コレクションをアレコレするのをLambdaで書いてみた

gistfile1.groovy

List list = [
    [color:"blue", weight:10],
    [color:"red", weight:30],
    [color:"blue", weight:50],
]

// groovy で Lambda を真似る
int weight = list
    .findAll { it.color == 'blue' }
    .collect { it.weight }
    .inject(0) { acc, e -> acc + e }
assert weight == 60

hoge.groovy

// Groovyのを何パターンか書いてみる

// filter して map して reduce する
list.findAll { it.color == 'blue' }
    .collect { it.weight }
    .inject(0) { acc, e -> acc + e }

// 足すだけだったら sum がありますね
list.findAll { it.color == 'blue' }
    .collect { it.weight }
    .sum()

// weight引っぱりだすくらいならcollectなんていらない
list.findAll { it.color == 'blue' }.weight.sum()

// ところでsumってクロージャ引数にとれるんですよ
list.findAll { it.color == 'blue' }.sum { it.weight }

// 開き直るとsumだけで済んだりも……
list.sum { it.color == 'blue' ? it.weight : 0 }

Hoge.java

import java.util.*;

public class Hoge {

  static class Fuga {
    final String color;
    final int weight;
    Fuga(String color, int weight) {
      this.color = color;
      this.weight = weight;
    }
  }

  public static void main(String... args) {
    List<Fuga> list = new ArrayList<>();
    list.add(new Fuga("blue", 10));
    list.add(new Fuga("red", 30));
    list.add(new Fuga("blue", 50));

    // Java SE 8 の Lambda で書くとこう
    int weight = list.stream()
      .filter( it -> it.color.equals("blue") )
      .map( it -> it.weight )
      .reduce(0, (acc, e) -> acc + e );

    System.out.println(weight);
  }
}

Hoge7.java

import java.util.*;

public class Hoge {

  static class Fuga {
    final String color;
    final int weight;
    Fuga(String color, int weight) {
      this.color = color;
      this.weight = weight;
    }
  }

  public static void main(String... args) {
    List<Fuga> list = new ArrayList<>();
    list.add(new Fuga("blue", 10));
    list.add(new Fuga("red", 30));
    list.add(new Fuga("blue", 50));

    // Java SE 7 以前の Java で書くとこんな感じ
    int weight1 = 0;
    for (Fuga e : list) {
        if ("blue".equals(e.color)) {
            weight1 += e.weight;
        }
    }

    System.out.println(weight);
  }
}