java - ストリーム groupingBy: リストの最初の要素に減らす

Question

私はList<Valuta>（簡略化された）JSONスタイルで表現できる を持っています：

[ { コード = ユーロ、説明 = ユーロ、比率 = 1 }、{ コード = 米ドル、説明 = ドル、比率 = 1.1 } ]

私はそれを次のように変換したいMap<String, Valuta>:

{ EUR={ codice=EUR, description=ユーロ, ratio=1 }, USD={ codice=USD, description=ドル, ratio=1.1 }}

私はこのワンライナーを書きました：

getValute().stream().collect(Collectors.groupingBy(Valuta::getCodice));

Map<String, List<Valuta>>しかし、これは必要なものの代わりにa を返します。

関数は私にとってはうまくいくと思いmapping()ますが、方法がわかりません。

Answer 1

例外をスローする代わりに、衝突で最初のtoMap値を選択することを選択するバージョンは次のとおりです。

Collectors.toMap(keyMapper, valueMapper, mergeFunction)元：

Map<String, Valuta> map = list
        .stream()
        .collect(Collectors.toMap(Valuta::getCodice, v -> v, (v1, v2) -> v1));

Answer 2

3つの方法があります。

public class Test1 {
  static class Foo {
    public int id, targetCost, actualCost;
    public String ref;

    public Foo(int id, String ref, int actualCost, int targetCost) {

      this.id = id;
      this.targetCost = targetCost;
      this.actualCost = actualCost;
      this.ref = ref;
    }

    public int getId() {
      return id;
    }

    public void setId(int id) {
      this.id = id;
    }

    public int getTargetCost() {
      return targetCost;
    }

    public void setTargetCost(int targetCost) {
      this.targetCost = targetCost;
    }

    public int getActualCost() {
      return actualCost;
    }

    public void setActualCost(int actualCost) {
      this.actualCost = actualCost;
    }

    public String getRef() {
      return ref;
    }

    public void setRef(String ref) {
      this.ref = ref;
    }

    @Override
    public String toString() {
      return " [id=" + id + ", targetCost="
    + targetCost + ", " + "actualCost=" 
          + actualCost + ", ref=" + ref
          + "]";
    }

  }// foo

  public static void main(String[] args) {

    List<Foo> list = Arrays.asList(

        new Foo(1, "P1", 300, 400), new Foo(2, "P2", 600, 400), new Foo(3, "P3", 30, 20),
        new Foo(3, "P3", 70, 20), new Foo(1, "P1", 360, 40), new Foo(4, "P4", 320, 200),
        new Foo(4, "P4", 500, 900)

    );
    // Method 1  : 
    Map<Integer, List<Foo>> collect = list.stream()
        .collect(
            Collectors.groupingBy(
                Foo::getId, 
                Collectors.collectingAndThen(

            Collectors.toList(),

            Function.identity()

        )// andthen

        )// gr

        );
    System.out.println(collect);
    /*
    {
        1=[ [id=1, targetCost=400, actualCost=300, ref=P1],
        id=1, targetCost=40, actualCost=360, ref=P1]],

         2=[ [id=2, targetCost=400, actualCost=600, ref=P2]],

          3=[ [id=3, targetCost=20, actualCost=30, ref=P3], 
           [id=3, targetCost=20, actualCost=70, ref=P3]], 

           4=[ [id=4, targetCost=200, actualCost=320, ref=P4], 
            [id=4, targetCost=900, actualCost=500, ref=P4]]

          }
  */

    // Method 2

    Map<Integer, List<Foo>> collect2 = list.stream().collect(
        Collectors.groupingBy(
            Foo::getId, 
            Collectors.mapping(
                Function.identity(), 
                Collectors.toList())));

    System.out.println(collect2);
    /*
     {
  1=[ [id=1, targetCost=400, actualCost=300, ref=P1], 
   [id=1, targetCost=40, actualCost=360, ref=P1]],

    2=[ [id=2, targetCost=400, actualCost=600, ref=P2]],

     3=[ [id=3, targetCost=20, actualCost=30, ref=P3],
       [id=3, targetCost=20, actualCost=70, ref=P3]],

        4=[ [id=4, targetCost=200, actualCost=320, ref=P4], 
         [id=4, targetCost=900, actualCost=500, ref=P4]]

       }

*/

    // Method 3 

    // If you need to compare something the you can use Compare.comparing

     Map<Integer, List<Foo>> collect3 = list
     .stream()
     .sorted( Comparator.comparing(Foo::getId)
         .thenComparing(Foo::getActualCost)
         .thenComparing(Foo::getTargetCost)    )

     .collect(                 

    Collectors.groupingBy(ch -> ch.id)     



         );

     System.out.println(collect3);


/*
{
  1=[ [id=1, targetCost=400, actualCost=300, ref=P1],  
  [id=1, targetCost=40, actualCost=360, ref=P1]],

   2=[ [id=2, targetCost=400, actualCost=600, ref=P2]],

    3=[ [id=3, targetCost=20, actualCost=30, ref=P3], 
     [id=3, targetCost=20, actualCost=70, ref=P3]],

      4=[ [id=4, targetCost=200, actualCost=320, ref=P4], 
       [id=4, targetCost=900, actualCost=500, ref=P4]]

     }
*/


  }// main

}