Convert String array to StringBuilder array

Arrays.setAll() seems to be the simplest one, it may be shorter to type than a loop:

    String[] str = new String[] {"hi","hello"};
    StringBuilder[] sbs = new StringBuilder[str.length];
    Arrays.setAll(sbs, i -> new StringBuilder(str[i]));

There is even parallelSetAll(), but for simply creating objects it would have to be a really long array to profit from parallelism I think.

You won't believe how it looks like on the inside:

public static <T> void setAll(T[] array, IntFunction<? extends T> generator) {
    Objects.requireNonNull(generator);
    for (int i = 0; i < array.length; i++)
        array[i] = generator.apply(i);
}

(The parallel one uses IntStream.range().parallel().forEach())

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Original answer
No loops were harmed during the making of these lines:

String[] str = new String[] {"hi","hello"};
StringBuilder[] sbs=Arrays.stream(str)
                          .map(s->new StringBuilder(s))
                          //.collect(Collectors.toList()) // not needed, see comments
                                                          // of @RealSkeptic and @Holger
                          .toArray(StringBuilder[]::new);

So yes of course, it can be done without writing the single loop statement. Just in the background there will be two loops now, one for creating the StringBuilders into a List, and one for converting it to an array. And I still would not call it particularly beautiful either, especially when considering that a similar bunch of lines will be there too at the end, for the other direction.

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If the modifications are independent, and the ends have to stay arrays, I would indeed go with a single, "classic" indexed for loop, doing everything in the body:

String[] str = new String[] {"hi","hello"};
for(int i = 0; i < str.length; i++) {
  StringBuilder sb = new StringBuilder(str[i]);
  ...magic...
  str[i] = sb.toString(); // or perhaps into a new array
}