stream分组API

末蓝、 2022-09-03 04:20 208阅读 0赞

public class YebTest {
        
    
        List<User> userList = new ArrayList<User>(){
        {
        
            add( new User("A","张三"));
            add( new User("A","李四"));
            add(new User("C","王五"));
        }};
    
    
        /**
         * A->张三
         * B->李四
         * C->王五
         *
         * java.lang.IllegalStateException: Duplicate key com.taimeitech.app.edc.app.api.web.cdisc.User@5e5792a0
         */
        @Test
        public void test1(){
        
            Map<String, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getName));
            map.forEach((key,value) -> {
        
                System.out.print(key + "->");
                System.out.println(value);
            });
        }
    
        /**
         * A->A张三
         * B->B李四
         * C->C王五
         *
         * java.lang.IllegalStateException: Duplicate key com.taimeitech.app.edc.app.api.web.cdisc.User@5e5792a0
         */
        @Test
        public void test2(){
        
    //        Map<String, User> map = userList.stream().collect(Collectors.toMap(User::getId, t -> t));
            Map<String, User> map = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));
            map.forEach((key,value) -> {
        
                System.out.print(key + "->");
                System.out.println(value.getId() + value.getName());
            });
        }
    
    
        /**
         * A->A张三
         * C->C王五
         */
        @Test
        public void test3(){
        
    //        Map<String, User> map = userList.stream().collect(Collectors.toMap(User::getId, t -> t));
            Map<String, User> map = userList.stream().collect(Collectors.toMap(User::getId, Function.identity(),(t1,t2) -> t1));
            map.forEach((key,value) -> {
        
                System.out.print(key + "->");
                System.out.println(value.getId() + value.getName());
            });
        }
    
        /**
         * A->张三
         * 李四
         * C->王五
         */
        @Test
        public void test4(){
        
    //        Map<String, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getName, (t1,t2) ->  t1 + t2));
            Map<String, List<User>> map = userList.stream().collect(Collectors.groupingBy(t -> t.getId()));
            map.forEach((key,value) -> {
        
                System.out.print(key + "->");
                List<String> collect = value.stream().map(user -> user.getName()).sorted().collect(Collectors.toList());
                collect.stream().forEach(System.out::println);
            });
        }
    }