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问题描述

我一直对我的头撞了一阵子,认为也许有些新鲜的眼睛会看到这个问题;感谢您的时间。

  b.b 

  Test.java:15:error:找不到添加的合适方法(Tbin< Derived>)
test2.add(new Tbin<衍生GT;());
$

如何写 Tbin 和 TbinList 所以最后一行可以接受?

请注意,我实际上会添加键入 Tbin s这就是为什么我在最后一行指定 Tbin< Derived> 的原因。

解决方案

好的,这里是答案:

  import java.util。* ; 

class Tbin< T>扩展ArrayList< T> {}
class TbinList< T>扩展ArrayList< Tbin<延伸T>> {}

class Base {}
class Derived extends Base {}
$ b public class Test {
public static void main(String [] args) {

TbinList< Base> test3 = new TbinList<>();
test3.add(new Tbin< Derived>());


$ b code
$ b $ p $正如我所预料的那样,一旦我看到它。但是很多人为了到这里而th ing不安。 Java的泛型看起来很简单,如果你只看代码。



感谢大家,作为一个声音板。


I've been beating my head against this one for awhile and thought that maybe some fresh eyes will see the issue; thanks for your time.

import java.util.*;

class Tbin<T> extends ArrayList<T> {}
class TbinList<T> extends ArrayList<Tbin<T>> {}

class Base {}
class Derived extends Base {}

public class Test {
  public static void main(String[] args) {
    ArrayList<Tbin<? extends Base>> test = new ArrayList<>();
    test.add(new Tbin<Derived>());

    TbinList<? extends Base> test2 = new TbinList<>();
    test2.add(new Tbin<Derived>());
  }
}

Using Java 8. It looks to me like the direct creation of the container in test is equivalent to the container in test2, but the compiler says:

Test.java:15: error: no suitable method found for add(Tbin<Derived>)
    test2.add(new Tbin<Derived>());
         ^

How do I write Tbin and TbinList so the last line is acceptable?

Note that I will actually be adding typed Tbins which is why I specified Tbin<Derived> in the last line.

解决方案

OK, here's the answer:

import java.util.*;

class Tbin<T> extends ArrayList<T> {}
class TbinList<T> extends ArrayList<Tbin<? extends T>> {}

class Base {}
class Derived extends Base {}

public class Test {
  public static void main(String[] args) {

    TbinList<Base> test3 = new TbinList<>();
    test3.add(new Tbin<Derived>());

  }
}

As I expected, kind of obvious once I saw it. But a lot of thrashing around to get here. Java generics seem simple if you only look at working code.

Thanks, everyone, for being a sounding board.

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09-12 08:22