blocks|key|923641|text|在Class类型的参数声明中使用T类型的位置进行限制会更有意义。在传入类型时，应该使用Class<?+extends+T>，而不是使用Class<?>之类的东西。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|923642|entityMap^0|1|5|G|1|17|I|1V|8|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|G|$]]

It would make more sense to put the restriction on where the type <code>T</code> is used to parametrise the <code>Class</code> type. When you pass the type in, instead of using something like <code>Class&lt;?&gt;</code>, you should use <code>Class&lt;? extends T&gt;</code>.

blocks|key|923701|text|执行此检查的唯一方法是使用表示类型的Class对象：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|923702|Class<T>+type;+//maybe+passed+into+the+method
if+(+type.isInstance(obj)+)+{
+++//...
}|code-block|syntax|javascript|923703|entityMap^0|I|5|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

The only way you can do this check is if you have the <code>Class</code> object representing the type:

<pre><code>Class&lt;T&gt; type; //maybe passed into the method
if ( type.isInstance(obj) ) {
 //...
}
</code></pre>

blocks|key|923772|text|只有当你有一个类型为T的对象时，这才能(部分地)起作用。然后你可以获取该对象的类，查看java.lang.Class<T>，看看它是否与有问题的对象相同。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|923773|但请注意，这与我们拥有genrics的原因背道而驰:使用泛型类型是一种方式，表示您并不关心它到底是什么类型(直到可能指定的上下限)。|923774|entityMap^0|17|I|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|N|8|@]|D|@]|E|$]]]|I|$]]

This will only work (partly) if you have an object of type T. Then you can get the class of that object, see <code>java.lang.Class&lt;T&gt;</code> and find if it's the same as the object in question.

But note that this goes counter the very reason we have genrics: using a generic type is a way to say that you don't care what type it really is (up to upper and lower bounds that may be specified).

blocks|key|707409|text|要扩展Mark+Peters的示例，通常需要执行以下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|707410|Class<T>+type;+//maybe+passed+to+the+method
if+(+type.isInstance(obj)+)+{
+++T+t+=+type.cast(obj);
+++//+...
}|code-block|syntax|javascript|707411|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

To extend the sample of Mark Peters, often you want to do something like:

<pre><code>Class&lt;T&gt; type; //maybe passed to the method
if ( type.isInstance(obj) ) {
 T t = type.cast(obj);
 // ...
}
</code></pre>

blocks|key|707482|text|你可以试试这个，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|707483|//+Cast+your+object+to+the+generic+type.
T+data+=+null;
try+{
++++data+=+(T)+obj;
}+catch+(ClassCastException+cce)+{
++++//+Log+the+error.
}

//+Check+if+the+cast+completed+successfully.
if(data+!=+null)+{
++++//+whatever....
}|code-block|syntax|javascript|707484|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You could try this, 

<pre><code>// Cast your object to the generic type.
T data = null;
try {
 data = (T) obj;
} catch (ClassCastException cce) {
 // Log the error.
}

// Check if the cast completed successfully.
if(data != null) {
 // whatever....
}
</code></pre>

blocks|key|923912|text|如果您不想像Mark+Peters提到的那样将Class类型作为参数传递，则可以使用以下代码。称赞大卫·奥米拉。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|923913|++Class<T>+type+=+(Class<T>)+((ParameterizedType)+getClass().getGenericSuperclass())
++++++++++++++++++.getActualTypeArguments()[0];
++if+(type.isInstance(obj))+{
++++++...
++}|code-block|syntax|javascript|923914|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you don't want to pass Class type as a parameter as mentioned by Mark Peters, you can use the following code. Kudos to David O'Meara.

<pre><code> Class&lt;T&gt; type = (Class&lt;T&gt;) ((ParameterizedType) getClass().getGenericSuperclass())
 .getActualTypeArguments()[0];
 if (type.isInstance(obj)) {
 ...
 }
</code></pre>

Is there a way to determine if an object is an instance of a generic type?

<pre><code>public &lt;T&gt; test(Object obj) {
 if (obj instanceof T) {
 ...
 }
}
</code></pre>

That clearly doesn't work. Is there an alternative? Like I want to use Java reflection to instantiate a class and then check to make sure it is of type generic <code>T</code>.

Test if object is instanceof a parameter type

Java

有没有办法确定一个对象是否是泛型类型的实例？public <T> test(Object obj) { if (obj instanceof T) { ... }}这显然是行不通的。有没有别的选择？例如，我希望使用Java反射来实例化一个类，然后进行检查以确保它是泛型T类型。

问测试对象是否为参数类型的实例
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问测试对象是否为参数类型的实例EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问测试对象是否为参数类型的实例
EN