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  1. import java.util.*;
  2. import java.util.stream.*;
  3.  
  4. /* Name of the class has to be "Main" only if the class is public. */
  5. public class Main {
  6.  
  7. 	static List<Foo> joinedCategoriesMap(List<Foo> input) {
  8. 	    return input
  9. 	            .stream()
  10. 	            .collect(Collectors.groupingBy(Foo::getId))
  11. 	            .entrySet().stream()
  12. 	            .map(e -> new Foo(
  13. 	                    e.getKey(),
  14. 	                    e.getValue().get(0).name,
  15. 	                    e.getValue().stream().map(Foo::getCategory).collect(Collectors.toList()))
  16. 	            )
  17. 	            .collect(Collectors.toList());
  18. 	}
  19.  
  20. 	static List<Foo> joinedCategoriesReduce(List<Foo> input) {
  21.         return input
  22.                 .stream()
  23.                 .collect(Collectors.groupingBy(Foo::getId))  // Map<Integer, List<Foo>>
  24.                 .entrySet().stream()
  25.                 .map(e -> e.getValue().stream() // Stream<Foo>
  26.                         .reduce(new Foo(e.getKey(), e.getValue().get(0).getName(), (String)null), Foo::merge)
  27.                 )
  28.                 .collect(Collectors.toList());
  29.     }
  30.  
  31. 	public static void main (String[] args) throws java.lang.Exception {
  32. 		final Foo f1 = new Foo(1L, "a", "c1");
  33. 		final Foo f2 = new Foo(1L, "a", "c2");
  34. 		final Foo f3 = new Foo(2L, "a", "c1");
  35.  
  36. 		final List<Foo> li = List.of(f1, f2, f3);
  37.  
  38. 		System.out.println("Map: ");
  39. 		joinedCategoriesMap(li).forEach(System.out::println);
  40.  
  41. 		System.out.println("Reduce: ");
  42.         joinedCategoriesReduce(li).forEach(System.out::println);
  43. 	}
  44.  
  45.     static class Foo {
  46.         private Long id;
  47.         private String name;
  48.         private String category;
  49.         private List<String> categories;
  50.  
  51.         Foo(Long id, String name, String cat) {
  52.             this.id = id;
  53.             this.name = name;
  54.             this.category = cat;
  55.         }
  56.  
  57.         Foo(Long id, String name, List<String> cats) {
  58.             this.id = id;
  59.             this.name = name;
  60.             this.categories = cats;
  61.         }
  62.  
  63.         static Foo merge(Foo accum, Foo other) {
  64.             if (null == accum.categories) {
  65.                 accum.categories = new ArrayList<>();
  66.                 if (null != accum.category) {
  67.                     accum.categories.add(accum.category);
  68.                     accum.category = null;
  69.                 }
  70.             }
  71.             accum.categories.add(other.category);
  72.  
  73.             return accum;
  74.         }
  75.  
  76.         public Long getId() { return id; }
  77.         public void setId(Long id) { this.id = id; }
  78.  
  79.         public String getName() { return name; }
  80.         public void setName(String name) { this.name = name; }
  81.  
  82.         public String getCategory() { return category; }
  83.  
  84.         @Override
  85.         public String toString() {
  86.             return String.format("Foo { id=%d, name=%s, category=%s, categories=%s }", id, name, category, categories);
  87.         }
  88.     }
  89. }
Success #stdin #stdout 0.15s 34784KB
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stdin
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Standard input is empty
stdout
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Map: 
Foo { id=1, name=a, category=null, categories=[c1, c2] }
Foo { id=2, name=a, category=null, categories=[c1] }
Reduce: 
Foo { id=1, name=a, category=null, categories=[c1, c2] }
Foo { id=2, name=a, category=null, categories=[c1] }
https://ideone.com/EJEJA6
language:
Java (HotSpot 12)
created:
4 years ago
visibility:
public

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