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The first thing to do is ban null from any of your code. You should never return null from a method, ever, ever, ever. If you are calling Java libraries that may return null or throw an exception because of input problems, convert these to Options. We did this for parsing Strings to Ints. The pattern is basic: no nulls. When you write code, ban null from your code. In the case of uninitialized instance variables, either assign a default value that is not null or, if there s a code path where the
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9. I view this book as a gateway drug to Scala. I tried to orient it to beginners. Programming in Scala by Martin Odersky, Lex Spoon, and Bill Venners (Artima, 2008) is an excellent next step for learning Scala.
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CHAPTER 9 SCALING YOUR TEAM
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variable could be used prior to initialization, use Option, and the default value becomes None. If there s no logical value that can be returned from a method given legal input, the return type should be Option. The get method should never be called on an Option. Instead, Options should be unpacked using map/flatMap, the for comprehension, or pattern matching. The first benefit using Option is the obvious avoidance of null pointer exceptions. The second benefit is a little more subtle. The use of Option and the transformative nature of mapping Options leads to a different style of approaching your code. The style is more transformative, more functional. The impact of repeatedly using immutable data structures will move your brain toward the functional side.
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Focus on Immutability
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In Java, mutability is the default. Variables are mutable unless they re marked final. JavaBeans have getters and setters. Data structures in Java are instantiated, set, and passed along to other methods. Try changing the paradigm in your Scala code. The first thing to do is use immutable collections classes by default. If you choose to use a mutable collections class, make a comment in your code as to why you chose mutability. There are times when mutable collections make sense. For example, in a method where you are building a List, using ListBuffer is more efficient, but don t return the ListBuffer, return the List. This is like using a StringBuilder in Java but ultimately returning a String. So, use immutable collections by default, and use mutable data structures with a justification. Use vals by default, and only use vars if there is a good reason that is justified by a comment. In your methods, use val unless there s going to be a significant performance hit. Using val in methods often leads to thinking recursively. Let s look at a mutable implementation of a method that consumes all the lines from a BufferedReader:
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def read1(in: java.io.BufferedReader): List[String] = { var ret: List[String] = Nil var line = in.readLine while (line != null) { ret ::= line line = in.readLine } ret.reverse }
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The above code is readable but uses a couple of vars. Let s rewrite the code without vars and see how we can use tail recursion to give us a while loop:
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def read2(in: java.io.BufferedReader): List[String] = { def doRead(acc: List[String]): List[String] = in.readLine match { case null => acc
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CHAPTER 9 SCALING YOUR TEAM
case s => doRead(s :: acc) } doRead(Nil).reverse }
Look ma, no vars. We defined the doRead method, which reads a line of input. If the line is null, we return the accumulated List. If the line is non-null, we call doRead with the accumulated List. Because doRead is in the scope of read2, it has access to all of read2 s variables. doRead calls itself on the last line, which is a tail call. The Scala compiler will optimize the tail call into a while loop, and there will only be one stack frame created no matter how many lines are read. The last line of read2 calls doRead with Nil as the seed value for the accumulator. Using vals in your code makes you think about alternative, immutable, functional code. This small example demonstrates that removing vars leads to refactoring. The refactoring leads to new coding patterns. The new coding patterns lead to a shift in your approach to coding. This shift in approach will yield transformative code that has fewer defects and is easier to maintain.
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