Java 8 thread concatenates and returns multiple values

Question

I am porting a piece of code from .NET to Java and stumbled upon a script in which I want to use a stream for matching and pruning.

class Content { private String propA, propB, propC; Content(String a, String b, String c) { propA = a; propB = b; propC = c; } public String getA() { return propA; } public String getB() { return propB; } public String getC() { return propC; } } List<Content> contentList = new ArrayList(); contentList.add(new Content("A1", "B1", "C1")); contentList.add(new Content("A2", "B2", "C2")); contentList.add(new Content("A3", "B3", "C3")); 

I want to write a function that can pass the contents of the contents of a content list and return a class with the result

 content { propA = "A1, A2, A3", propB = "B1, B2, B3", propC = "C1, C2, C3" } 

I am new to Java, so you can find code similar to C # than to java

Answer 1

You can use the correct lambda for BinaryOperator in the reduction function.

 Content c = contentList .stream() .reduce((t, u) -> new Content( t.getA() + ',' + u.getA(), t.getB() + ',' + u.getB(), t.getC() + ',' + u.getC()) ).get(); 

Answer 2

 static Content merge(List<Content> list) { return new Content( list.stream().map(Content::getA).collect(Collectors.joining(", ")), list.stream().map(Content::getB).collect(Collectors.joining(", ")), list.stream().map(Content::getC).collect(Collectors.joining(", "))); } 

---

EDIT: Extending the built-in Federico collector, here is a specific class dedicated to combining Content objects:

 class Merge { public static Collector<Content, ?, Content> collector() { return Collector.of(Merge::new, Merge::accept, Merge::combiner, Merge::finisher); } private StringJoiner a = new StringJoiner(", "); private StringJoiner b = new StringJoiner(", "); private StringJoiner c = new StringJoiner(", "); private void accept(Content content) { a.add(content.getA()); b.add(content.getB()); c.add(content.getC()); } private Merge combiner(Merge second) { a.merge(second.a); b.merge(second.b); c.merge(second.c); return this; } private Content finisher() { return new Content(a.toString(), b.toString(), c.toString()); } } 

Used as:

 Content merged = contentList.stream().collect(Merge.collector()); 

Answer 3

The most common way to solve such problems is to combine the result of several collectors into one.

Using the jOOL library, you can get the following:

 Content content = Seq.seq(contentList) .collect( Collectors.mapping(Content::getA, Collectors.joining(", ")), Collectors.mapping(Content::getB, Collectors.joining(", ")), Collectors.mapping(Content::getC, Collectors.joining(", ")) ).map(Content::new); 

This creates Seq from the input list and combines the 3 given collectors to create Tuple3 , which is simply a holder for 3 values. Then these 3 values ​​are displayed in Content using the new Content(a, b, c) constructor. The collectors themselves simply map each Content to its value a , b or c and combine the results together, separating them with ", " .

---

Without third-party help, we could create our own combiner collector (this is based on StreamEx pairing , which does the same for 2 collectors). It takes 3 collectors as arguments and performs a finisher operation based on the result of 3 collected values.

 public interface TriFunction<T, U, V, R> { R apply(T t, U u, V v); } public static <T, A1, A2, A3, R1, R2, R3, R> Collector<T, ?, R> combining(Collector<? super T, A1, R1> c1, Collector<? super T, A2, R2> c2, Collector<? super T, A3, R3> c3, TriFunction<? super R1, ? super R2, ? super R3, ? extends R> finisher) { final class Box<A, B, C> { A a; B b; C c; Box(A a, B b, C c) { this.a = a; this.b = b; this.c = c; } } EnumSet<Characteristics> c = EnumSet.noneOf(Characteristics.class); c.addAll(c1.characteristics()); c.retainAll(c2.characteristics()); c.retainAll(c3.characteristics()); c.remove(Characteristics.IDENTITY_FINISH); return Collector.of( () -> new Box<>(c1.supplier().get(), c2.supplier().get(), c3.supplier().get()), (acc, v) -> { c1.accumulator().accept(acc.a, v); c2.accumulator().accept(acc.b, v); c3.accumulator().accept(acc.c, v); }, (acc1, acc2) -> { acc1.a = c1.combiner().apply(acc1.a, acc2.a); acc1.b = c2.combiner().apply(acc1.b, acc2.b); acc1.c = c3.combiner().apply(acc1.c, acc2.c); return acc1; }, acc -> finisher.apply(c1.finisher().apply(acc.a), c2.finisher().apply(acc.b), c3.finisher().apply(acc.c)), c.toArray(new Characteristics[c.size()]) ); } 

and finally use it with

 Content content = contentList.stream().collect(combining( Collectors.mapping(Content::getA, Collectors.joining(", ")), Collectors.mapping(Content::getB, Collectors.joining(", ")), Collectors.mapping(Content::getC, Collectors.joining(", ")), Content::new )); 

Answer 4

If you do not want to repeat three times in the list or do not want to create too many intermediate Content objects, then you will need to assemble a stream with your own implementation:

 public static Content collectToContent(Stream<Content> 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 three empty StringBuilder objects. It then defines a battery that adds each property of the Content element to the corresponding StringBuilder . Then it defines the merge function, which is used only when the stream is processed in parallel, which combines the two previously accumulated partial results. Finally, it also defines a finisher function that converts 3 StringBuilder objects to a new Content instance with each property corresponding to the accumulated lines of the previous steps.

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