How do you assign a lambda to a variable in Java 8?

You have forgotten the generics on your BiFunction:

public static void main(final String[] args) throws Exception {
    final Map<String, Integer> map = new HashMap<>();
    map.put("A", 1);
    map.put("B", 2);
    map.put("C", 3);
    final BiFunction<String, Integer, Integer> remapper = (k, v) -> v == null ? 42 : v + 41;
    map.compute("A", remapper);
}

Running:

PS C:\Users\Boris> java -version
java version "1.8.0-ea"
Java(TM) SE Runtime Environment (build 1.8.0-ea-b120)
Java HotSpot(TM) 64-Bit Server VM (build 25.0-b62, mixed mode)

As Boris The Spider points out, the specific problem you have is the generics. Depending on context, adding an explicit {} block around the lambda will help add clarity and may be required.

With that, you would get something like:

BiFunction<String, Integer, Integer> x = (String k, Integer v) -> {v == null ? 42 : v + 41};

It would be pretty helpful for future readers who have the same problem if you posted your syntax errors so they can be indexed.

This link has some additional examples that might help.