Tuesday, April 12, 2016

Two little things I wish Java would add

When geeking out about language design, it's tempting to focus on the things that require learning something new to even understand how it works. SAM types require understanding target typing, and type members require understanding path-dependent types. Fun stuff.

Aside from these things that are fun to talk about over beers, I really wish Java would pick up a few things from Scala that are just plain more convenient.

Multi-line string literals

A great way to structure a unit test is to feed in a chunk of text, run some processing that you want to verify, convert the actual output to text, and then compare it against another chunk of text that's included in the test case. Compared to a dense string of assertEquals calls, this testing pattern tends to be much easier to read and understand at a glance. When such a test fails, you can read a text diff at a glance and possibly see multiple different kinds of failure that happened with the test, rather than stare into the thicket of assertEquals calls and try to deduce what is being tested by the particular one that failed.

The biggest weakness of this style is very mundane: it's hard to encode a multi-line chunk of text in Java. You have to choose between putting the text in an external file, or suffering through strings that have a lot of "\n" escapes in them. Both choices have problems, although the latter option could be mitigated with a little bit of IDE support.

In Scala, Python, and many other languages, you can write a multi-line string by opening it with triple quotes (""") rather than a single quote mark ("). It's a trivial feature that adds a lot to the day to day convenience of using the language.

As one trick to be aware of, it's important to help people out with indentation when using triple quotes. In Scala, I lobbied for the stripMargin approach to dealing with indentation, where you put a pipe on each continuation line, and anything up to the pipe is considered leading indentation and removed. In retrospect, I wish I had pushed for that to simply be the default behavior. If you need to insert a literal continuation character, you can always write it twice. Making people write stripMargin on almost every multi-line string is a form of boilerplate.

Case classes

There are philosophers who disagree, but I find them a little too philosophical for my taste. Sometimes you really want to write a class that has no hidden internal state. Sometimes it would be a breach of the API to retain any internal state, or to implement the public API as anything other than plain old final fields. Some motivating examples are: tiny types, data structure nodes such as links in a linked list, and data-transfer objects.
In such a case, it takes a tremendous amount of code in Java to implement all the odds and ends you would really like for such a class. You would really like all of the following, and they are all completely mechanical:
  • Constructors that copy their parameters to a series of final fields.
  • A toString() implementation.
  • Comparison operations: equals(), hashCode(), and compareTo(). Ideally also helpers such as isLessThan().
  • Copy constructors that make a new version by replacing just one of the fields with a new value.
The equals() method is particularly painful in Java because there is a lot of advice going around about how to write them that is not consistent. I've been drug into multi-day debates on equals() methods where people cite things I published in the past to try and use against me; I'm pretty sure I meant what I said then and mean what I say now. Above all, though, I'd rather just have a reasonable equals() method and not spend time talking about it.