Proposal: Block Expressions for Java (original) (raw)

Neal Gafter neal at gafter.com
Fri Mar 6 12:58:40 PST 2009

The places I would have found this most useful are in a super() invocation, where you simply can't write a local variable before, and in a control-flow expression of one sort or another, like ?:, &&, ||, or on the right-hand-side of an assert statement. This kind of thing is particularly useful in automatically-generated code.

On Fri, Mar 6, 2009 at 12:50 PM, Tim Peierls <tim at peierls.net> wrote:

I kind of like Neal's block expression proposal, but I am struggling to find more compelling examples for it. Instead of

super((double pi = Math.PI ; pipi)**);* it would be clearer to write super(Math.PI * Math.PI); Instead of public static final Map<Integer,Integer> primes = (  Map<Integer,Integer> t = new HashMap<Integer,Integer>();  t.put(1, 2); t.put(2, 3); t.put(3, 5); t.put(4, 7);  Collections.unmodifiableMap(t)); it would be better to use a Builder like the one in Google Collections' ImmutableMap: public static final ImmutableMap<Integer, Integer> PRIMES =     new ImmutableMap.Builder<Integer, Integer>()  .put(1, 2).put(2, 3).put(3, 5).put(4, 7)  .build(); That particular example could in turn be rewritten more simply as public static final ImmutableMap<Integer, Integer> PRIMES =  ImmutableMap.of(1, 2, 2, 3, 3, 5, 4, 7); So while I understand the intention behind these examples, I think they could be improved.  Any ideas? What I like about the proposal is the ability to add side-effects in places I currently can't without distorting the existing logic. For example, let's say I want to add logging of the value computed in the while-loop test of this fragment: while (jobsRemaining() > 0) {  processSomeJobs(); } I could write: int j = jobsRemaining(); log.info("jobs remaining = %d", j); while (j > 0) {  processSomeJobs();  j = jobsRemaining();  log.info("jobs remaining = %d", j); } but that means repeating the logging statement and introducing a variable j with scope greater than the while loop. If I don't introduce a variable I have to repeat the call to jobsRemaining(), which might not be appropriate if it's an expensive call (and I'd still have to repeat the logging call). I could write a helper method instead: int computeAndLogJobsRemaining() {  int j = jobsRemaining();  log.info("jobs remaining = %d", j);  return j; } ... while (computeAndLogJobsRemaining() > 0) {  processSomeJobs(); } but that's adding a lot of machinery for something as conceptually simple as this. Similarly for solutions involving method interception. The proposed enhancement would (I think) let me write something like this: while ((  int j = jobsRemaining();  log.info("jobs remaining = %d", j);  j > 0 )) {                    // are two levels of parens necessary?  processSomeJobs(); } which is exactly what I wanted to say, without having to change the structure of the original code. Could I get along without this? Absolutely.  Would I find it useful?  Yes, occasionally it would be quite handy. --tim

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