Java: Assign a variable within lambda

Question

I cannot do this in Java:

Optional<String> optStr = Optional.of("foo");
String result;
optStr.ifPresent(s -> result = s);

The doc says that the variables used in lambdas must be effectively final. Then how to extract and store something from a lambda into a variable nicely?

Actually the real use case is more complex.
I want to apply several regular expressions to a string one after another with matcher.replaceAll. I'm doing this in a forEach lambda and wanted to store intermediate results somewhere.

Answer 1

The answer is simple: you can't (directly) do that.

A very ugly hack would be to mutate an external object instead of assigning to a variable:

Optional<String> optStr = Optional.of("foo");
StringBuilder sb = new StringBuilder();
optStr.ifPresent(s -> sb.append(s));
String result = sb.toString();

This works because sb is effectively final here

But I want to emphasize the point that this is probably not what you really want to do.

• If you want to have a default value, use optStr.orElse or optStr.orElseGet instead.
• If you want to map the result only if it is present, use optStr.map

Since you did not provide your whole use-case, these are just guesses but the key point is that I really do not recommend the above snippet of code: it goes against the concept of functional programming (by mutating an object).