JDK1.8 stream流再会
一、最大值、最小值、平均值、个数
import lombok.Data;
import lombok.NoArgsConstructor;
import java.util.*;
import java.util.stream.Collectors;
@Data
@NoArgsConstructor
public class Dish {
private Double calories;
public Dish(Double calories) {
this.calories = calories;
}
public static void main(String[] args) {
List<Dish> list = new ArrayList<>();
list.add(new Dish(100.0));
list.add(new Dish(200.0));
list.add(new Dish(300.0));
//查找流中的最大值和最小值
Optional<Dish> max1 = list.stream().collect(Collectors.maxBy(Comparator.comparing(Dish::getCalories)));
Optional<Dish> min2 = list.stream().min(Comparator.comparing(Dish::getCalories));
max1.ifPresent(dish -> System.out.println("最大值:" + dish.getCalories()));
min2.ifPresent(dish -> System.out.println("最小值:" + dish.getCalories()));
//求和
Double sum1 = list.stream().collect(Collectors.summingDouble(Dish::getCalories));
Double sum2 = list.stream().mapToDouble(Dish::getCalories).sum();
System.out.println("求和:" + sum1);
System.out.println("求和:" + sum2);
//求平均
Double avg1 = list.stream().collect(Collectors.averagingDouble(Dish::getCalories));
System.out.println("平均值:" + avg1);
//总体分析
DoubleSummaryStatistics statistics = list.stream().collect(Collectors.summarizingDouble(Dish::getCalories));
//分析结果DoubleSummaryStatistics{count=3, sum=600.000000, min=100.000000, average=200.000000, max=300.000000}
System.out.println("分析结果" + statistics);
System.out.println("最大" + statistics.getMax());
System.out.println("最小" + statistics.getMin());
System.out.println("平均" + statistics.getAverage());
System.out.println("总和" + statistics.getSum());
System.out.println("个数" + statistics.getCount());
}
}
二、分区
//超过150卡路里的算true,否则算false
Predicate<Dish> predicate = (Dish e) -> e.getCalories().compareTo(150.0) > 0;
Map<Boolean, List<Dish>> map = list.stream().collect(Collectors.partitioningBy(predicate));
//分区后:{false=[Dish(calories=100.0)], true=[Dish(calories=200.0), Dish(calories=300.0)]}
System.out.println("分区后:" + map);
分组带排序
使用LinkedHashMap而不是Map接口接收最后的分组结果。
三、排序
@Data
@NoArgsConstructor
public class Dish {
private Double calories;
private String name;
public Dish(Double calories) {
this.calories = calories;
}
public Dish(Double calories, String name) {
this.calories = calories;
this.name = name;
}
public static void main(String[] args) {
List<Dish> list = new ArrayList<>();
list.add(new Dish(100.0, "A"));
list.add(new Dish(200.0, "C"));
list.add(new Dish(300.0, "B"));
list.add(new Dish(400.0, "A"));
//提取出名称:[A, C, B, A]
List<String> names = list.stream().map(Dish::getName).collect(Collectors.toList());
System.out.println("提取出名称" + names);
//按照名称排序:按照名称排序
// [
// Dish(calories=100.0, name=A),
// Dish(calories=400.0, name=A),
// Dish(calories=300.0, name=B),
// Dish(calories=200.0, name=C)
// ]
List<Dish> names2 = list.stream().sorted(Comparator.comparing(Dish::getName)).collect(Collectors.toList());
System.out.println("按照名称排序" + names2);
//按照名称倒序[C, B, A, A]
List<String> names3 = list.stream()
.sorted(Comparator.comparing(Dish::getName).reversed())
.map(Dish::getName).collect(Collectors.toList());
System.out.println("按照名称倒序" + names3);
四、截断
@Data
@NoArgsConstructor
public class Dish {
private Double calories;
private String name;
public Dish(Double calories) {
this.calories = calories;
}
public Dish(Double calories, String name) {
this.calories = calories;
this.name = name;
}
public static void main(String[] args) {
List<Dish> list = new ArrayList<>();
list.add(new Dish(100.0, "A"));
list.add(new Dish(200.0, "C"));
list.add(new Dish(300.0, "B"));
list.add(new Dish(400.0, "A"));
//截断前2个[A, C]
List<String> twoNameList = list.stream().limit(2).map(Dish::getName).collect(Collectors.toList());
System.out.println("截断前2个" + twoNameList);
//忽略两个后卡路里之和700.0
Double skipTwoSum = list.stream().skip(2).collect(Collectors.summingDouble(Dish::getCalories));
System.out.println("忽略两个后卡路里之和" + skipTwoSum);