Dart:不重写泛型方法具有与返回类型相似的泛型参数类型 [英] Dart: doesn't override generic method has generic parameter type similar to the return type

问题描述

我有一个问题要从抽象类中重写泛型方法.

I have an issue to override a generic method from an abstract class.

这是我的抽象课:

abstract class A {
  String getData<Type>(Type key);
}

当我创建一个类(B)来实现类(A)时，如下所示:

when I created a class (B) to implement class (A) as showing below:

class B implements A {
  @override
  String getData<String>(String key) {     //Error <= B.getData' ('String Function<String>(String)') isn't a valid override of 'A.getData' ('String Function<Type>(Type)')

    return "";
  }
}

在(getData)方法中显示以下编译错误:

Shown the below compilation error at (getData) method:

"B.getData"(字符串函数(字符串)")不是对"A.getData"(字符串函数(类型)")的有效替代.dart(invalid_override)

'B.getData' ('String Function(String)') isn't a valid override of 'A.getData' ('String Function(Type)'). dart(invalid_override)

返回语句中的错误:

无法从方法"getData"返回"String"类型的值，因为它的返回类型为"String".

A value of type 'String can't be returned from method 'getData' because it has a return type of 'String'.

当通用类型类似于返回类型时，为什么我会得到与覆盖相关的错误，这使我感到困惑.当我用 getData< int>(int键)而不是 getData< String>(String键)创建另一个类(C)时，一切按预期进行:

It's confusing for me why I am getting this error related to the override when the generic type similar to return type. as when I created another class (C) with getData<int>(int key) instead of getData<String>(String key) everything works as expected:

class C implements A {
  @override
  String getData<int>(int key) {

    return "";
  }
}

如果我删除了如下所示的返回类型，则使用(B)类，一切都会按预期进行:

Also with the class (B) if I deleted the return type as shown below everything will work as expected:

class B implements A {

  @override
  getData<String>(String key) {

     return "";
  }
}

这在Dart设计中是否是一个问题，所以我可以意识到，因为这种行为例如在c#语言中不存在?

Is it an issue in Dart design so I can be aware of it because this behavior does not exist in c# language for example?

DartPad链接: https://dartpad.dev/ebc7e6361d0cf1c6afad2f52ab8ebcaa

推荐答案

方法签名

String getData<Type>(Type key);

由于 Type 此签名用于允许为 T 填写任何类型的方法.这意味着每个子类都必须具有允许 any 类型 T 的方法.

This signature is for a method that allows any type to be filled in for T. This means that every subclass must have a method that allows for any type T.

我怀疑您想要的是将子类专用于单个泛型类型-在这种情况下，您希望将泛型从方法级别移至类级别.

I suspect that what you want is for subclasses to be specialized to a single generic type - in that case you want to move the generic from the method level to the class level.

abstract class A<T> {
  String getData(T key);
}

class B implements A<String> {
  @override
  String getData(String key) => '';
}

class C implements A<int> {
  @override
  String getData(int key) => '';
}

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