这是什么意思< S延伸T>保存（S实体）;在Spring Repository中？ [英] What does it mean <S extends T> save (S entity); in Spring Repository?

问题描述

在Spring Data项目中，CrudRepository为正在管理的实体类提供了复杂的CRUD功能。

  public interface CrudRepository< T，ID扩展了Serializable> 
扩充知识库< T，ID> {
 
< S延伸T> S保存（S实体）; 
 
 T findOne（ID primaryKey）; 
 
可重用< T>找到所有（）; 
 
 Long count（）; 
 
 void delete（T entity）; 
 
布尔值存在（ID primaryKey）; 
 
 // ...省略了更多功能。 
} 
  

一般而言，我知道S延伸T的含义，即S ，保存操作的返回类型，必须是T的子类型。为什么需要添加如约束？我认为可以这样做：

  T save（T entity）; 
  

与

  void delete（T entity）; 
  

我已经搜索到了更多的帮助，并且我已经找到了以下关于stackoverflow本身的问题，但它对我来说不是很清楚：

Spring CrudRepository：为什么要发明一个新的泛型类型S

谢谢。

解决方案

如果您将其作为

  T保存（T实体）; 
  

然后，您可以分配结果的唯一变量必须是 T 。

因此，如果您有 CrudRepository< Animal，AnimalID> repository ，并且你有

  Dog dog = getDog（）; 
 Dog savedDog = repository.save（dog）; 
  

你会得到一个编译错误 - 你不能将结果赋给狗，因为它必须是 T 类型，在这种情况下， Animal 。

你需要检查返回的值是否确实是 Dog 类型，如果是这样，cast它将 Dog 放入 savedDog 。

如果是这样的声明，就意味着你可以将它分配给一个与原始参数类型相同的变量，因为类型解析会允许这样做。

声明本身并没有说明如何保存狗的非动物部分。如果它最初是 Dog 。

$，它允许将结果分配给 Dog b $ b

In Spring Data project the CrudRepository provides sophisticated CRUD functionality for the entity class that is being managed.

public interface CrudRepository<T, ID extends Serializable>
extends Repository<T, ID> {

    <S extends T> S save(S entity);

    T findOne(ID primaryKey);

    Iterable<T> findAll();

    Long count();

    void delete(T entity);

    boolean exists(ID primaryKey);

    // … more functionality omitted.
}

In general, I know what "S extends T" means, i.e. that S, the return type of save operation, must be subtype of T. Why is it necessary to add such as constraint? I think that would be fine doing something like this:

T save (T entity);

As in

void delete(T entity);

I've googled to find more help and I've figured out following question on stackoverflow itself but it isn't really clear for me:

Spring CrudRepository: why to invent a new generic type S

thank you.

解决方案

If you were to have it as

T save (T entity);

Then the only variable you could assign the result to would have to be of type T.

So, if you have a CrudRepository<Animal,AnimalID> repository, and you have

Dog dog = getDog();
Dog savedDog = repository.save(dog);

You'd get a compile error - you can't assign the result to Dog, as it has to be of type T, in this case, Animal.

You'd need to check if the returned value was indeed of type Dog, and if so, cast it to Dog to put it in savedDog.

With the declaration as it is, it means that you can assign it to a variable of the same type as the original argument, as type resolution will allow that.

The declaration itself doesn't specify how the non-animal parts of the dog are saved if at all. All it does is allow assigning the result back to a Dog if it was originally a Dog.

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