static <T> Collector<T,?,Double> averagingWeighted(ToDoubleFunction<T> valueFunction, ToIntFunction<T> weightFunction) {
class Box {
double num = 0;
long denom = 0;
}
return Collector.of(
Box::new,
(b, e) -> {
b.num += valueFunction.applyAsDouble(e) * weightFunction.applyAsInt(e);
b.denom += weightFunction.applyAsInt(e);
},
(b1, b2) -> { b1.num += b2.num; b1.denom += b2.denom; return b1; },
b -> b.num / b.denom
);
}
这个自定义收集器以两个函数作为参数: 第一个函数返回给定流元素的值 (作为 ToDoubleFunction
), 第二个函数返回权重 (作为 ToIntFunction
)。它使用辅助本地类在收集过程中存储分子和分母。每次接受一个条目时,分子会增加值与权重的乘积结果,并且分母会增加权重。最后,完成者将两者相除后返回一个 Double
。
一个示例用法如下:
Map<Double,Integer> map = new HashMap<>();
map.put(0.7, 100);
map.put(0.5, 200);
double weightedAverage =
map.entrySet().stream().collect(averagingWeighted(Map.Entry::getKey, Map.Entry::getValue));
/**
* Calculates the weighted average of a map.
*
* @throws ArithmeticException If divide by zero happens
* @param map A map of values and weights
* @return The weighted average of the map
*/
static Double calculateWeightedAverage(Map<Double, Integer> map) throws ArithmeticException {
double num = 0;
double denom = 0;
for (Map.Entry<Double, Integer> entry : map.entrySet()) {
num += entry.getKey() * entry.getValue();
denom += entry.getValue();
}
return num / denom;
}
您可以查看其单元测试以了解用例。
/**
* Tests our method to calculate the weighted average.
*/
@Test
public void testAveragingWeighted() {
Map<Double, Integer> map = new HashMap<>();
map.put(0.7, 100);
map.put(0.5, 200);
Double weightedAverage = calculateWeightedAverage(map);
Assert.assertTrue(weightedAverage.equals(0.5666666666666667));
}
你需要这些导入来进行单元测试:
import org.junit.Assert;
import org.junit.Test;
您需要这些导入代码:
import java.util.HashMap;
import java.util.Map;
public static double weightedAvg(Collection<Map.Entry<? extends Number, ? extends Number> data) {
var sumWeights = data.stream()
.map(Map.Entry::getKey)
.mapToDouble(Number::doubleValue)
.sum();
var sumData = data.stream()
.mapToDouble(e -> e.getKey().doubleValue() * e.getValue().doubleValue())
.sum();
return sumData / sumWeights;
}
static float weightedMean(List<Double> value, List<Integer> weighted, int n) {
int sum = 0;
double numWeight = 0;
for (int i = 0; i < n; i++) {
numWeight = numWeight + value.get(i).doubleValue() * weighted.get(i).intValue();
sum = sum + weighted.get(i).intValue();
}
return (float) (numWeight) / sum;
}
Collectors.averagingDouble
方法吗? - Aaron