方法返回类型包含子类定义 [英] Method return type contains subclass definition

问题描述

  public< A extends B> double [] [] foo（C arg）{
 ....} 
  

I
$ b a） foo（C arg）的返回类型是< A扩展B>双[] [] ？这是什么意思？例如，我会理解返回类型，比如 double [] [] >，但我无法确定前一个修饰符（可能？）< A b）为什么在返回类型中有一个子类声明？由于A是B的一个子类，我们在哪里重写或添加任何方法/成员等？对我来说，它似乎是一个只包含基类A的相同方法/成员的子类。不是吗？那么在编写 public< A> c）最后，我认为<>必须处理Java泛型，但是甚至那么我已经看到类似于 D 的声明，其中T用于参数化原始类型 D 。在这里，我尝试删除<>（因为我不明白它们代表的是什么），但是编译器抱怨了。


返回类型是 double [] [] 。 < A扩展B> 部分是通用方法

restriction 扩展B 意味着只有泛型类型 A 扩展或者实际上在这个方法中实现 B 是允许的。

无法回答第三个问题，因为它需要方法体中的代码，更多上下文。

I come up with a code that uses a syntax like this:

public <A extends B> double[][] foo(C arg) {
.... }

I get a couple of questions by viewing it.

a) The return type of foo(C arg) is <A extends B> double[][]? What does this mean? I would understood a return type like double[][] for example, but I cannot determine what does the previous modifier (maybe?) <A extends B> does?

b) Why there is a subclass declaration inside a return type? Since A is a subclass of B where do we override or add any methods/members etc? To me it seems that it's a subclass containing just the same methods/members of the base class A. Isn't so? So is there any difference in writing public <A> double[][] foo(C arg)?

c) Finally I suppose that <> have to do with Java generics but even then I have seen declaration like D<T> which T is used to parametrize the raw type D. Here I tried to remove the <> (since I don't understand what they stand for) but compiler complains.

解决方案

1. The return type is double[][]. The <A extends B> part is the parametrization of the generic method
2. The restriction extends B implies that only generic types A extending or actually implementing B are allowed in this method.
3. Cannot answer your third question as it would require the code in the method's body and more context.

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