两层For循环的一些性能优化

本是古典何须时尚 2021-12-16 00:43 529阅读 0赞

import JavaBase.User;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.function.Function;
    import java.util.stream.Collectors;
    
    /**
     * @author 633805 LYH
     * @version V1.0
     * @description 对类的描述
     * @create 2019-07-02 20:37
     * @since 1.7
     */
    public class ForTest {
    
        public static void main(String[] args) {
    
    
            List<User> list1 = new ArrayList<>();
    
    
    
            List<User> list2 = new ArrayList<>();
    
            long start = System.currentTimeMillis();
    
            for (int i =0 ; i< 5000; i++){
                User user = User.builder().name("lyh"+i).age(12 + i).build();
    
                list1.add(user);
            }
    
            for (int i =0 ; i< 5000; i++) {
    
                User user2 = User.builder().age(12 + i).build();
    
                list2.add(user2);
            }
    
            //方法一
            /*for (User us : list1 ) {
    
                for(User user : list2) {
                    if (user.getAge() == us.getAge()) {
                        user.setName(us.getName());
                    }
                }
            }*/
    
            //方法二
            /*for(int i =0 ; i < list1.size(); i++) {
                for (int j = 0; j < list2.size(); j++) {
                    if (list1.get(i).getAge().equals(list2.get(j).getAge())) {
                        list2.get(j).setName(list1.get(i).getName());
                    }
                }
            }*/
    
            //方法三
            /*int i, j, leni, lenj;
            for( i =0 , leni = list1.size(); i < leni; i++) {
                for ( j = 0, lenj = list2.size(); j < lenj; j++) {
                    if (list1.get(i).getAge().equals(list2.get(j).getAge())) {
                        list2.get(j).setName(list1.get(i).getName());
                    }
                }
            }*/
    
            //方法四
            /*Map<Integer, User> map = new HashMap<>();
            for (int i = 0; i < list1.size(); i++) {
                map.put(list1.get(i).getAge(), list1.get(i));
            }
    
            for (int j =0; j < list2.size(); j ++) {
                //Map<User, Integer> map1 = new HashMap<>();
                //map1.put(list2.get(j), list2.get(j).getAge());
    
                if (map.get(list2.get(j).getAge()) != null) {
                    list2.get(j).setName( map.get(list2.get(j).getAge()).getName() );
                }
            }*/
    
            //方法五
            Map<Integer, User> map = list1.stream().collect(Collectors.toMap(User::getAge, Function.identity() , (key1, key2) -> key2));
    
            list2 = list2.stream().map(user -> {
                Integer age = user.getAge();
                User user1 = map.get(age);
                user.setName(user1.getName());
                return user;
            }).collect(Collectors.toList());
    
            list1.addAll(list2);
    
            list1.forEach(System.out::println);
    
            System.out.println(list1.size());
    
            System.out.println(System.currentTimeMillis() - start);
    
    
        }
    }