在Java8中我们可以使用Stream.reduce()
来对List<BigDecimal>
进行求和。
1. Stream.reduce()
下面例子演示了使用循环语句和Stream.reduce()
来对BigDecimal求和。
JavaBigDecimal.java
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));
// 使用for循环对BigDecimal求和
BigDecimal sum = BigDecimal.ZERO;
for (BigDecimal amt : invoices) {
sum = sum.add(amt);
}
System.out.println("Sum = " + sum);
// 使用Stream求和
BigDecimal sum2 = invoices.stream().reduce(BigDecimal.ZERO, BigDecimal::add);
System.out.println("Sum (Stream) = " + sum2);
}
}
输出
Sum = 36.59
Sum (Stream) = 36.59
2. Map与Reduce
下面这个例子对所有发票的金额进行求和。
JavaBigDecimalObject.java
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;
}
}