Make a string from an IntStream of code point numbers?

or more direct to String by using an array passed to new String(…)

IntStream intStream = "input_goes_here".codePoints();

int[] arr;
String output = new String( (arr = intStream.toArray()), 0, arr.length );

and here's the original short solution without the superfluous IntStream intStream assignment:

int[] arr;
String output = new String( (arr = "input_goes_here".codePoints().toArray()), 0, arr.length );

not to forget the Java IO library:
use IntStream::collect with a StringWriter

String output = 
    "input_goes_here".codePoints() // Generates an IntStream of Unicode code points,
                                   //  one Integer for each character in the string.
    .collect(                      // Collect the results of processing each code point.
        StringWriter::new,         // Supplier<R> supplier
        StringWriter::write,       // ObjIntConsumer<R> accumulator
        (w1, w2) -> w1.write(      // BiConsumer<R,R> combiner
            w2.toString() ) )
    .toString();

Use IntStream::collect with a StringBuilder.

String output = 
    "input_goes_here"
    .codePoints()                            // Generates an `IntStream` of Unicode code points, one `Integer` for each character in the string.
    .collect(                                // Collect the results of processing each code point.
        StringBuilder::new,                  // Supplier<R> supplier
        StringBuilder::appendCodePoint,      // ObjIntConsumer<R> accumulator
        StringBuilder::append                // BiConsumer<R,​R> combiner
    )                                        
    .toString()
;

If you prefer the more general CharSequence interface over concrete String, simply drop the toString() at the end. The returned StringBuilder is a CharSequence.

IntStream codePointStream = "input_goes_here".codePoints ();
CharSequence output = codePointStream.collect ( StringBuilder :: new , StringBuilder :: appendCodePoint , StringBuilder :: append );