怎么在Java8中使用Collectors实现一个求和功能？相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
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;
}
}@FunctionalInterface
public interface ToBigDecimalFunction<T> {
BigDecimal applyAsInt(T value);
}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
创建ToBigDecimalFunction接口
import java.math.BigDecimal;
@FunctionalInterface
public interface ToBigDecimalFunction<T> {
BigDecimal applyAsBigDecimal(T value);
}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);
}
}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);
}
}看完上述内容，你们掌握怎么在Java8中使用Collectors实现一个求和功能的方法了吗？如果还想学到更多技能或想了解更多相关内容，欢迎关注亿速云行业资讯频道，感谢各位的阅读！
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