测试 object 是否为参数类型的 instanceof

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) {
...
}
}

这显然行不通。还有别的选择吗？就像我想使用 Java 反射来实例化一个类，然后检查它是否是泛型 T类型。

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小开

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

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最佳答案

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

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

小开

This will only work (partly) if you have an object of type T. Then you can get the class of that object, see java.lang.Class<T> 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).

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

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

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You could try this,

// 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....
}

小开

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.

  Class<T> type = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass())
.getActualTypeArguments()[0];
if (type.isInstance(obj)) {
...
}