lambda 表达式：

        Map<WeightLevel, List<Human>> map = humans.stream().collect(groupingBy((human) -> {if(human.getWeight() < 80) return WeightLevel.LIGHT;else if(human.getWeight() >= 80 && human.getWeight() <= 140) return WeightLevel.MEDIUM;else return WeightLevel.HEAVY;}));

方法引用：

Map<WeightLevel, List<Human>> map = humans.stream().collect(groupingBy(Human::getWeightLevel));

完整代码。根据体重分类：

public class ReafctorTest {public static void main(String[] args) {List<Human> humans = asList(new Human(70), new Human(180), new Human(190));//        Map<WeightLevel, List<Human>> map = humans.stream().collect(groupingBy((human) -> {
//            if(human.getWeight() < 80) return WeightLevel.LIGHT;
//            else if(human.getWeight() >= 80 && human.getWeight() <= 140) return WeightLevel.MEDIUM;
//            else return WeightLevel.HEAVY;
//        }));Map<WeightLevel, List<Human>> map = humans.stream().collect(groupingBy(Human::getWeightLevel));System.out.println(map);}}
enum WeightLevel {LIGHT, MEDIUM, HEAVY}
class Human{private int weight;public int getWeight() {return weight;}Human(int weight){this.weight = weight;}public void setWeight(int weight) {this.weight = weight;}public WeightLevel getWeightLevel(){if(weight < 80) return WeightLevel.LIGHT;else if(weight >= 80 && weight <= 140) return WeightLevel.MEDIUM;else return WeightLevel.HEAVY;}
}

可用jdk静态方法替代部分常用lambda

比较lambda 表达式：

humans.sort((o1, o2) -> o1.getWeight().compareTo(o2.getWeight()) );

替换成静态方法：

humans.sort(comparing(Human::getWeight));

完整代码：

按体重排序

public class ReafctorTest {public static void main(String[] args) {List<Human> humans = asList(new Human(70), new Human(180), new Human(190), new Human(50));humans.sort(comparing(Human::getWeight));
//        humans.sort((o1, o2) -> o1.getWeight().compareTo(o2.getWeight()) );System.out.println(humans);}}
class Human{private Integer weight;public Integer getWeight() {return weight;}Human(int weight){this.weight = weight;}public String toString(){return "my weight is " +weight;}
}

除了comparing静态方法，还有maxBy也是：

import java.util.Arrays;
import java.util.List;import static java.util.Comparator.comparing;
import static java.util.stream.Collectors.maxBy;public class TestMaxBy {public static void main(String[] args) {List<Human> humans = Arrays.asList(new Human(100), new Human(199), new Human(89));Human human = humans.stream().collect(maxBy(comparing(Human::getWeight))).get();System.out.println(human);}private static class Human{private int weight;Human(int weight){this.weight = weight;}public int getWeight(){return weight;}public String toString(){return "my weight is: " + weight;}}
}

使用sumingInt:

import java.util.Arrays;
import java.util.List;public class TestMaxBy {public static void main(String[] args) {List<Human> humans = Arrays.asList(new Human(100), new Human(199), new Human(89));
//        int sumWeight = humans.stream().collect(summingInt(Human::getWeight));//method referenceint sumWeight = humans.stream().map(e -> e.getWeight()).reduce(0, (e1, e2) -> e1 + e2);//用reduce以及lambdaSystem.out.println(sumWeight);}private static class Human{private int weight;Human(int weight){this.weight = weight;}public int getWeight(){return weight;}public String toString(){return "my weight is: " + weight;}}
}