Java 8 stream join and return multiple values

Question

I\'m porting a piece of code from .NET to Java and stumbled upon a scenario where I want to use stream to map & reduce.

class Content
{
  private String          


        

Answer 1

If you don't want to iterate 3 times over the list, or don't want to create too many Content intermediate objects, then you'd need to collect the stream with your own implementation:

public static Content collectToContent(Stream stream) {
    return stream.collect(
        Collector.of(
            () -> new StringBuilder[] {
                    new StringBuilder(),
                    new StringBuilder(),
                    new StringBuilder() },
            (StringBuilder[] arr, Content elem) -> {
                arr[0].append(arr[0].length() == 0 ? 
                        elem.getA() : 
                        ", " + elem.getA());
                arr[1].append(arr[1].length() == 0 ? 
                        elem.getB() : 
                        ", " + elem.getB());
                arr[2].append(arr[2].length() == 0 ? 
                        elem.getC() : 
                        ", " + elem.getC());
            },
            (arr1, arr2) -> {
                arr1[0].append(arr1[0].length() == 0 ?
                        arr2[0].toString() :
                        arr2[0].length() == 0 ?
                                "" :
                                ", " + arr2[0].toString());
                arr1[1].append(arr1[1].length() == 0 ?
                        arr2[1].toString() :
                        arr2[1].length() == 0 ?
                                "" :
                                ", " + arr2[1].toString());
                arr1[2].append(arr1[2].length() == 0 ?
                        arr2[2].toString() :
                        arr2[2].length() == 0 ?
                                "" :
                                ", " + arr2[2].toString());
                return arr1;
            },
            arr -> new Content(
                    arr[0].toString(), 
                    arr[1].toString(), 
                    arr[2].toString())));
}

This collector first creates an array of 3 empty StringBuilder objects. Then defines an accumulator that appends each Contentelement's property to the corresponding StringBuilder. Then it defines a merge function that is only used when the stream is processed in parallel, which merges two previously accumulated partial results. Finally, it also defines a finisher function that transforms the 3 StringBuilder objects into a new instance of Content, with each property corresponding to the accumulated strings of the previous steps.

Please check Stream.collect() and Collector.of() javadocs for further reference.