How to append the elements of one Set to the elements of another Set using Java 7

Question

I have 2 sets:

Set<String> set1 = new TreeSet<String>();
Set<String> set2 = new TreeSet<String>();
Set<String> set3 = new TreeSet<String>();

set1 = [A, C, E];
set2 = [B, D, F];

I am looking for a way to append the value of set2 to set1 and store them in set3

Set 3 Output

set3 = [AB, CD, EF]

Answer 1

You need to use an Iterator to keep the order of the TreeSet. You can get it by calling TreeSet.iterator():

Returns an iterator over the elements in this set in ascending order.

Assuming that both sets have the same length, you could have:

public static void main(String[] args) {
    Set<String> set1 = new TreeSet<String>(Arrays.asList("A", "C", "E"));
    Set<String> set2 = new TreeSet<String>(Arrays.asList("B", "D", "F"));
    Set<String> set3 = new TreeSet<String>();

    Iterator<String> it1 = set1.iterator();
    Iterator<String> it2 = set2.iterator();
    while (it1.hasNext() && it2.hasNext()) {
        set3.add(it1.next() + it2.next());
    }

    System.out.println(set3); // prints "[AB, CD, EF]"
}

If the set have different size and that is an error, you could add the following check before the while loop:

if (set1.size() != set2.size()) {
    throw new IllegalArgumentException("Can't merge sets of different size");
}

---

For reference, you could make this using Java 8, using the zip utility from this answer:

Set<String> set3 = zip(set1.stream(), set2.stream(), String::concat)
                            .collect(Collectors.toCollection(TreeSet::new));

Answer 2

Here's a way to do this the imperative way, with Java 7:

Set<String> set1 = new TreeSet<String>(Arrays.asList(new String[]{"A", "C", "E"}));
Set<String> set2 = new TreeSet<String>(Arrays.asList(new String[]{"B", "D", "F"}));
Set<String> set3 = new TreeSet<String>();

// Java 7: imperative
// edit: checking sizes beforehand
if (set1.size() != set2.size()) 
    throw new IllegalStateException("Sets aren't the same size!");
Iterator<String> iterator1 = set1.iterator();
Iterator<String> iterator2 = set2.iterator();
while (iterator1.hasNext()) {
    String s = iterator1.next();
    // assuming iterator2.hasNext() as sizes were checked previously
    s = s.concat(iterator2.next());
    set3.add(s);
}
System.out.println(set3);

Output

[AB, CD, EF]

Java 8 variant with functional idiom (a little ugly)

// Java 8: functional (but ugly)
Iterator<String> iterator2new = set2.iterator();
// edited, ensuring a `TreeSet` is returned
set3 = 
    set1
    .stream()
    .map((s) -> s.concat(iterator2new.next()))
    .collect(Collectors.toCollection(TreeSet::new));
System.out.println(set3);

Output

[AB, CD, EF]

Answer 3

Another way to do this is to use a library that adds zip to Java 8 streams such as https://github.com/poetix/protonpack or https://github.com/jOOQ/jOOL

Answer 4

I quite dislike while (it1.hasNext()), so my ways:

1)

    Iterator<String> iterator2 = set2.iterator();
    for(String s : set1) {
            set3.add(s.concat(iterator2.next()));
    }
    System.out.println(set3);

2) or

    List<String> list = new ArrayList<String>(set2);
    int i = 0;
    for(String s : set1) {
            set3.add(s.concat(list.get(i)));
            i++;
    }
    System.out.println(set3);

3) (1) with check:

    Iterator<String> iterator2 = set2.iterator();
    for(String s : set1) {
        set3.add(s.concat(iterator2.hasNext()? iterator2.next(): ""));
    }
    System.out.println(set3);

4) (2) with check:

    List<String> list = new ArrayList<String>(set2);
    int i = 0;
    for(String s : set1) {
            set3.add(s.concat(list.size() > i? list.get(i): ""));
            i++;
    }
    System.out.println(set3);