I am learning Stream api and I am stuck at reduce method of Stream. I have stream of some string and I want to concate them. Below is my code snippet:

```java
Stream<String> data = Stream.of("a", "b", "c", "d", "e", "f");
String col = data.parallel().reduce("dummy",
        (t, u) -> {
            return t + " : " + u;
        },
        (a, b) -> {
            return a + b;
        });
System.out.println(col);
//Output: dummy : adummy : bdummy : cdummy : ddummy : edummy : f
//Expected: dummy : a : b : c : d : e : f

```

I don't want them in sequence but at least they should not have repeated "dummy" String.

It results in a weird output. Please help me to find what's wrong here.