Java8 Collectors求和功能的自定义扩展操作

业务中需要将一组数据分类后收集总和，原本可以使用collectors.summingint()，但是我们的数据源是bigdecimal类型的，而java8原生只提供了summingint、summinglong、summingdouble三种基础类型的方法。

于是就自己动手丰衣足食吧。。

自定义工具类

public class mycollectors {
  private mycollectors() {
  }
//  public static <t> collector<t, ?, bigdecimal> summingbigdecimal(function<? super t, bigdecimal> mapper) {}
 	// bigdecimal 类型的集合求和
  public static <t> collector<t, ?, bigdecimal> summingbigdecimal(tobigdecimalfunction<? super t> mapper) {
    return new collectorimpl<>(
        () -> new bigdecimal[] { bigdecimal.zero },
        (a, t) -> a[0] = a[0].add(mapper.applyasint(t)),
        (a, b) -> {
          a[0] = a[0].add(b[0]);
          return a;
        },
        a -> a[0],
        collections.emptyset()
    );
  }
  static class collectorimpl<t, a, r> implements collector<t, a, r> {
    // 创建一个计算用的容器
    private final supplier<a> supplier;
    // 计算逻辑
    private final biconsumer<a, t> accumulator;
    // 合并逻辑
    private final binaryoperator<a> combiner;
    // 返回最终计算值
    private final function<a, r> finisher;
    // 空set
    private final set<characteristics> characteristics;
    collectorimpl(supplier<a> supplier, biconsumer<a, t> accumulator, binaryoperator<a> combiner,
           function<a, r> finisher, set<characteristics> characteristics) {
      this.supplier = supplier;
      this.accumulator = accumulator;
      this.combiner = combiner;
      this.finisher = finisher;
      this.characteristics = characteristics;
    }
    collectorimpl(supplier<a> supplier, biconsumer<a, t> accumulator, binaryoperator<a> combiner,
           set<characteristics> characteristics) {
      this(supplier, accumulator, combiner, castingidentity(), characteristics);
    }
    @override
    public biconsumer<a, t> accumulator() {
      return accumulator;
    }
    @override
    public supplier<a> supplier() {
      return supplier;
    }
    @override
    public binaryoperator<a> combiner() {
      return combiner;
    }
    @override
    public function<a, r> finisher() {
      return finisher;
    }
    @override
    public set<characteristics> characteristics() {
      return characteristics;
    }
  }
  @suppresswarnings("unchecked")
  private static <i, r> function<i, r> castingidentity() {
    return i -> (r) i;
  }
}

自定义函数式接口

@functionalinterface
public interface tobigdecimalfunction<t> {
  bigdecimal applyasint(t value);
}

测试入口

public class answerapp {
 public static void main(string[] args) {
    list<bigdecimal> list = lists.newarraylist();
    for (int i = 0; i < 24; i++) {
      list.add(bigdecimal.valueof(i + 10.2121543));
    }
    // 方式1
    bigdecimal sum = list.stream().collect(mycollectors.summingbigdecimal(e -> e));
    system.out.println(sum.doublevalue());
    // 方式2
    optional<bigdecimal> reduce = list.stream().reduce(bigdecimal::add);
    system.out.println(reduce.orelse(bigdecimal.valueof(0)));
 }    
}
// output: 521.0917032

补充：collectors扩展接口 实现bigdecimal的相加

第一步

创建tobigdecimalfunction接口

import java.math.bigdecimal;
@functionalinterface
public interface tobigdecimalfunction<t> {
  bigdecimal applyasbigdecimal(t value);
}

第二步

创建工具类 实现接口

import java.math.bigdecimal;
import java.util.collections;
import java.util.set;
import java.util.function.biconsumer;
import java.util.function.binaryoperator;
import java.util.function.function;
import java.util.function.supplier;
import java.util.stream.collector;
public class collectorsutil {
  static final set<collector.characteristics> ch_noid = collections.emptyset();
  private collectorsutil() {
  }
  @suppresswarnings("unchecked")
  private static <i, r> function<i, r> castingidentity() {
    return i -> (r) i;
  }
  /**
   * simple implementation class for {@code collector}.
   *
   * @param <t>
   *      the type of elements to be collected
   * @param <r>
   *      the type of the result
   */
  static class collectorimpl<t, a, r> implements collector<t, a, r> {
    private final supplier<a> supplier;
    private final biconsumer<a, t> accumulator;
    private final binaryoperator<a> combiner;
    private final function<a, r> finisher;
    private final set<characteristics> characteristics;
    collectorimpl(supplier<a> supplier, biconsumer<a, t> accumulator, binaryoperator<a> combiner,
           function<a, r> finisher, set<characteristics> characteristics) {
      this.supplier = supplier;
      this.accumulator = accumulator;
      this.combiner = combiner;
      this.finisher = finisher;
      this.characteristics = characteristics;
    }
    collectorimpl(supplier<a> supplier, biconsumer<a, t> accumulator, binaryoperator<a> combiner,
           set<characteristics> characteristics) {
      this(supplier, accumulator, combiner, castingidentity(), characteristics);
    }
    @override
    public biconsumer<a, t> accumulator() {
      return accumulator;
    }
    @override
    public supplier<a> supplier() {
      return supplier;
    }
    @override
    public binaryoperator<a> combiner() {
      return combiner;
    }
    @override
    public function<a, r> finisher() {
      return finisher;
    }
    @override
    public set<characteristics> characteristics() {
      return characteristics;
    }
  }
  public static <t> collector<t, ?, bigdecimal> summingbigdecimal(tobigdecimalfunction<? super t> mapper) {
    return new collectorimpl<>(() -> new bigdecimal[1], (a, t) -> {
      if (a[0] == null) {
        a[0] = bigdecimal.zero;
      }
      a[0] = a[0].add(mapper.applyasbigdecimal(t));
    }, (a, b) -> {
      a[0] = a[0].add(b[0]);
      return a;
    }, a -> a[0], ch_noid);
  }
}

使用测试

import com.example.javademo.javademoapplicationtests;
import com.example.javademo.pojo.student;
import com.example.javademo.utils.datautils;
import org.junit.test;
import java.math.bigdecimal;
import java.util.stream.collectors;
public class testbigdecimal extends javademoapplicationtests {
  @test
  public void testgroupbyafterbigdecimal(){
    /*
    自定义实现对分组后的集合，属性为bigdecmal进行相加
     */
    system.out.println(datautils.getdata().stream().collect(collectors.groupingby(student::getschool,collectorsutil.summingbigdecimal(student::getmoney))));
    //归约造作
    bigdecimal reduce = datautils.getdata().stream().map(student::getmoney).reduce(bigdecimal.zero, bigdecimal::add);
    system.out.println(reduce);
    int sum = datautils.getdata().stream().maptoint(student::getage).sum();
    system.out.println(sum);
  }
}

以上为个人经验，希望能给大家一个参考，也希望大家多多支持。如有错误或未考虑完全的地方，望不吝赐教。