Java - Concatenate Consecutive Elements in a Stream

I am trying to concatenate two consecutive elements in an array. I could do this iteratively, but I am trying to learn Java Streams, and thought this would be a good exercise.

If I have an array of Strings:
String exampleArray[] = ["a1", "junk", "a2", "b1", "junk", "b2", "c1", "junk", "junk", "junk", "c2", "d1", "junk", "d2", "junk-n"]

I want to get:
["a1 - a2", "b1 - b2", "c1 - c2", "d1 - d2"] as output.

I tried this:

Arrays.asList(exampleArray)
	.stream()
	.filter(s -> s.length() > 0)   // gets rid of blanks
	.filter(s -> !s.contains("junk"))
	.collect(Collectors.groupingBy(it -> counter.getAndIncrement() / 2))
	.values();

This returns a Collection<List<String>> like [ [a1, a2], [b1, b2], [c1, c2], [d1, d2] ]

But I am not sure how to get to: ["a1 - a2", "b1 - b2", "c1 - c2", "d1 - d2"]

Any help is appreciated!

You were almost there. You already had the pairs, all you have to do now is smash them with a "-" in the middle into a String.

Give this a try:

Arrays.stream(exampleArray)
                .filter(s -> s.length() > 0)   // gets rid of blanks
                .filter(s -> !s.contains("junk"))
                .collect(Collectors.groupingBy(it -> counter.getAndIncrement() / 2))
                .values()
                .stream()                        //stream the pairs
                .map(l -> String.join("-", l))   //and put a "-" between them & into a string
                .collect(Collectors.toList())    //collect all your joined String

Collectors.groupingBy has a overloaded method where you can pass the result to a downstream collector. In fact, by default it has used toList collector and so you got List<String>. You can use joiningBy to concatenating strings.

.collect(Collectors.groupingBy(it -> counter.getAndIncrement() / 2
       , Collectors.joining(" - ")))

Result is

[a1-a2, b1-b2, c1-c2, d1-d2]

Code

public static void main(String[] args) {
    String exampleArray[] = new String[] {"a1", "junk", "a2", "b1", "junk", "b2", "c1", "junk", "junk", "junk", "c2", "d1", "junk", "d2", "junk-n"};
    AtomicInteger counter = new AtomicInteger(0);
    Collection<String> ans = (Arrays.asList(exampleArray)
            .stream()
            .filter(s -> s.length() > 0)   // gets rid of blanks
            .filter(s -> !s.contains("junk"))
            .collect(Collectors.groupingBy(it -> counter.getAndIncrement() / 2, Collectors.joining(" - ")))
            .values());

    System.out.println(ans);
}

A simple way to achieve this output is:

String a[] = {"a1", "junk", "a2", "b1", "junk", "b2", "c1", "junk", "junk", "junk", "c2", "d1", "junk", "d2", "junk-n"};

	Arrays.parallelSort(a); //sorting the array from a to z
	
	List<String> output = new ArrayList<>();
	
	for (int i = 0; i < a.length - 1; i++) {
		
		if(a[i].matches(".*\\d.*")) //verifying if have a number in string
		   output.add(a[i] + " - " + a[++i]);
		
	}
	
	System.out.println(output); //["a1 - a2", "b1 - b2", "c1 - c2", "d1 - d2"]