Java programmer, and in recent years I have been using Haskell, I am learning Scala now. My question is:

How to program without side effects in Java?

i.e. How can I do manually what Scala does for me?

Scala Odersky's book often explains functional concepts in terms of OO terminology (e.g. val = final, if = trernary op.).

Is there any manual available for programming without side effects in Java (as little as possible)? I assume that you can get many well-known advantages with plain old java and a little discipline (for example, immutable collections, final values, hard methods that do not change the state of the object / application).

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4 answers

See Functional Java .

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  • Make each method, field and class either abstract or final .
  • Make each parameter of the variable and method final .
  • If you use mutable structures such as Array or I / O resources such as files and database connections, never let them hide the area in which they are created.
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One way to program without side effects is to use only immutable classes. You can apply this to your own classes, but you cannot change the standard Java work environment without rewriting from scratch.

In any case, making the free Java effect free does not make sense, because it is an imperative programming language with encapsulation of OO data. In other words, side effects form part of the tongue and are not evil per se.

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As a longtime Java programmer, and in recent years I have been using Haskell, I am learning Scala now.

Congratulations :)

How to program without side effects in Java? How can I do manually what Scala does for me?

Well, Scala doesn't do so much for you (compared to Haskell) in terms of providing or supporting a free-style side effect. For example, you always have the Scala option to use var , wherever you are, whereas in Haskell, if you want to change IORef or STRef , you need to specify this in the type system using the appropriate monad.

In other words, although you are asking how to simulate the Scala style in Java, the first step to consider is how to simulate the Haskell style in Scala, which of course is Scalaz .

All that was said, I think that you basically answered your question:

I assume that many of the well-known advantages can be obtained using simple old java and a little discipline (for example, non-modifiable collections, final values, hard methods that do not change the state of the object / application).

Discipline is at least half the story. Just using final and immutable collections (à la Guava ) has all sorts of consequences for what your code looks like. The other half of the story, of course, is what you do when your program requires side effects (such as I / O)? In Haskell you have to use the IO monad, and in Scala you can use scalaz.effects.IO , but in Java, to be honest, I think the battle is not worth the fight.

In my opinion, the two most important lines of functional style code that you will write in any Java program are as follows:

 import static com.google.common.collect.Iterables.filter; import static com.google.common.collect.Iterables.transform;

The rest is an exercise for the reader;)

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