Java8：使用Lambda表达式增强版Comparator排序

旧城等待， 2021-06-22 15:36 383阅读 0赞

package com.server.model;
    
    /**
     * Created by jstarseven on 2016/3/21.
     */
    public class Human {
    
        private String name;
        private int age;
    
        @Override
        public String toString() {
            return "Human{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    
        public Human() {
            super();
        }
    
        public Human(final String name, final int age) {
            super();
            this.name = name;
            this.age = age;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public int getAge() {
            return age;
        }
    
        public void setAge(int age) {
            this.age = age;
        }
    
        public static int compareByNameThenAge(Human lhs, Human rhs) {
            if (lhs.getName().equals(rhs.getName())) {
                return lhs.getAge() - rhs.getAge();
            } else {
                return lhs.getName().compareTo(rhs.getName());
            }
        }
    
    
    }

package com.server.lambda;
    
    import com.server.model.Human;
    import org.junit.Test;
    
    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.Comparator;
    import java.util.List;
    
    /**
     * Created by jstarseven on 2016/3/21.
     */
    public class TestLambda {
    
        //    不使用Lambda表达式的基本排序
        @Test
        public void givenPreLambda_whenSortingEntitiesByName_thenCorrectlySorted() {
    
            List<Human> humans = new ArrayList();
            humans.add(new Human("A", 10));
            humans.add(new Human("C", 14));
            humans.add(new Human("C", 12));
            humans.add(new Human("B", 12));
            Collections.sort(humans, new Comparator<Human>() {
                public int compare(Human o1, Human o2) {
                    return o1.getName().compareTo(o2.getName());
                }
            });
            for (Human bean : humans) {
                System.out.println(bean.toString());
            }
        }
    
    
        //    使用Lambda表达式的基本排序
        @Test
        public void whenSortingEntitiesByName_thenCorrectlySorted() {
            List<Human> humans = new ArrayList();
            humans.add(new Human("A", 10));
            humans.add(new Human("C", 14));
            humans.add(new Human("C", 12));
            humans.add(new Human("B", 12));
            humans.sort((Human h1, Human h2) -> h1.getName().compareTo(h2.getName()));
            for (Human bean : humans) {
                System.out.println(bean.toString());
            }
        }
    
    
        //    没有类型定义（ Type Definitions）的基本排序
        @Test
        public void givenLambdaShortForm_whenSortingEntitiesByName_thenCorrectlySorted() {
            List<Human> humans = new ArrayList();
            humans.add(new Human("A", 10));
            humans.add(new Human("C", 14));
            humans.add(new Human("C", 12));
            humans.add(new Human("B", 12));
            humans.sort((h1, h2) -> h1.getName().compareTo(h2.getName()));
            for (Human bean : humans) {
                System.out.println(bean.toString());
            }
        }
    
        //    使用静态方法的引用来排序
        @Test
        public void givenMethodDefinition_whenSortingEntitiesByNameThenAge_thenCorrectlySorted() {
            List<Human> humans = new ArrayList();
            humans.add(new Human("A", 10));
            humans.add(new Human("C", 14));
            humans.add(new Human("C", 12));
            humans.add(new Human("B", 12));
            humans.sort(Human::compareByNameThenAge);
            for (Human bean : humans) {
                System.out.println(bean.toString());
            }
        }
    
        //    提取Comparator进行排序
        @Test
        public void givenInstanceMethod_whenSortingEntitiesByNameThenAge_thenCorrectlySorted() {
            List<Human> humans = new ArrayList();
            humans.add(new Human("A", 10));
            humans.add(new Human("C", 14));
            humans.add(new Human("C", 12));
            humans.add(new Human("B", 12));
            Collections.sort(humans, Comparator.comparing(Human::getName));
            for (Human bean : humans) {
                System.out.println(bean.toString());
            }
        }
    
        //    反转排序
        @Test
        public void whenSortingEntitiesByNameReversed_thenCorrectlySorted() {
            List<Human> humans = new ArrayList();
            humans.add(new Human("A", 10));
            humans.add(new Human("C", 14));
            humans.add(new Human("C", 12));
            humans.add(new Human("B", 12));
            Comparator<Human> comparator = (h1, h2) -> h1.getName().compareTo(h2.getName());
    
            humans.sort(comparator.reversed());
            for (Human bean : humans) {
                System.out.println(bean.toString());
            }
        }
    
        //    多条件排序
        @Test
        public void whenSortingEntitiesByNameThenAge_thenCorrectlySorted() {
            List<Human> humans = new ArrayList();
            humans.add(new Human("A", 10));
            humans.add(new Human("C", 14));
            humans.add(new Human("C", 12));
            humans.add(new Human("B", 12));
            humans.sort((lhs, rhs) -> {
                if (lhs.getName().equals(rhs.getName())) {
                    return lhs.getAge() - rhs.getAge();
                } else {
                    return lhs.getName().compareTo(rhs.getName());
                }
            });
            for (Human bean : humans) {
                System.out.println(bean.toString());
            }
        }
    
        //    多条件组合排序
        @Test
        public void givenComposition_whenSortingEntitiesByNameThenAge_thenCorrectlySorted() {
            List<Human> humans = new ArrayList();
            humans.add(new Human("A", 10));
            humans.add(new Human("C", 14));
            humans.add(new Human("C", 12));
            humans.add(new Human("B", 12));
            humans.sort(Comparator.comparing(Human::getName).thenComparing(Human::getAge));
            for (Human bean : humans) {
                System.out.println(bean.toString());
            }
        }
    }