blocks|key|1560497|text|首先，在Java中，它是泛型，而不是C%2B%2B中的模板。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1560498|在类T中声明泛型类型参数B，但没有使用它。您应该在整个T类定义过程中使用T而不是A。|offset|length|style|CODE|1560499|public+static+class+B<T>+{
++++List<T>+aList+=+new+ArrayList<T>();

++++public+List<T>+getAList()+{
++++++++return+aList;
++++}
}|code-block|syntax|javascript|1560500|然后，在声明类B的实例时，应该使用类型参数。|1560501|B<A>+b+=+new+B<A>();|1560502|但是，如果您知道aList变量总是保存A类型的对象，就像方法名getAList建议的那样，那么就没有理由使类B泛化。|1560503|entityMap^0|0|2|1|C|1|R|1|10|1|14|1|0|0|7|1|0|0|8|5|J|1|V|8|1I|1|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]|$D|Y|E|Z|F|G]|$D|10|E|11|F|G]|$D|12|E|13|F|G]|$D|14|E|15|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|16|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|17|8|@$D|18|E|19|F|G]]|9|@]|A|$]]|$1|O|3|P|5|J|7|1A|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|1B|8|@$D|1C|E|1D|F|G]|$D|1E|E|1F|F|G]|$D|1G|E|1H|F|G]|$D|1I|E|1J|F|G]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]]|T|$]]

First, in Java, it's Generics, not Templates like it is in C++.

You are declaring a generic type parameter <code>T</code> in your class <code>B</code> but you aren't using it. You should use <code>T</code> instead of <code>A</code> throughout your <code>B</code> class defintion.

<pre><code>public static class B&lt;T&gt; {
 List&lt;T&gt; aList = new ArrayList&lt;T&gt;();

 public List&lt;T&gt; getAList() {
 return aList;
 }
}
</code></pre>

Then you should use a type parameter when you declare your instance of class <code>B</code>.

<pre><code>B&lt;A&gt; b = new B&lt;A&gt;();
</code></pre>

But if you know that your <code>aList</code> variable will always hold objects of type <code>A</code> as the method name <code>getAList</code> suggests, then there would be no reason to make class <code>B</code> generic.

blocks|key|2351548|text|这与这个bug非常相似：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2351549|6760983+:不相关方法中未使用的类型参数触发错误|offset|length|2351550|报告如下：|2351551|未使用的泛型引发问题|2351552|entityMap|0|LINK|mutability|MUTABLE|url|http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6760983|1|http://www.coderanch.com/t/385316/java/java/Unused-Generic-causing^0|0|0|R|0|0|0|0|A|1|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@]|9|@$D|V|E|W|1|X]]|A|$]]|$1|F|3|G|5|6|7|Y|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@]|9|@$D|10|E|11|1|12]]|A|$]]|$1|J|3|-4|5|6|7|13|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|Q]]|R|$5|M|N|O|A|$P|S]]]]

This is really similar to this bug:

<a href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6760983" rel="nofollow">6760983 : Unused type parameter triggering error in unrelated method</a>

Which is reported here:

<a href="http://www.coderanch.com/t/385316/java/java/Unused-Generic-causing" rel="nofollow">Unused Generic causing problem</a>

