关于java泛型的小问题？

Question

大家好，请看演示代码及问题：

先定义一个类：

public class Arr<S> {
    private S entity;
    private IService<S> service;
    public Arr(S entity,IService<S> service){
        this.entity = entity;
        this.service = service;
    }
}

然后创建一个List,来存放上面类的多个实例：

ArrayList<Arr<?>> list = new ArrayList<>();

现在开始创建实例，并添加到list中：

@Autowired
private IUserService userService;

@Autowired
private IDogService dogService;

@Autowired
private ICarService carService;


Arr<User> a = new Arr<>(new User(),userService);
Arr<Dog> b = new Arr<>(new Dog(),dogService);
Arr<Car> c = new Arr<>(new Car(),carService);
list.add(a);
list.add(b);
list.add(c);

最后，也是问题所在，我开始遍历list:

for (Arr<?> item :list) {
    //我的问题就是:怎么将注释部分的代码改成泛型形式的代码。

    //LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    //wrapper.eq(User::getId,"1");
    //User user = item.service.getOne(wrapper);
    //System.out.println(user);
}

我想在上面遍历代码中，每次取出item的entity属性的类型，然后通过该类型创建Mybatis plus的
LambdaQueryWrapper，但是我怎么能获取到这个属性对应的类型呢？也就是上面注释的User,只不过每次遍历分别是Uesr、Dog、Car。谢谢！~

Answer 1

使用如下方法：

// 获取类
item.getClass();
// 获取类名
item.getClass().getName();

Answer 2

补充 如果只是构造查询条件，其实 mp有多种方式，你这种情况，建议这种玩法

 for (Arr<?> item : list) {
            Class<?> entityClass = item.getEntity().getClass();
            if (entityClass.equals(ChenXiModelSyncCount.class)) {
                ChenXiModelSyncCount chenXiModelSyncCount = (ChenXiModelSyncCount) item.getEntity();
                IChenXiService chenXiService= (IChenXiService) item.getService();
        //重点在这里
                LambdaQueryChainWrapper<ChenXiModelSyncCount> lambdaQueryChainWrapper = chenXiService.lambdaQuery();
                List<ChenXiModelSyncCount> list1 = lambdaQueryChainWrapper.list();
                ChenXiModelSyncCount one = lambdaQueryChainWrapper.eq(ChenXiModelSyncCount::getId, 1).one();
                // ....

                
                //..
            } else if (entityClass.equals(BsDirectoryTreeDo.class)) {
                BsDirectoryTreeDo bsDirectoryTreeDo = (BsDirectoryTreeDo) item.getEntity();
                IBsDirectoryTreeService bsDirectoryTreeService= (IBsDirectoryTreeService) item.getService();
                //...
            }
        }

@Service
public class ArrService {
    @Autowired
    IChenXiService chenXiService;
    @Autowired
    IBsDirectoryTreeService bsDirectoryTreeService;

    public void test() {
        ArrayList<Arr<?>> list = new ArrayList<>();
        Arr<ChenXiModelSyncCount> a = new Arr<>(new ChenXiModelSyncCount(), chenXiService);
        Arr<BsDirectoryTreeDo> c = new Arr<>(new BsDirectoryTreeDo(), bsDirectoryTreeService);
        list.add(a);
        list.add(c);
        for (Arr<?> item : list) {
            Class<?> entityClass = item.getEntity().getClass();
            if (entityClass.equals(ChenXiModelSyncCount.class)) {
                ChenXiModelSyncCount chenXiModelSyncCount = (ChenXiModelSyncCount) item.getEntity();
                IChenXiService chenXiService= (IChenXiService) item.getService();
                //..
            } else if (entityClass.equals(BsDirectoryTreeDo.class)) {
                BsDirectoryTreeDo bsDirectoryTreeDo = (BsDirectoryTreeDo) item.getEntity();
                IBsDirectoryTreeService bsDirectoryTreeService= (IBsDirectoryTreeService) item.getService();
                //...
            }
        }
    }
}

Answer 3

public class App {
    public static void main(String[] args) throws Exception {
        List<Parent> list = Arrays.asList(new User(new UserService()), new Dog(new DogService()));
        list.forEach(Parent::fun);
    }

    interface Service {
        void query();
    }

    static class UserService implements Service {

        @Override
        public void query() {
            System.out.println("User query");
        }
    }

    static class DogService implements Service {
        @Override
        public void query() {
            System.out.println("Dog query");
        }
    }


    static abstract class Parent {
        abstract void fun();
    }

    static class User extends Parent {
        private Service service;

        public User(Service service) {
            this.service = service;
        }

        @Override
        void fun() {
            System.out.println("this is user");
            service.query();
        }
    }

    static class Dog extends Parent {
        private Service service;

        public Dog(Service service) {
            this.service = service;
        }

        @Override
        void fun() {
            System.out.println("this is dog");
            service.query();
        }
    }
}

Answer 4

思路不对，泛型不是这么用的。
除非 Arr<?> 有公共的方法，或者 User、Dog、Car 继承自某个共同的父类
否则for循环List<Arr<?>>没有任何意义 和for循环List<Object>没区别。
你应该先想好自己想做什么，或者明确一下具体的使用场景