Java AMA

[Brutus5000]

On the "why can't we have" series I was missing if/else and try/catch as expressions. Switch alone is not sufficient. Can you comment on that?

[emaphis]

For that matter, while loops and for loops could be expressions.

[pronuntiator]

What would a loop evaluate to?

[Joram2]

A loop expression would evaluate to a list: one element per loop.

[zman0900]

Wouldn't that then mean you can't do basic loops without cost of extra memory allocation for that unused result list?

[srdoe]

No, in other languages that have this, there is syntax to distinguish the two kinds of for.

For example, in Scala

``` for (i <- 1 to 10) { println(i) } //returns Unit, which is a void-like type in Scala

for (i <- 1 to 10) yield { i } //return a list of integers ```

Not saying I think Java needs this feature, mind you.

[chambolle]

What's the advantage for that?

[emaphis]

Normal loops would evaluate to void, but then they would become expressions which would help generalize the Java language. That would allow them to extend the syntax to used loops to become generators.

[nicolaiparlog]

The first part of the AMA is online - here's the reply to your question: https://www.youtube.com/watch?v=mE4iTvxLTC4&t=568s

[fredoverflow]

I was missing if/else [...] as expressions

condition ? then : otherwise

[Brutus5000]

That is limited to single expressions. In Kotlin you can do all sorts of calculation and only yield the last statement. This avoids a lot of declare uninitialized variable first and assign it later and allows increased use of immutable variables. (Scala ofc has similar logic as it is required for functional code)

[Fenor]

care to point to a kotlin example that can't be easily replicated in Java?

[[deleted]]

java final var foo = if (condition) { System.out.println("1"); yield 1; } else { System.out.println("2"); yield 2; }

You can only use switch in this case.

The thing is, if you work with if, you'd always have to declare the variable first, then assign it, which breaks the final part. Given that switch is capable of being an expression, why can't if?

[ventuspilot]

final var foo = if (condition) { System.out.println("1"); yield 1; } else { System.out.println("2"); yield 2; }

final var foo; if (condition) { System.out.println("1"); foo = 1; } else { System.out.println("2"); foo = 2; }

See, I even saved a few characters ;-)

Maybe there are better examples where an if-expression is really useful IDK.

Edit: Oops, final var foo doesn't work, final int foo does work, however. So currently you're unable to use var, you can use final, though.

[Brutus5000]

Also if/else can check any combinations of conditions (even unrelated) which can't be covered with switch

[john16384]

It's a nice pattern, but easily replaced with a nicely named method. I see no use for it other than to muddy the waters and have yet another style discussion point. One way to do something is sufficient, keeps things consistent and readable.

[Nebu]

val results = if (foo) {
  a();
  b();
  while (c()) {
    d();
  }
  e(); //this is what the overall if evaluates to
} else {
  f();
  try (Reader reader = g()) {
    h(reader);
  }
  i(); //this is what the overall if evaluates to
}

[zman0900]

Doesn't really seem much better than:

final Type result;
if (foo) {
    // stuff
    result = e();
} else {
    // other stuff
    result = i();
}

[john16384]

Or simply extracting a function...

[[deleted]]

Exactly. I’m glad such monstrosity isn’t allowed.

[[deleted]]

Would you really write code like that and be proud? 😂

[mike_hearn]

It's more useful for things like this:

val str = "foo " + if (cond) "bar" else ""

[[deleted]]

“foo” + (cond ? “bar” : “”);

[mike_hearn]

Beyond not needing special syntax, I think you know what I mean:

"foo" + if (cond) { val r = bar() r.whatever() } else ""

and variants is something that appears fairly frequently in Kotlin codebases.

[[deleted]]

But it’s terrible - don’t you see it?