How to reference the result of reduce() operation in Java 8?

Question

I was trying to write a mkString function in Java8, a la Scala's useful mkString and ran into 2 issues that I could use some help on:

1. I am unable to make the first argument of mkString a generic Collection reference like Collection<Object> c and have invokers call with ANY type of collection.

2. Unable to reference the returned result of reduce() in-line to access the result's length to remove the extra leading separator.

Here's the code :

public static void main(String[] args) {
    List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
    System.out.println(mkString(numbers, ","));

}

public static String mkString(Collection<Integer> c, String sep) {
    return c.stream()
            .map(e -> String.valueOf(e))
            .reduce("", (a, b) -> a + sep + b)
            .substring(1, <<>>.length);
}

Answer 1

Note that if you're doing this not for self-education but to actually use it in some production code, you might want to consider the built-in Collectors.joining collector:

String result = numbers.stream()
    .map(Object::toString)
    // or
    //   .map(x -> x.toString())  // exactly the same
    // or
    //   .map(String::valueOf)    // handles nulls by turning them to the string "null"
    .collect(Collectors.joining(","));

It has several overloads, similar to Scala's mkString. Still, this collector only accepts CharSequences, so you need to convert your values to strings explicitly as a separate map step.

Additionally, there is the String.join method, which also works for a collection of CharSequences. If you specifically have one of those (e.g. List<String>), it might be more convenient to use this method rather than converting the collection to a stream first:

List<String> strings = ...;

String result = String.join(",", strings);

// vs

String result = strings.stream().collect(Collectors.joining(","))

Answer 2

If I remember my java correctly, you can declare the argument type as Collection<?> to be able to pass a collection of any objects.

As to biting the separator off, I think, just .substring(1) will do what you want.