使用stream api替代sql

假设mysql数据库中有两张表：

user用户表

  company企业表

当这以上两种类型的数据不是存放在数据库中，而是分别来自两个接口，如果想要对分别来自两个不同的接口的数据做一些join，group(sum)，order，limit等操作的时候，我们就需要使用stream api来进行处理

在java中用list来封装用户，企业信息：

    static List<User> buildUserList(){
        ArrayList list = new ArrayList();
        list.add(new User(1,"yc",31,2));
        list.add(new User(2,"yf",32,2));
        list.add(new User(3,"yy",29,1));
        list.add(new User(4,"yl",26,1));
        list.add(new User(5,"ygf",27,3));
        return  list;
    }
    static List<Company> buildCompanyList(){
        ArrayList list = new ArrayList();
        list.add(new Company(1,1,"H001","WN"));
        list.add(new Company(2,2,"H001","WN"));
        list.add(new Company(3,3,"H002","CXY"));
        return  list;
    }

@Data
public class Company {
    private Integer id;
    private Integer userId;
    private String code;
    private String name;

    public Company(Integer id, Integer userId, String code, String name) {
        this.id = id;
        this.userId = userId;
        this.code = code;
        this.name = name;
    }
}

@Data
public class User {
    private Integer id;
    private String name;
    private Integer age;
    private Integer type;

    public User(Integer id, String name, Integer age, Integer type) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.type = type;
    }
}

 

order

用mysql实现：

SELECT * FROM user order by age DESC;

 

 用stream api（sorted）实现：

    static List<User> processOrder(List<User> userList){
       return userList.stream().sorted(Comparator.comparing(User::getAge).reversed()).collect(Collectors.toList());
    }

 limit

 用mysql实现：

SELECT * FROM user order by age DESC limit 2;

 用stream api（limit）实现：

    static List<User> processOrderLimit(List<User> userList){
        return userList.stream().sorted(Comparator.comparing(User::getAge).reversed()).limit(2).collect(Collectors.toList());
    }

  

limt分页

 用mysql实现：

SELECT * FROM user order by age DESC limit 0,2;
SELECT * FROM user order by age DESC limit 2,2;
SELECT * FROM user order by age DESC limit 4,2;

 其中limit offset,length其中 offset= (pageIndex-1)*pageSize ,length=pageSize

用stream api（skip,limit）实现：

List<User> userList = processOrderLimitPage(buildUserList(),1,2);
List<User> userList = processOrderLimitPage(buildUserList(),2,2);
List<User> userList = processOrderLimitPage(buildUserList(),3,2);

    static List<User> processOrderLimitPage(List<User> userList,Integer pageIndex,Integer pageSize){
        return userList.stream().sorted(Comparator.comparing(User::getAge).reversed()).skip((pageIndex-1)*pageSize).limit(pageSize).collect(Collectors.toList());
    }

group(sum)，order

 用sql实现：

SELECT SUM(age) num,type FROM user GROUP BY type ORDER BY num DESC;

 用stream api（groupingBy,sorted）实现：

    static Map<Integer,Integer> processGroupSum(List<User> userList){
        return userList.stream().collect(Collectors.groupingBy(User::getType,Collectors.summingInt(User::getAge))).entrySet().stream().sorted(Map.Entry.comparingByValue(Comparator.reverseOrder())).collect(Collectors.toMap(Map.Entry::getKey,
                Map.Entry::getValue,
                (oldVal, newVal) -> oldVal,
                LinkedHashMap::new));
    }

  用stream api（merge,sorted）实现：

    static Map<Integer,Integer> processMergeSum(List<User> userList){
        Map<Integer,Integer> map = new HashMap<>();
        userList.stream().forEach(x->map.merge(x.getType(),x.getAge(),Integer::sum));
        return map.entrySet().stream().sorted(Map.Entry.comparingByValue(Comparator.reverseOrder())).collect(Collectors.toMap(Map.Entry::getKey,
                Map.Entry::getValue,
                (oldVal, newVal) -> oldVal,
                LinkedHashMap::new));
    }

 inner join,left join,right join

 用mysql实现：

select * from `user` INNER JOIN company ON `user`.id = company.user_id;
select * from `user` left  JOIN company ON `user`.id = company.user_id;
select * from `user` right JOIN company ON `user`.id = company.user_id;

用stream api实现：

List<UserCompany> userCompanyList = processJoin(buildUserList(),buildCompanyList(),"inner");
List<UserCompany> userCompanyList = processJoin(buildUserList(),buildCompanyList(),"left");
List<UserCompany> userCompanyList = processJoin(buildUserList(),buildCompanyList(),"right");

    static List<UserCompany> processJoin(List<User> userList, List<Company> companyList,String type) {
        Map<Integer, List<User>> userMap = userList.stream().collect(Collectors.groupingBy(User::getId));
        Map<Integer, List<Company>> companyMap = companyList.stream().collect(Collectors.groupingBy(Company::getUserId));
        Set<Integer> integerList = new HashSet<>();
        if("join".equals(type)){
            integerList = userMap.keySet().stream().filter(companyMap.keySet()::contains).collect(Collectors.toSet());
            Set<Integer> finalIntegerList = integerList;
            userMap = userMap.entrySet().stream().filter(x -> finalIntegerList.contains(x.getKey())).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
            companyMap = companyMap.entrySet().stream().filter(x -> finalIntegerList.contains(x.getKey())).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        }

        if("left".equals(type)){
            integerList = userMap.keySet();
        }else if("right".equals(type)){
            integerList = companyMap.keySet();
        }
        List<UserCompany> userCompanyList = new ArrayList<>();
        Map<Integer, List<User>> finalUserMap = userMap;
        Map<Integer, List<Company>> finalCompanyMap = companyMap;
        integerList.forEach(x -> {
            List<User> userList1 = finalUserMap.get(x);
            List<Company> companyList1 = finalCompanyMap.get(x);
            userList1.forEach(m -> {
                if(!CollectionUtils.isEmpty(companyList1)) {
                    companyList1.forEach(n -> {
                        userCompanyList.add(new UserCompany(m, n));
                    });
                }else{
                    userCompanyList.add(new UserCompany(m,null));
                }
            });
        });
        return userCompanyList;
    }

@Data
public class UserCompany {
    private Integer userId;
    private String name;
    private Integer age;
    private Integer type;

    private Integer companyId;
    private String code;
    private String companyName;


    public UserCompany(User user,Company company) {
        if(user != null){
            this.userId = user.getId();
            this.name = user.getName();
            this.age = user.getAge();
            this.type = user.getType();
        }

        if(company != null){
            this.companyId = company.getId();
            this.code = company.getCode();
            this.companyName = company.getCode();
        }
    }
}