blocks|key|241000|text|是的，已知的行为是，如果使用原始类型，那么类上的所有类型参数都会丢失，而不仅仅是没有声明的类型级别参数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|241001|这个问题的部分原因是：|241002|如果我指定B的模板(如B<String>+)，或者从B中完全删除该模板，那么一切都正常。|blockquote|offset|length|style|CODE|241003|这不是一个选项，您不能选择是否指定类型参数。它在没有指定参数的情况下编译的唯一原因是为了向后兼容性。编写缺少类型参数的新代码是编程错误。|241004|List<A>+list+=+b.getList()没有成功地解释类型，它只是有效地停留在任意的类型中，并且相信分配是正确的。如果您查看编译器警告，它实际上正在为不安全的转换生成警告。|241005|for(A+a+:+b.getList())+{}将该警告升级为错误，因为插入的强制转换将在编译器生成的代码中，因此它完全拒绝自动生成不安全的代码，而不只是发出警告。|241006|来自java语言规范：|241007|只允许使用原始类型作为对遗留代码兼容性的让步。在Java编程语言中引入泛型之后编写的代码中使用原始类型是非常不可取的。Java编程语言的未来版本有可能不允许使用原始类型。|241008|实际上，java泛型与C%2B%2B模板共享的唯一重要内容是<>语法:)|241009|更多细节：什么是原始类型，我们为什么不使用它？|241010|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/2770321/what-is-a-raw-type-and-why-shouldnt-we-use-it?answertab=votes#tab-top^0|0|0|B|9|0|0|0|Q|0|0|P|0|0|0|0|5|I|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|17|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|18|8|@$G|19|H|1A|I|J]]|9|@]|A|$]]|$1|K|3|L|5|6|7|1B|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1C|8|@$G|1D|H|1E|I|J]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1F|8|@$G|1G|H|1H|I|J]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1I|8|@]|9|@]|A|$]]|$1|S|3|T|5|F|7|1J|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1K|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1L|8|@]|9|@$G|1M|H|1N|1|1O]]|A|$]]|$1|Y|3|-4|5|6|7|1P|8|@]|9|@]|A|$]]]|Z|$10|$5|11|12|13|A|$14|15]]]]

Yes, it is known behaviour that if you use a raw type, then all type parameters on the class are lost, not just the type-level parameter that you failed to declare.

The issue is partly here:

<blockquote>
 If I specify the template of B like <code>B&lt;String&gt;</code>, or if I remove the
 template from B completely, then everything is ok.
</blockquote>

That's not an option, you aren't to choose if you want to specify the type parameter or not. The only reason it compiles at all with no parameter specified is for backward compatibility. Writing new code with missing type parameters is a programming error.

<code>List&lt;A&gt; list = b.getList()</code> does not successfully interpret the type, it is just effectively sticking in an arbitrary cast and trusting you that the assignment is correct. If you look at the compiler warnings it is in fact generating a warning for an unsafe conversion.

<code>for(A a : b.getList()) {}</code> upgrades that warning to an error because the inserted cast would be inside compiler generated code, so it refuses to auto-generate unsafe code at all, rather than just give a warning.

From the java language specification:

<blockquote>
 The use of raw types is allowed only as a concession to compatibility
 of legacy code. The use of raw types in code written after the
 introduction of genericity into the Java programming language is
 strongly discouraged. It is possible that future versions of the Java
 programming language will disallow the use of raw types.
</blockquote>

Bottom line really is that the only significant thing java generics share with C++ templates is the &lt;> syntax :)

More details: <a href="https://stackoverflow.com/questions/2770321/what-is-a-raw-type-and-why-shouldnt-we-use-it?answertab=votes#tab-top">What is a raw type and why shouldn&#39;t we use it?</a>

Why do collections that are not related to the template class drop their type? Here is an example: (Sorry, it will not compile because of the error I'm confused about.)

<pre><code>package test;

import java.util.ArrayList;
import java.util.List;

public class TemplateTest {

 public static class A { }

 public static class B&lt;T extends Comparable&gt; {
 List&lt;A&gt; aList = new ArrayList&lt;A&gt;();

 public List&lt;A&gt; getAList() {
 return aList;
 }

 public int compare(T t, T t1) {
 return t.compareTo(t1);
 }
 }

 public static void main(String[] args) {
 B b = new B();
 for (A a : b.getAList()) { //THIS DOES NOT WORK

 }
 List&lt;A&gt; aList = b.getAList(); //THIS WORKS
 for (A a : aList) {

 }
 }
}
</code></pre>

This code throws an error upon compilation:

<pre><code>test/TemplateTest.java:24: incompatible types
 found : java.lang.Object
 required: test.TemplateTest.A
 for (A a : b.getAList()) {
</code></pre>

If I specify the template of <code>B</code> like <code>B&lt;String&gt;</code>, or if I remove the template from B completely, then everything is ok.

What's going on?

EDIT: people pointed out there was no need to make B generic so I added to B

Generic screws up non-related collection

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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为什么与模板类无关的集合会删除其类型？这里有一个例子：(对不起，由于我很困惑的错误，它不会编译。)package test;import java.util.ArrayList;import java.util.List;public class TemplateTest {    public static clas...

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