java8 利用reduce实现将列表中的多个元素的属性求和并返回操作

A a = list.stream()
        .reduce(
            (x , y) -> new A( (x.getPrincipal() + y.getPrincipal()), (x.getFee() + y.getFee()) ) )
        .orElse( new A(0, 0) );

package org.byron4j.eight;

import java.util.ArrayList;
import java.util.List;

import org.junit.Test;

public class ReduceTwoObjectAddProp {


  class A{
    int principal = 0;
    int fee = 0;
    public A(int principal, int fee) {
      super();
      this.principal = principal;
      this.fee = fee;
    }
    public A() {
      super();
      // TODO Auto-generated constructor stub
    }
    public int getPrincipal() {
      return principal;
    }
    public void setPrincipal(int principal) {
      this.principal = principal;
    }
    public int getFee() {
      return fee;
    }
    public void setFee(int fee) {
      this.fee = fee;
    }
    @Override
    public String toString() {
      return "A [principal=" + principal + ", fee=" + fee + "]";
    }

  }


  @Test
  public void test() {


    List<A> list = new ArrayList<A>();
    list.add(new A(1, 2));
    list.add(new A(100, 200));

    A a = list.stream()
        .reduce(
            (x , y) -> new A( (x.getPrincipal() + y.getPrincipal()), (x.getFee() + y.getFee()) ) )
        .orElse( new A(0, 0) );
    System.out.println(a);
  }

}

package com.mkyong;
 
import java.math.BigDecimal;
import java.util.LinkedList;
import java.util.List;
 
public class JavaBigDecimal {
 
  public static void main(String[] args) {
 
    List<BigDecimal> invoices = new LinkedList<>();
    invoices.add(BigDecimal.valueOf(9.9));
    invoices.add(BigDecimal.valueOf(1.0));
    invoices.add(BigDecimal.valueOf(19.99));
    invoices.add(BigDecimal.valueOf(0.2));
    invoices.add(BigDecimal.valueOf(5.5));
 
    // sum using a for loop
    BigDecimal sum = BigDecimal.ZERO;
    for (BigDecimal amt : invoices) {
      sum = sum.add(amt);
    }
    System.out.println("Sum = " + sum);
 
    // sum using stream
    BigDecimal sum2 = invoices.stream().reduce(BigDecimal.ZERO, BigDecimal::add);
    System.out.println("Sum (Stream) = " + sum2);
 
  }
 
}

package com.mkyong;
 
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.Arrays;
import java.util.List;
 
public class JavaBigDecimalObject {
 
  public static void main(String[] args) {
 
    List<Invoice> invoices = Arrays.asList(
        new Invoice("I1001", BigDecimal.valueOf(9.99), BigDecimal.valueOf(1)),
        new Invoice("I1002", BigDecimal.valueOf(19.99), BigDecimal.valueOf(1.5)),
        new Invoice("I1003", BigDecimal.valueOf(4.888), BigDecimal.valueOf(2)),
        new Invoice("I1004", BigDecimal.valueOf(4.99), BigDecimal.valueOf(5)),
        new Invoice("I1005", BigDecimal.valueOf(.5), BigDecimal.valueOf(2.3))
    );
 
    BigDecimal sum = invoices.stream()
        .map(x -> x.getQty().multiply(x.getPrice()))  // map
        .reduce(BigDecimal.ZERO, BigDecimal::add);   // reduce
 
    System.out.println(sum);  // 75.851
    System.out.println(sum.setScale(2, RoundingMode.HALF_UP)); // 75.85
 
  }
}
 
class Invoice {
 
  String invoiceNo;
  BigDecimal price;
  BigDecimal qty;
 
  public Invoice(String invoiceNo, BigDecimal price, BigDecimal qty) {
    this.invoiceNo = invoiceNo;
    this.price = price;
    this.qty = qty;
  }
 
  public String getInvoiceNo() {
    return invoiceNo;
  }
 
  public void setInvoiceNo(String invoiceNo) {
    this.invoiceNo = invoiceNo;
  }
 
  public BigDecimal getPrice() {
    return price;
  }
 
  public void setPrice(BigDecimal price) {
    this.price = price;
  }
 
  public BigDecimal getQty() {
    return qty;
  }
 
  public void setQty(BigDecimal qty) {
    this.qty = qty;
  }
}