generics

问题 Libraries like Jackson can create objects from JSON if we provide enough information about generics. In Jackson, we can do Class<?>[] parameterTypes; JavaType type = objectMapper.getTypeFactory().constructParametricType(ObjectClass, parameterTypes); objectMapper.readValue(json, type); In java, a Generic class can be defined in many ways, like one generic class having another generic class and that can have another generic class, for simple illustration consider these three classes. public

问题 I suppose foreach method in Traversable trait is defined as follows: def foreach[U](f: Elem => U) Should it be defined as foreach[U,Elem](f:Elem =>U) as there are two types, Elem and U ? 回答1: foreach[U](A => U) is a method in the trait Traversable[+A] . trait Traversable[+A] extends TraversableLike[A, ...] { ... } trait TraversableLike[+A, ...] extends ... { ... def foreach[U](f: A => U): Unit ... } https://github.com/scala/scala/blob/2.12.x/src/library/scala/collection/TraversableLike.scala

问题 Say I have a class CoolStorageClass , which inherits from StorageClassBase : public abstract class StorageClassBase { } public class CoolStorageClass : StorageClassBase { } Then I have a generic abstract BaseClass<T> . It is important, that T can only be of type StorageClassBase . public abstract class BaseClass<T> where T : StorageClassBase { } Then I have the implementation of the BaseClass with T as CoolStorageClass in the form of CoolClass : public class CoolClass : BaseClass

问题 Say I have a class CoolStorageClass , which inherits from StorageClassBase : public abstract class StorageClassBase { } public class CoolStorageClass : StorageClassBase { } Then I have a generic abstract BaseClass<T> . It is important, that T can only be of type StorageClassBase . public abstract class BaseClass<T> where T : StorageClassBase { } Then I have the implementation of the BaseClass with T as CoolStorageClass in the form of CoolClass : public class CoolClass : BaseClass

问题 In dart you can do class Preference<T extends int> to define a type constraint. But is there a way to define multiple constrains? I tried class Preference<T extends int, String> But an error get's thrown when I try to pass a argument of type T to a function that excepts a String saying The argument type 'T' can't be assigned to the parameter type 'String' 回答1: No, Dart type parameters can only have a single constraint. There is no workaround. 来源： https://stackoverflow.com/questions/61820181

问题 I am trying to assert in a function if an object is instance of a class with type predicate. function assertClass<CL extends new (...args: any[]) => any>(v: any, theClass: CL, message: string): v is CL { let is = v instanceof theClass ; assert(is, message); return is; } (credit) And it works except when the constructor of the class is private class Cat{} class Dog{ private constructor(){ } static async build(){ let ret = new Dog(); await somethingElse(); return ret; } } let animal: Cat | Dog;

问题 I'm writing code for a command shell that looks like this: interface Context<C extends Context<C>> {} interface RecursiveContext<C extends RecursiveContext<C>> extends Context<C> { Shell<C> getShell(); default Result execute(Command cmd) { return getShell().execute(cmd, this); } } interface Shell<C extends Context<C>> { C newContext(); Result execute(Command cmd, C context); } I'm getting an error in the default method saying The method execute(Command, C) in the type Shell<C> is not

问题 I'm writing code for a command shell that looks like this: interface Context<C extends Context<C>> {} interface RecursiveContext<C extends RecursiveContext<C>> extends Context<C> { Shell<C> getShell(); default Result execute(Command cmd) { return getShell().execute(cmd, this); } } interface Shell<C extends Context<C>> { C newContext(); Result execute(Command cmd, C context); } I'm getting an error in the default method saying The method execute(Command, C) in the type Shell<C> is not

问题 I am writing a custom hook to fetch some data from an API. I would like the returned data to be type-safe if possible. Can this be done with generics? type Action = { type: 'PENDING' } | { type: 'SUCCESS'; payload: any } | { type: 'FAIL' }; interface State { isLoading: boolean; isError: boolean; data: any; } const dataFetchReducer = (state: State, action: Action): State => { switch (action.type) { case 'PENDING': return { ...state, isLoading: true, }; case 'SUCCESS': { return { ...state,

问题 Let's suppose I have the following task: var task = _entityManager.UseRepositoryAsync(async (repo) => { IEnumerable<Entity> found = //... Get from repository return new { Data = found.ToList() }; } What is the type of task ? Actually, it turns out to be: System.Threading.Tasks.Task<'a> , where 'a is anonymous type: { List<object> Data } How can I explicitly state this type without using var ? I have tried Task<a'> task = ... or Task<object> task = ... but can't manage it to compile. Why do I