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Bind/Flat map nested stream
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| import lombok.extern.slf4j.Slf4j; | |
| import java.util.ArrayList; | |
| import java.util.List; | |
| import java.util.function.Function; | |
| import java.util.stream.Collectors; | |
| import java.util.stream.Stream; | |
| import static java.lang.String.format; | |
| import static java.util.Arrays.asList; | |
| @Slf4j | |
| public class BindNestedStreamSample { | |
| public static void main(String[] args) { | |
| List<ClassA> aLists = asList( | |
| new ClassA("1", | |
| asList( | |
| new ClassB("a", asList( | |
| new ClassC("I"))))), | |
| new ClassA("2", | |
| asList( | |
| new ClassB("a", asList( | |
| new ClassC("I"), | |
| new ClassC("II") | |
| )))) | |
| ); | |
| System.out.println("Imperative collect: "+imperativeCollect(aLists)); | |
| System.out.println("Declarative collect: "+declarativeCollect(aLists)); | |
| } | |
| private static List<String> imperativeCollect(List<ClassA> aList){ | |
| List<String> strings = new ArrayList<>(); | |
| for (ClassA a: aList){ | |
| for(ClassB b: a.bList){ | |
| for(ClassC c: b.cList) { | |
| String string = stringify(a).apply(b).apply(c); | |
| strings.add(string); | |
| } | |
| } | |
| } | |
| return strings; | |
| } | |
| private static List<String> declarativeCollect(List<ClassA> aList){ | |
| return bindNestedStream(BindNestedStreamSample::stringify) | |
| .apply(aList.stream()) | |
| .apply(a -> a.bList.stream()) | |
| .apply(b -> b.cList.stream()) | |
| .collect(Collectors.toList()); | |
| } | |
| private static Function<ClassB, Function<ClassC, String>> stringify(ClassA a){ | |
| return b -> c -> format("%s-%s-%s", a.name, b.name, c.name); | |
| } | |
| //flattening nested streams with reference to parent. A comprehension pattern | |
| //equivalent to | |
| // | |
| //Stream aStream; | |
| //Stream bStream = a -> stream(a) | |
| //Stream cStream = b -> stream(b) | |
| // | |
| //for(A a: aStream){ | |
| // for(B b: bStream(a)){ | |
| // for(C c: cStream(b)){ | |
| // f(a, b, c) | |
| // } | |
| // } | |
| //} | |
| private static <A, B, C, D> Function<Stream<A>, | |
| Function<Function<A, Stream<B>>, | |
| Function<Function<B, Stream<C>>, Stream<D>>>> bindNestedStream( | |
| Function<A, Function<B, Function<C, D>>> f) { | |
| return a -> b -> c -> | |
| a.flatMap(x -> | |
| b.apply(x).flatMap(y -> | |
| c.apply(y).map(z -> | |
| f.apply(x).apply(y).apply(z) | |
| ) | |
| ) | |
| ); | |
| } | |
| private static class ClassA { | |
| private final String name; | |
| final List<ClassB> bList; | |
| private ClassA(String name, List<ClassB> bList) { | |
| this.name = name; | |
| this.bList = bList; | |
| } | |
| } | |
| private static class ClassB { | |
| private final String name; | |
| final List<ClassC> cList; | |
| private ClassB(String name, List<ClassC> cList) { | |
| this.name = name; | |
| this.cList = cList; | |
| } | |
| } | |
| private static class ClassC { | |
| private final String name; | |
| ClassC(String name) { | |
| this.name = name; | |
| } | |
| } | |
| } |
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